bash

# 1) (claude code) add time, space complexity to java class
claude

/add-time-space BinarySearch
/add-time-space BFS

# ....

Resource

LC classics problems
- Blind Curated 75
- Grind 75
- Grind 169
- leetcode wiki repo
- leetcode wiki
- LC 官神Github題目分類整理
- LC top 100 likes
- neetcode 150 LC list
  - My Brain after 569 Leetcode Problems
- jiakaobo LC : LC code & video
- LC pattern @ blind : Curated-List-of-Top-100-LeetCode-Questions-to-Save-Your-Time
- LC Algorithm Problem Classification
- cheatsheet-leetcode-a4
- 14-patterns-to-ace-any-coding-interview-question
- grokking-the-coding-interview
LC experiences
- LC難度表
  - 數字越高, 題目越難, 挑與自己LC排名接近的題庫
  - (e.g. LC rank ~= 1600, pick 1600 problem)
- 代碼隨想錄
- Leetcode cookbook
- fucking-algorithm
- fucking-algorithm website
- FAANG 面試準備經驗與建議（一）
- FAANG 面試準備經驗與建議（二）
- LC 小知識
- Meta SWE isnterview prep
- 0到100的軟體工程師面試之路
- 來和大家聊聊我是如何刷題的 : pt1, pt2, pt3
LC Flow
- high level idea : data structure, algorithm
- offer time & space complexity
- code implementation
- offer test case (consider edge case)
- discussion & follow up

Resource.md - Resource for coding interview (keep updating)
Teach yourself CS
MindMapCodeInterview - Mind map for coding interview
CodeInterviewCheatsheet - Coding interview cheetsheet
repl.it - Coding online!

Visualization
- Algorithms viz
- visualgo - DFS / BFS - DFS, BFS visualization
- visualgo - linkedlist - Linkedlist visualization
- visualgo - BST - binary search tree visualization
- toptal-sorting-algorithms- sorting algorithms online

How to: Work at Google — Example Coding/Engineering Interview
bit_manipulation.md - Bit Manipulation Cheat Sheet
Py TimeComplexity - Py basic data structure Time Complexity ref
Py data model - Python data model doc
pgexercises - Postgre exercises
sqlservertutorial
Books
freecodecamp - data-structures
LC interview-experience
Cheatsheet

Data structure
- py core data structure interview
System design
- system_design readme
- sys_design_resource
Tools
LC SQL resources
- LC SQL resource

Algorithms content

Database

Shell

Shell Script

Progress

Data Structure

Title	Solution	Use case	Comment	Status
`Linear`
Array	Py			AGAIN*
Queue	Py (array), Py (linkedlist), JS			AGAIN*
Stack	Py, JS (linkedlist), JS (array)			OK
Hash table	Py, JS	usually for improving `time complexity B(O)` via extra space complexity (time-space tradeoff)	`good basic`	AGAIN****
`Linear, Pointer`
LinkedList	Py, JS, Java			OK**
Doubly LinkedList	Python, JS			AGAIN
`Non Linear, Pointer`
Tree	Py			AGAIN**
Binary search Tree (BST)	Python, JS, Java			AGAIN
Binary Tree	Py			AGAIN**
Trie	Py			AGAIN
Heap	heap.py, MinHeap.py, MaxHeap.py, MinHeap.java, MaxHeap.java			AGAIN
Priority Queue (PQ)	Py 1, Py 2, Py 3			AGAIN
`Graph`
Graph	Py, JS. Java1, Java2			OK***
DirectedEdge	Java			AGAIN

Algorithm

Title	Solution	Use case	Comment	Time complexity	Space complexity	Status
Binary search	Python	complexity ref	`Best : O(1), Avg : O(log N), Worst : O(log N)`			AGAIN
Linear search	Python					AGAIN
Breadth-first search (BFS)	Python	`FIND SHORTEST PATH`				AGAIN***
Depth-first search (DFS)	Python	`TO CHECK IF SOMETHING EXIST`	`inorder`, `postorder`, `postorder (can recreate a tree)`	Adjacency List: `O(V+E)`, Adjacency Matrix: `O(V^2)`	`O(V)` (V:Vertices, E:Edges)	AGAIN***
Bubble sort	Python					OK* (3)
Insertion sort	Python, InsertionSort.java	`stable` sort	work very fast for `nearly sorted` array	`Best :O(n), Average : O(n^2), Worst : O(n^2)`	Worst : O(1)	AGAIN
Bucket sort	Python, BucketSort.java					AGAIN
Quick sort	quick_sort.py, quick_sort_v2.py, QuickSort.java, QuickSortV2.java, QuickSortV3.java	NOTE !!!, `avg is NLogN, worst is N**2`, big O ref, big O ref 2	`Best : O(N Log N), Avg : O(N Log N), Worst : O(N^2)`		AGAIN***
Heap sort	Python	big O ref	`Best : O(N Log N), Avg : O(N Log N), Worst : O(N Log N)`		AGAIN**
Merge sort	merge_sort.py, merge_sort_topdown.py, mergesort_bottomup.py, MergeSortTopDown.java, MergeSort.java SQL			`Best : O(N Log N), Avg : O(N Log N), Worst : O(N Log N)`	O(N)	OK* (2)
Pancake sort	Python					AGAIN
Selection sort	Python, JS					AGAIN
Topological sort	Python, Java, Java V2		Topological Sort is a algorithm can find “ordering” on an “order dependency” graph			AGAIN
md5	Python					AGAIN
Union Find	Python 1, Python 2, Java 1, Java 2, Java 3			AGAIN
Dynamic programming	JS 1, fibonacci_dp JS					AGAIN
Dijkstra	Python,Java 1, Java 2			AGAIN***
Floyd-Warshall	Python, Java 1, Java 2				LC 2642	not start
Bellman-Ford	Python					not start
Quick Find	Python, Java	init : O(N), union : O(N), find : O(1)	simple, but slow			AGAIN
Quick Union	Java, Java v2	init : O(N), union : O(N), find : O(N)	lazy approach, route compression, optimized Quick Find			AGAIN
Quick Union (Improvements)		lazy approach, path compression			AGAIN
Priority Queue (`unsorted`)	Java				AGAIN
LRU cache	Python		LC 146			AGAIN
LFU Cache	Python		LC 460			AGAIN
DifferenceArray	Java		LC 1109, 370			AGAIN
Kadane Algo	Java		LC 53, 152,918			AGAIN

Array

#	Title	Solution	Time	Space	Difficulty	Note	Status
015	3 Sum	Python, Java	O(n^2)	O(1)	Medium	two pointers, Curated Top 75, good basic, check with `# 001 Two Sum`, `#018 4 sum`, `amazon`, `fb`	OK******* (7)
16	3 Sum Closest	Python, Java	O(n^2)	O(1)	Medium	two pointers, `basic`, `good trick`, `3 sum`, google, fb, apple, m$, GS, amazon	AGAIN******* (3)
018	4 Sum	Python, Java	O(n^3)	O(1)	Medium	two pointers, `k sum`, check `#016 3 sum`, `# 454 4 SUM II`, good trick, `fb`	AGAIN******** (4)
26	Remove Duplicates from Sorted Array	Python, Scala, Java	O(n)	O(1)	Easy	two pointers, MUST, `basic`, `good trick`, `M$`, `fb`	AGAIN************** (11)
27	Remove Element	Python, Java	O(n)	O(1)	Easy	two pointers, MUST, `amazon`	AGAIN****** (2)(MUST)
31	Next Permutation	Python, Scala, Java	O(n)	O(1)	Medium	array, good trick, 2 pointers, Uber, GS, `google`, `amazon`, `fb`	AGAIN****************** (9) (MUST)
41	First Missing Positive	Python, Java	O(n)	O(1)	Hard	array, good trick, hash key, LC top 100 like, `amazon`, `fb`, google, apple, uber	OK** (2)
048	Rotate Image	Python, Java	O(n^2)	O(1)	Medium	array, Curated Top 75, `basic`, `i, j->j, i` `transpose matrix`, garena, `amazon`, `apple`	AGAIN************* (5) (MUST)
054	Spiral Matrix	Python, Java	O(m n)*	O(1)	Medium	array, Curated Top 75, good basic, array boundary, matrix, `amazon`	AGAIN**** (4)
057	Insert Interval	Python, Java	O(n)	O(n)	Medium	interval, Curated Top 75, good basic, `056 Merge Intervals`, java array, list basic	AGAIN********** (5) (MUST!)
059	Spiral Matrix II	Python, Java	O(n^2)	O(1)	Medium	array, check `# 054 Spiral Matrix`, `amazon`	AGAIN** (2)
066	Plus One	Python, Java	O(n)	O(1)	Easy	array, `basic`, `digit`, `google`	AGAIN** (3)
073	Set Matrix Zeroes	Python, Java	O(m n)*	O(1)	Medium	array, Curated Top 75, `matrix`, `amazon`	OK* (4)
080	Remove Duplicates from Sorted Array II	Python, Java	O(n)	O(1)	Medium	array, `two pointers`, AGAIN, `basic`, `trick`, `fb`, `#26 Remove Duplicates from Sorted Array`	AGAIN************ (8)
118	Pascal’s Triangle	Python, Java	O(n^2)	O(1) excluding output	Easy	array, `trick`, `basic`	AGAIN** (2)
119	Pascal’s Triangle II	Python, Java	O(n^2)	O(n)	Easy	array, check with `# 118 Pascal's Triangle`, `amazon`	OK** (4)
121	Best Time to Buy and Sell Stock	Python, Java	O(n)	O(1)	Easy	array, Curated Top 75, `dp`, `basic`, `greedy`, `UBER`, `M$`, `amazon`, `fb`	OK****** (7) (but again)
157	Read N Characters Given Read4	Python, Java	O(n)	O(1)	Easy	array, 🔒, buffer, `google`, `amazon`, `fb`	AGAIN**** (3)
163	Missing Ranges	Python, Java	O(n)	O(1)	Medium	array, two pointer, 🔒, `google`, `amazon`	OK******* (4)
169	Majority Element	Python, Java	O(n)	O(1)	Easy	array, Boyer-Moore majority vote, LC 229, `basic`, `amazon`	OK*
189	Rotate Array	Python, Java	O(n)	O(1)	Medium	array, k % len(nums), good basic, `amazon`	OK* (7) (but again)
209	Minimum Size Subarray Sum	Python, Java	O(n)	O(1)	Medium	array, good basic, sliding window, `Binary Search`, `trick`, `2 pointers`, `fb`	OK********** (7) (but again)
215	Kth Largest Element in an Array	Python, Java	O(n) ~ O(n^2)	O(1)	Medium	array, EPI, `quick sort`, bubble sort, sort, `amazon`, `fb`	OK* (2)
228	Summary Ranges	Python, Java	O(n)	O(1)	Easy	array, check `# 163 Missing Ranges`, `good basic`, `google`	OK********* (5)
229	Majority Element II	Python, Java	O(n)	O(1)	Medium	array, Boyer-Moore majority vote (2 candidates), LC 169, `good trick`, `amazon`	OK
238	Product of Array Except Self	Python, Java	O(n)	O(1)	Medium	array, Curated Top 75, left, right array, LintCode, `good trick`, `Apple`, `M$`, `amazon`, `fb`	OK******* (but again)(6)
240	Search a 2D Matrix II	Python, Java	O(m + n)	O(1)	Medium	array, LintCode, `basic`, `matrix`, `trick`, `binary search`, check `# 74 Search a 2D Matrix`, `amazon`, `apple`	OK***** (7) (but again)
243	Shortest Word Distance	Python, Java	O(n)	O(1)	Easy	array, 🔒, `basic`	OK*
245	Shortest Word Distance III	Python, Java	O(n)	O(1)	Medium	array, 🔒, check `Shortest Word Distance I II III`	OK*
251	Flatten 2D Vector	Python, Java	ctor: O(1), next: O(1) amortized	O(1)	Medium	design, 🔒, `good basic`, check`# 341 Flatten Nested List Iterator`, `google`, `airbnb`, `twitter`, `amazon`	AGAIN******** (5)
277	Find the Celebrity	Python, Java	O(n)	O(1)	Medium	array, graph, 🔒, `fb`, `amazon`, `trick`	AGAIN********** (5)
287	Find the Duplicate Number	Python, Java	O(n)	O(1)	Medium	two pointers, good trick, Floyd cycle detection, binary search on value, check `# 142 Linked List Cycle II`, `amazon`	OK* (2)
289	Game of Life	Python, Java	O(m n)*	O(1)	Medium	array, simultaneous, matrix, `complex`, `google`, `amazon`	AGAIN*** (4)
293	Flip Game	Python, Java	O(n^2)	O(n)	Easy	array, 🔒, `basic`	AGAIN*
308	Range Sum Query 2D - Mutable	Python, Java	ctor: O(m n), update: O(logm * logn), query: O(logm * logn)*	O(m n)*	Hard	array, LC 303, LC 304, pre-sum, binary indexed tree (BIT), segment tree, 🔒, google	AGAIN(1)***** not start
311	Sparse Matrix Multiplication	Python, Java	O(m n * l)*	O(m l)*	Medium	array, 🔒, matrix, `basic`, `fb`, google	OK**** (5)
334	Increasing Triplet Subsequence	Python, Java	O(n)	O(1)	Medium	array, AGAIN, check LC 300, `trick`, `fb`, google	OK***** (but again) (4)
335	Self Crossing	Java	O(n)	O(1)	Hard	array, geometry, google	Again (not start)
370	Range Addition	Python, Java	O(k + n)	O(n)	Medium	prefix sum, prefix, diff array, check `# 598 Range Addition II`, `amazon`, LC 1109, LC 1094, google	AGAIN************** (4) (MUST)
384	Shuffle an Array	Python, Java	ctor: O(n), shuffle: O(n)	O(n)	Medium	design, EPI, `trick`	OK*
396	Rotate Function	Python, Java	O(n)	O(1)	Medium	array, can pass, math, `trick`, `amazon`	AGAIN*** (3)
412	Fizz Buzz	Python, Scala, Java	O(n)	O(1)	Easy	array, `basic`, string, simulation, LC 1195	OK
414	Third Maximum Number	Python, Java	O(n)	O(1)	Easy	array, `good basic`, `amazon`	OK* (4)
419	Battleships in a Board	Python, Java	O(m n)*	O(1)	Medium	array, `matrix`, count only top-left cell of each ship, no extra space, LC 200, `amazon`	AGAIN*
422	Valid Word Square	Python, Java	O(m n)*	O(1)	Easy	array, 🔒, `basic`, `matrix`	AGAIN*
442	Find All Duplicates in an Array	Python, Java	O(n)	O(1)	Medium	array, index-as-hash (negative marking), LC 448, LC 41, `good trick`, `amazon`	OK*
448	Find All Numbers Disappeared in an Array	Python, Java	O(n)	O(1)	Easy	array, index-as-hash (negative marking), LC 442, LC 41, `good basic`	OK
498	Diagonal Traverse	Python, Java	O(m n)*	O(1) excluding output	Medium	array, `matrix`, Diagonal, `google`, `fb`	AGAIN****** (5)
531	Lonely Pixel I	Python, Java	O(m n)*	O(m + n)	Medium	array, 🔒, `matrix`, `basic`	AGAIN*
533	Lonely Pixel II	Python, Java	O(m n)*	O(m n)*	Medium	array, 🔒, `matrix`, row-pattern hashing, LC 531	AGAIN (not start*)
565	Array Nesting	Python, Java	O(n)	O(1)	Medium	array, `union find`, `dfs`, `apple`	AGAIN**** (1)
566	Reshape the Matrix	Python, Java	O(m n)*	O(m n)*	Easy	array, `basic`, `matrix`	AGAIN**
581	Shortest Unsorted Continuous Subarray	Python, Java	O(n)	O(1)	Easy	array, `basic`	AGAIN*
605	Can Place Flowers	Python, Java	O(n)	O(1)	Easy	array, `pass vs continue`	OK*
624	Maximum Distance in Arrays	Python, Java	O(n)	O(1)	Easy	array, 🔒, track running max excluding the current array, `good trick`	AGAIN*
643	Maximum Average Subarray I	Python, Java	O(n)	O(1)	Easy	array, `Math`, `basic`	AGAIN*
661	Image Smoother	Python, Java	O(m n)*	O(m n)*	Easy	array, `matrix`, `basic`, `amazon`	OK***** (4) (but again!)
665	Non-decreasing Array	Python, Java	O(n)	O(1)	Easy	array, `greedy`, at most one modification, decide nums[i-1] vs nums[i+1], `good trick`	AGAIN (not start)
667	Beautiful Arrangement II	Python, Java	O(n)	O(1)	Medium	array, `math`, construct k distinct diffs then run out, `trick`	AGAIN (not start)
670	Maximum Swap	Python, Java	O(n)	O(n)	Medium	array, `good basic`, n = number of digits, last-occurrence index, string + pointers, `fb`	AGAIN************ (9) (MUST)
674	Longest Continuous Increasing Subsequence	Python, Java	O(n)	O(1)	Easy	array, `good basic`, `dp`, `2 pointers`, `fb`	OK*** (5)
697	Degree of an Array	Python, Java	O(n)	O(n)	Easy	array, `hash table`, degree + first/last index, `good basic`	AGAIN (not start)
713	Subarray Product Less Than K	Python, Java	O(n)	O(1)	Medium	array, `basic`, `sliding window`, good trick	OK****** (3) (but again)
717	1-bit and 2-bit Characters	Python, Java	O(n)	O(1)	Easy	array, Greedy	AGAIN (not start)
723	Candy Crush	Python, Java	O((R C)^2)*	O(1)	Medium	array, `complex`	AGAIN (not start)
724	Find Pivot Index	Python, Java	O(n)	O(1)	Easy	array, good basic, prefix-sum	OK**** (2)
729	My Calendar I	Python, Java	O(nlogn)	O(n)	Medium	array, good basic, interval, google	AGAIN**** (1)
731	My Calendar II	Python, Java	O(n^2)	O(n)	Medium	array, `trick`, good trick, scanning line, google	AGAIN******** (4)
747	Largest Number At Least Twice of Others	Python, Java	O(n)	O(1)	Easy	array, `good basic`, `data structure`	OK*
755	Pour Water	Python, Java	O(v n)*	O(1)	Medium	array, `complex`	AGAIN (not start)
766	Toeplitz Matrix	Python, Java	O(m n)*	O(1)	Easy	array, `basic`, `matrix`, `google`	AGAIN* (2)
769	Max Chunks To Make Sorted	Python, Java	O(n)	O(1)	Medium	prefix sum, LC 768, good trick, stack, array, google	AGAIN*** (2)
792	Number of Matching Subsequences	Python, Java	O(n + w)	O(w)	Medium	hash table, basic, hash map, google	AGAIN**** (2)
794	Valid Tic-Tac-Toe State	Python, Java	O(1)	O(1)	Medium	array, `complex`	AGAIN
795	Number of Subarrays with Bounded Maximum	Python, Java	O(n)	O(1)	Medium	array, count subarrays with max in [left, right], `two pointers`, `trick`	AGAIN (not start*)
807	Max Increase to Keep City Skyline	Python, Java	O(n^2)	O(n)	Medium	array, `matrix`, row max + col max, `greedy`, `good basic`	AGAIN*
821	Shortest Distance to a Character	Python, Java	O(n)	O(1) excluding output	Easy	array, `basic`, `trick`	AGAIN**
830	Positions of Large Groups	Python, Java	O(n)	O(1)	Easy	array, `basic`	AGAIN*
832	Flipping an Image	Python, Java	O(n^2)	O(1)	Easy	array, `matrix`, reverse each row then invert, `two pointers`, `basic`	AGAIN*
835	Image Overlap	Python, Java	O(n^4)	O(n^2)	Medium	array, `matrix`, brute-force all shifts, count overlaps with a delta hashmap	AGAIN (not start)
840	Magic Squares In Grid	Python, Java	O(m n)*	O(1)	Easy	array, `complex`	AGAIN (not start)
842	Split Array into Fibonacci Sequence	Python, Java	O(n^3)	O(n)	Medium	array, check `# 306 Addictive Number`, `basic`, `dfs`, `fibonacci`	AGAIN* (not start)
845	Longest Mountain in Array	Python, Java	O(n)	O(1)	Medium	array, `basic`, 2 pointers expansion, google	AGAIN************ (2)(MUST)
849	Maximize Distance to Closest Person	Python, Java	O(n)	O(1)	Medium	array, `basic`, 2 pointers, LC 855, google	AGAIN********* (2)
860	Lemonade Change	Python, Java	O(n)	O(1)	Easy	array, amazon	OK* (2)
868	Transpose Matrix	Python, Java	O(r c)*	O(r c)*	Easy	array, `basic`	OK*
885	Spiral Matrix III	Python, Java	O(max(m, n)^2)	O(1)	Medium	array, `basic`	AGAIN* (not start)
888	Fair Candy Swap	Python, Java	O(m + n)	O(m + n)	Easy	array, `basic`, `trick`	OK*
896	Monotonic Array	Python, Java	O(n)	O(1)	Easy	array, `all`, `fb`	OK
905	Sort Array By Parity	Python, Java	O(n)	O(1)	Easy	array, custom sort, 2 pointers	AGAIN***** (1)(MUST)
909	Snakes and Ladders	Python, Java	O(n^2)	O(n^2)	Medium	array, matrix, bfs, `complex`, `amazon`	AGAIN***** (3)
915	Partition Array into Disjoint Intervals	Python, Java	O(n)	O(n)	Medium	array, `basic`, `trick`	AGAIN* (not start)
918	Maximum Sum Circular Subarray	Python, Java	O(n)	O(1)	Medium	array, check `# 053 Maximum Subarray`, `basic`, `trick`	AGAIN** (2)
921	Minimum Add to Make Parentheses Valid	Python, Java	O(n)	O(1)	Medium	array, `basic`, `trick`	AGAIN**
922	Sort Array By Parity II	Python, Java	O(n)	O(1)	Easy	array, `basic`	AGAIN*
923	3Sum With Multiplicity	Python, Java	O(n^2)	O(n)	Medium	array, LC 15, count pairs with a Counter, combination math, `trick`	AGAIN (not start)
932	Beautiful Array	Python, Java	O(nlogn)	O(n)	Medium	array, divide and conquer, odd/even split, `trick`, google	AGAIN** (not start) (1)
941	Valid Mountain Array	Python, Java	O(n)	O(1)	Easy	array, `basic`, `good basic`	AGAIN*
945	Minimum Increment to Make Array Unique	Python, Java	O(nlogn)	O(n)	Medium	array, `trick`, `good`	AGAIN**
947	Most Stones Removed with Same Row or Column	Python, Java	O(n a(n))*	O(n)	Medium	array, Union Find on row/col keys, dfs, `trick`, google	AGAIN** (1) (not start)
949	Largest Time for Given Digits	Python, Java	O(1)	O(1)	Easy	array, brute force all 4! digit permutations, `math`, `basic`	OK*
950	Reveal Cards In Increasing Order	Python, Java	O(nlogn)	O(n)	Medium	array, sort + deque simulation, queue, 2 pointers, google	AGAIN (not start) (2)
954	Array of Doubled Pairs	Python, Java	O(n + klogk)	O(k)	Medium	array, `counter`, `dict`, `trick`	AGAIN**
961	N-Repeated Element in Size 2N Array	Python, Java	O(n)	O(1)	Easy	array, LC 217, hash table / count, `basic`	OK
989	Add to Array-Form of Integer	Python, Java	O(n)	O(1)	Easy	array, add xxx to sum, fb	AGAIN (not start)
004	Median of Two Sorted Arrays	Python, Java	O(log(min(m, n)))	O(1)	Hard	binary search, binary search on partition, median, heapq (brute), LC top 100 like, amazon, google, apple	AGAIN* (1)
10	Regular Expression Matching	Python, Java	O(m n)*	O(m n)*	Hard	dp, recursion, 2D dp, LC 44, `fb`	AGAIN (not start)
056	Merge Intervals	Python, Java	O(nlogn)	O(n)	Medium	interval, Curated Top 75, `good trick`, 057 Insert Interval, `twitter`, `M$`, `UBER`, `google`, `amazon`, `fb`	OK********* (6) (but AGAIN)
759	Employee Free Time	Python, Java	O(nlogn)	O(n)	Hard	interval, LC 56 and LC 986, heap, merger intervals, scanning line, `amazon`, fb, google, uber, airbnb	AGAIN********** (2) (not start)
1007	Minimum Domino Rotations For Equal Row	Java	O(n)	O(1)	Medium	array, google, dp	AGAIN (1)
1014	Best Sightseeing Pair	Python, Java	O(n)	O(1)	Medium	array, dp, good basic, Spotify	AGAIN (not start)
1027	Longest Arithmetic Subsequence	Python, Java	O(n^2)	O(n^2)	Medium	array, dp, hash table, trick, google, amazon	AGAIN*** (not start)
1031	Maximum Sum of Two Non-Overlapping Subarrays	Python, Java	O(n)	O(n)	Medium	array, prefix sum+slide window, good trick, `google`	AGAIN*************** (3)(MUST)
1041	Robot Bounded In Circle	Python, Java	O(n)	O(1)	Medium	array, math, `amazon`	AGAIN** (2)
1074	Number of Submatrices That Sum to Target	Java	O(m^2 n)*	O(n)	Hard	array, hashmap, prefix sum	AGAIN (not start)
1089	Duplicate Zeros	Java	O(n)	O(1)	Easy	array, `two pointers`, in-place shift from the back, `good trick`	AGAIN (not start)
1094	Car Pooling	Python, Java	O(n + k)	O(k)	Medium	array, range addition, prefix sum, LC 370	AGAIN********** (5) (MUST)
1109	Corporate Flight Bookings	Python, Java	O(n + k)	O(n)	Medium	array, LC 370, difference array, good basic, `amazon`, google	AGAIN********* (MUST) (3)
1248	Count Number of Nice Subarrays	Python, Java	O(n)	O(n)	Medium	array, LC 828, map, Prefix sum, exactly K with at most K, `amazon`	AGAIN********************* (4) (MUST)
1272	Remove Interval	Java	O(n)	O(n)	Medium	array, google	AGAIN (not start)
1275	Find Winner on a Tic Tac Toe Game	Python, Java	O(1)	O(1)	Easy	array, `amazon`	AGAIN (not start)
1288	Remove Covered Intervals	Java	O(nlogn)	O(1)	Medium	array, greedy, interval	AGAIN (not start)
1314	Matrix Block Sum	Java	O(m n)*	O(m n)*	Medium	array, prefix sum, dp, matrix sum	AGAIN (not start)
1431	Kids With the Greatest Number of Candies	Java	O(n)	O(1) excluding output	Easy	array, compare each candy count with the max, `basic`	AGAIN (not start)
1470	Shuffle the Array	Java	O(n)	O(1) excluding output	Easy	array, index mapping, 2 pointers, `basic`	AGAIN (not start)
1564	Put Boxes Into the Warehouse I	Java	O(nlogn)	O(m)	Medium	array, `google`	AGAIN (not start)
1567	Maximum Length of Subarray With Positive Product	Python, Java	O(n)	O(1)	Medium	array, good trick, dp, 2 pointers, amazon	AGAIN**** (1) (not start)
1672	Richest Customer Wealth	Java	O(m n)*	O(1)	Easy	array, row sum then max, `matrix`, `basic`	OK
1806	Minimum Number of Operations to Reinitialize a Permutation	Java	O(n)	O(n)	Medium	array, brute force, google	AGAIN (not start)
1914	Cyclically Rotating a Grid	Python, Java	O(m n)*	O(m n)*	Medium	array, brute force, dequeue`amazon`	AGAIN (not start)
1920	Build Array from Permutation	Java	O(n)	O(1) excluding output	Easy	array, index mapping, `basic`	AGAIN (not start)
1929	Concatenation of Array	Java	O(n)	O(1) excluding output	Easy	array, concatenate, `basic`	AGAIN (not start)
1991	Find the Middle Index in Array	Java	O(n)	O(1)	Easy	array, prefix sum, LC 724, `good basic`	AGAIN (not start)
2079	Watering Plants	Java	O(n)	O(1)	Medium	array, google	AGAIN (not start)
2640	Find the Score of All Prefixes of an Array	Java	O(n)	O(1) excluding output	Medium	array, lc weekly 102	OK (1)
2644	Find the Maximum Divisibility Score	Java	O(n m)*	O(1)	Easy	array, sort	Again (1)
2553	Separate the Digits in an Array	Java	O(n d)*	O(1) excluding output	Easy	array, weekly 97	OK
3195	Find the Minimum Area to Cover All Ones I	Java	O(m n)*	O(1)	Medium	array, LC weekly	AGAIN (1)
3964	Minimum Lights to Illuminate a Road	Python, Java	O(n)	O(n)	Medium	array, diff array, LC bi weekly	AGAIN (1)

Set

#	Title	Solution	Time	Space	Difficulty	Note	Status
1436	Destination City	Java	O(n)	O(n)	Easy	set, set of outgoing cities, find the city with no outgoing path, `basic`	Again (1)
2357	Make Array Zero by Subtracting Equal Amounts	Java	O(n)	O(n)	Easy	set, count distinct non-zero values, `good trick`	Again (1)
1316	Distinct Echo Substrings	Java	O(n^2)	O(n^2)	Hard	set, set of substrings, rolling hash / Rabin-Karp for O(n^2), `trick`	Again (not start)
2657	Find the Prefix Common Array of Two Arrays	Java	O(n)	O(n)	Medium	set, frequency array + running common count, good trick, weekly_103	Again**** (1)
2682	Find the Losers of the Circular Game	Java	O(n)	O(n)	Easy	set, simulate the circular jumps until a friend repeats, hashset, `basic`	Again (1)

Slide Window

#	Title	Solution	Time	Space	Difficulty	Note	Status
30	Substring with Concatenation of All Words	Java	O(n l)*	O(m l)*	Hard	sliding window, hashmap word count, fixed-size window of `l` chars, LC 76, `good trick`	AGAIN(not start)
768	Max Chunks To Make Sorted II	Java	O(n)	O(n)	Hard	sliding window, LC 769, prefix max vs suffix min, monotonic stack, `good trick`	AGAIN(not start)
1004	Max Consecutive Ones III	Java	O(n)	O(1)	Medium	sliding window, longest window with at most k zeros, LC 487, LC 424, `good basic`	Again (1)
1658	Minimum Operations to Reduce X to Zero	Python, Java	O(n)	O(1)	Medium	sliding window, turn “remove from both ends” into “longest middle subarray with sum total-x”, `good trick`, google	AGAIN******** (1)(MUST)
1838	Frequency of the Most Frequent Element	Java, Python	O(nlogn)	O(1)	Medium	sliding window, sort + sliding window with sum, binary search on the answer, `good trick`	AGAIN******** (1)(good)
1839	Longest Substring Of All Vowels in Order	Python, Java	O(n)	O(1)	Medium	sliding window, window must be non-decreasing and cover all 5 vowels, `basic`	AGAIN** (1)(good)
2555	Maximize Win From Two Segments	Java	O(n)	O(n)	Medium	sliding window, best window ending at i + best window before i, prefix dp, weekly 97	AGAIN (not start)
2962	Count Subarrays Where Max Element Appears at Least K Times	Java	O(n)	O(1)	Medium	sliding window, count windows containing the max at least k times, `good trick`, weekly 375	AGAIN************ (3)
3090	Maximum Length Substring With Two Occurrences	Java	O(n)	O(1)	Easy	sliding window, longest window where every char appears at most twice, weekly 390	AGAIN (1)

Hash Table

#	Title	Solution	Time	Space	Difficulty	Note	Status
001	Two Sum	Python, Java,Scala	O(n)	O(n)	Easy	hash table, `good basic`, `apple`, `amazon`, `UBER`, `grab`, `fb`	OK*** (6) (but again)
003	Longest Substring Without Repeating Characters	Python, Java	O(n)	O(1)	Medium	hash table, Curated Top 75, 2 pointers, optimized (SLIDING WINDOW + DICT), `good trick`, `hashMap`, MUST, `apple`, `amazon`, `fb`	AGAIN**************** (12) (but AGAIN)
36	Valid Sudoku	Python, Java	O(9^2)	O(9^2)	Medium	hash table, array, hashset, `UBER`, `apple`, `amazon`	OK**** (3) (again)
49	Group Anagrams	Python, Scala, Java	O(n glogg)*	O(n g)*	Medium	hash table, Curated Top 75, `good basic`, dict, `trick`, `dict + sort`, `UBER`, `amazon`, `fb`	OK ********** (6)
170	Two Sum III - Data structure design	Python , Java	add: O(1), find: O(n)	O(n)	Easy	hash table, 🔒, `trick`, `good basic`, `linkedin`, `fb`	OK*** (4)
187	Repeated DNA Sequences	Python , Java	O(n)	O(n)	Medium	hash table, `basic`, `trick`, `set`, `linkedin`, google	AGAIN**
202	Happy Number	Python , Java	O(k)	O(k)	Easy	hash table, `twitter`, `airbnb`, `UBER`	AGAIN** (4)
204	Count Primes	Python , Java	O(nloglogn)	O(n)	Medium	hash table, Sieve of Eratosthenes, `good trick`, cache, `M$`, `amazon`	AGAIN******** (5)
205	Isomorphic Strings	Python, Java	O(n)	O(1)	Easy	hash table, `basic`, `trick`, `bloomberg`	AGAIN**
217	Contains Duplicate	Python , Java	O(n)	O(n)	Easy	hash table, Curated Top 75, `yahoo`, `airbnb`	OK
219	Contains Duplicate II	Python ,Java	O(n)	O(n)	Easy	hash table, `basic`, `trick`	AGAIN*
244	Shortest Word Distance II	Python , Java	ctor: O(n), lookup: O(a + b)	O(n)	Medium	hash table, 🔒	AGAIN (not start*)
246	Strobogrammatic Number	Python, Java	O(n)	O(1)	Easy	hash table, 🔒, `google`, `fb`	OK* (5)
249	Group Shifted Strings	Python , Java	O(n l)*	O(n l)*	Medium	hash table, LC 49, 🔒, hashmap, char, `apple`, `UBER`, `google`	OK *****(6) (but again)
266	Palindrome Permutation	Python , Java	O(n)	O(1)	Easy	hash table, 🔒	OK
288	Unique Word Abbreviation	Python , Java	ctor: O(n), lookup: O(1)	O(k)	Medium	hash table, 🔒, ?, google	AGAIN (2)(not start)
290	Word Pattern	Python , Java	O(n)	O©	Easy	hash table, `basic`, chcek `# 205 Isomorphic Strings`, `good basic`, `for ptn, word in zip(pattern, words)`, `dropbox`, `UBER`	AGAIN* (3)
299	Bulls and Cows	Python , Java	O(n)	O(1)	Easy	hash table, `trick`, `map(operator.eq, a, b)`, `amazon`, `airbnb`, `google`	AGAIN* (3)
314	Binary Tree Vertical Order Traversal	Python , Java	O(n)	O(n)	Medium	hash table, LC 987, 🔒, `BFS`, `DFS`, `tree`, `trick`, `google`, `amazon`, `fb`, m$	OK**** (6)
325	Maximum Size Subarray Sum Equals k	Python, Java	O(n)	O(n)	Medium	hash table, prefix sum, 🔒, `hashmap`, `good trick`, `dict`, `fb`	AGAIN***************** (8) (MUST)
336	Palindrome Pairs	Java	O(n l^2)*	O(n l)*	Hard	hash table, LC 49, Anagrams, string op, hashmap, sub-string	Again******* (2)(again)
356	Line Reflection	Python , Java	O(n)	O(n)	Medium	hash table, 🔒, Hash, Two Pointers, `math`, `google`	AGAIN (not start)
359	Logger Rate Limiter	Python, Java	O(1), amortized	O(k)	Easy	hash table, 🔒, google	OK
387	First Unique Character in a String	Python , Java	O(n)	O(n)	Easy	hash table, `amazon`, `apple`, `fb`	OK
388	Longest Absolute File Path	Python , Java	O(n)	O(d)	Medium	hash table, stack, good trick, file system, `google`	AGAIN******* (3)
409	Longest Palindrome	Python , Java	O(n)	O(1)	Easy	hash table, count char pairs, add 1 if any odd count remains, `basic`	OK*
424	Longest Repeating Character Replacement	Python, Java	O(n)	O(1)	Medium	hash table, Curated Top 75, hash map, max_freq, `silding window`, `good trick`, LC blind pattern	AGAIN**************** (10) (MUST)
438	Find All Anagrams in a String	Python , Java	O(n)	O(1)	Medium	hash table, `trick`, `AGAIN`, `sliding window`, `acc Counter`, `amazon`, `fb`, apple, google, uber, yahoo	AGAIN************** (11)(must)
447	Number of Boomerangs	Python , Java	O(n^2)	O(n)	Easy	hash table, `trick`, `google`	AGAIN (3) (not start)
454	4Sum II	Python , Java	O(n^2)	O(n^2)	Medium	hash table, check LC 018 4SUM, `trick`, `basic`, `amazon`	AGAIN**
463	Island Perimeter	Python , Java	O(m n)*	O(1)	Easy	hash table, count cells then subtract shared edges, `matrix`, `basic`, `google`, `fb`, amazon	AGAIN** (2)
470	Implement Rand10() Using Rand7()	Python , Java	O(1)	O(1)	Medium	hash table, `trick`, `google`	AGAIN** (3)
473	Matchsticks to Square	Python , Java	O(n s * 2^n)*	O(n (2^n + s))*	Medium	hash table, good trick, backtrack	AGAIN****** (2)
523	Continuous Subarray Sum	Python , Java	O(n)	O(k)	Medium	hash table, sub array sum, check `# 560 Subarray Sum Equals K`, `good trick`, `substring`, `AGAIN`, `M$`, `fb`, `apple`	AGAIN*************** (10) (MUST)
525	Contiguous Array	Python, Java	O(n)	O(n)	Medium	hash table, sub-array sum, LC 560, 525, 974, `good basic`, `array`, `hashmap`, `fb`, amazon	AGAIN****************** (12) (MUST)
532	K-diff Pairs in an Array	Python , Java	O(n)	O(n)	Medium	hash table, `basic`, `collections.Counter()`, `a-b =k -> a = k + b` , `amazon`	AGAIN********** (6)
554	Brick Wall	Python , Java	O(n)	O(m)	Medium	hash table, LC 252, gap map, `trick`, `hash map`, `bloomberg`, `fb`, grab	AGAIN******** (6)
560	Subarray Sum Equals K	Python, Java	O(n)	O(n)	Medium	hash table, LC 560, 525, 974, prefix sum, `must check`, check `# 523 Continuous Subarray Sum`, LC 1268, `basic`, `substring`, `good trick`, `google`, `fb`	AGAIN***************** (8) (MUST)
561	Array Partition I	Python , Java	O(nlogn)	O(1)	Easy	hash table, sort then sum every other element, `greedy`, `good trick`	OK
575	Distribute Candies	Python , Java	O(n)	O(n)	Easy	hash table, min(distinct kinds, n / 2), hashset, `basic`	OK
594	Longest Harmonious Subsequence	Python , Java	O(n)	O(n)	Easy	hash table, `basic`, `good trick`	OK* (3)
599	Minimum Index Sum of Two Lists	Python , Java	O((m + n) l)*	O(m l)*	Easy	hash table, `yelp`	OK*
609	Find Duplicate File in System	Python , Java	O(n l)*	O(n l)*	Medium	hash table, `collections.defaultdict(list)`, `dropbox`	OK*
657	Robot Return to Origin	Python , Java	O(n)	O(1)	Easy	hash table, `amazon`	OK (2)
710	Random Pick with Blacklist	Java	ctor: O(b), pick: O(1)	O(b)	Hard	hash table, hashmap, google	AGAIN (not start)
748	Shortest Completing Word	Python , Java	O(n)	O(1)	Easy	hash table, `collections.Counter`, `google`	AGAIN* (3)
760	Find Anagram Mappings	Python , Java	O(n)	O(n)	Easy	hash table, `basic`, `collections.defaultdict`	OK
771	Jewels and Stones	Python , Java	O(m + n)	O(n)	Easy	hash table, `amazon`	OK (2)
811	Subdomain Visit Count	Python , Java	O(n)	O(n)	Easy	hash table, `indeed`, `bloomberg`	AGAIN (not start)
822	Card Flipping Game	Python , Java	O(n)	O(n)	Medium	hash table, `good basic`	AGAIN* (3)
825	Friends Of Appropriate Ages	Python , Java	O(a^2 + n)	O(a)	Medium	hash table, `good basic`, `counter`, `AGAIN`, `fb`	AGAIN***** (5)
869	Reordered Power of 2	Python , Java	O(1)	O(1)	Medium	hash table, `trick`, `basic`, `bit manipulation`, `bit`	AGAIN*
873	Length of Longest Fibonacci Subsequence	Python , Java	O(n^2)	O(n)	Medium	hash table, `trick`, `DP`, `Fibonacci`, `set`	AGAIN**
957	Prison Cells After N Days	Python , Java	O(1)	O(1)	Medium	hash table, `trick`, `DP`, `mod`	AGAIN (not start)
966	Vowel Spellchecker	Python , Java	O(n)	O(w)	Medium	hash table, `trick`, `dict`, `set`	AGAIN (not start)
974	Subarray Sums Divisible by K	Python , Java	O(n)	O(k)	Medium	hash table, LC 560, 525, 974, prefix, hashmap	AGAIN*************** (7)(MUST)
992	Subarrays with K Different Integers	Python , Java	O(n)	O(k)	Hard	hash table, trick, hashmap, slide window, amazon	AGAIN**** (1)
1010	Pairs of Songs With Total Durations Divisible by 60	Python , Java	O(n)	O(1)	Medium	hash table, good basicm dict, array, `amazon`	AGAIN********* (4) (MUST)
1099	Two Sum Less Than K	Python , Java	O(nlogn)	O(1)	Medium	hash table, dict, sort, `amazon`	AGAIN* (1) (not start)
1121	Divide Array Into Increasing Sequences	Java	O(n)	O(1)	Hard	hash table, dict, google	Again (not start)
1131	Rank Transform of an Array	Python , Java	O(nlogn)	O(n)	Easy	hash table, dict, array, `amazon`	OK* (1)
1170	Compare Strings by Frequency of the Smallest Character	Java	O((m + n) l + nlogn)*	O(n)	Medium	hash table, hashmap, binary search, `google`	AGAIN*** (1)s
1257	Smallest Common Region	Java	O(n)	O(n)	Medium	hash table, hashmap, good trick, lowest common ancestor (LCA)	AGAIN (not start)
1296	Divide Array in Sets of K Consecutive Numbers	Python , Java	O(nlogn)	O(n)	Medium	hash table, LC 846, dict, `google`	AGAIN (not start)
1726	Tuple with Same Product	Python , Java	O(n^2)	O(n^2)	Medium	hash table, hashmap, sort, 2 pointers, brute force, good basic, `google`	AGAIN**** (1)(MUST)
1794	Count Pairs of Equal Substrings With Minimum Difference	Java	O(m + n)	O(1)	Medium	hash table, first, last idx, hashmap, trick, `google`	AGAIN**** (1)(not start)
1923	Longest Common Subpath	Python, Java	O(m nlogn)*	O(n)	Hard	hash table, hash, bit, `amazon`	AGAIN (not start)
2506	Count Pairs Of Similar Strings	Java	O(n l)*	O(n)	Easy	hash table, hashmap, LC weekly	AGAIN (1)
2592	Maximize Greatness of an Array	Java	O(nlogn)	O(1)	Medium	hash table, Treemap, 2 pointers, LC weekly	AGAIN (1)
2615	Sum of Distances	Java	O(n)	O(n)	Medium	hash table, good trick, prefix sum, hashmap, LC weekly	AGAIN**** (1)
2766	Relocate Marbles	Java	O(n + m)	O(n)	Medium	hash table, hashset, hashmap LC weekly, good trick	AGAIN******(1)
2768	Number of Black Blocks	Java	O(n)	O(n)	Medium	hash table, hashmap, LC weekly	AGAIN(1) (not srart)
2870	Minimum Number of Operations to Make Array Empty	Java	O(n)	O(n)	Medium	hash table, LC 2244, hashmap, count, LC weekly	AGAIN (1)
2963	Count the Number of Good Partitions	Java	O(n)	O(n)	Hard	hash table, LC weekly	AGAIN (1) (not start)
3121	Count the Number of Special Characters II	Java	O(n)	O(1)	Medium	hash table, greedy, hashmap	AGAIN(1) (not srart)
4007	Minimum Initial Strength to Defeat All Monsters	Python, Java	O(nlogM)	O(1)	Medium	hash table, good trick, hashmap pair, LC weekly	AGAIN(1) (not srart)

Linked list

#	Title	Solution	Time	Space	Difficulty	Note	Status
002	Add Two Numbers	Python, Java	O(n)	O(1)	Medium	linked list, Curated Top 75, check `# 445 Add Two Numbers II`, `basic`, `airbnb`, `amazon`, `fb`	OK**** (6) (but again)
021	Merge Two Sorted Lists	Python, Java	O(n)	O(1)	Easy	linked list, Curated Top 75, `UBER`, `amazon`, `apple`, `fb`	OK******** (6) (but again)
023	Merge k sorted lists	Python, Java	O(nlogk)	O(k)	Hard	linked list, Curated Top 75, check `#21 Merge Two Sorted Lists`, `amazon`	OK**** (2) (but again !!!
024	Swap Nodes in Pairs	Python, Java	O(n)	O(1)	Medium	linked list, `GOOD basic`, LC 100 like, `UBER`, `amazon`, `fb`	AGAIN**************** (8) (MUST)
025	Reverse Nodes in k-Group	Python, Java	O(n)	O(1)	Hard	linked list, good trick, reverse linkedlist, reverse K linkedlist, amazon, google, fb, m$	AGAIN********** (4)
061	Rotate List	Python, Java	O(n)	O(1)	Medium	linked list, `basic`	AGAIN (2)
082	Remove Duplicates from Sorted List II	Python, Java	O(n)	O(1)	Medium	linked list, good basic, check `# 083 Remove Duplicates from Sorted List`	AGAIN (3) *****
083	Remove Duplicates from Sorted List	Python, Java	O(n)	O(1)	Easy	linked list, `basic`	OK*
92	Reverse Linked List II	Python, Java	O(n)	O(1)	Medium	linked list, LC 206, reverse k nodes, `good trick`, fb, apple, google, amazon, m$	AGAIN*************** (10) (MUST)
138	Copy List with Random Pointer	Python, Java	O(n)	O(n)	Medium	linked list, trick, recursive, `hash table`, `UBER`, `M$`, `amazon`, `fb`	AGAIN******* (7)
160	Intersection of Two Linked Lists	Python, Java	O(m + n)	O(1)	Easy	linked list, basic, hash table, 2 pointers, `airbnb`, `amazon`, `fb`	OK******* (7) (again!)
203	Remove Linked List Elements	Python , Java	O(n)	O(1)	Easy	linked list, linked list basic, `amazon`	AGAIN**** (3)
206	Reverse Linked List	Python, Java	O(n)	O(1)	Easy	linked list, Curated Top 75, `good basic`, `amazon`, `fb`	OK*********** (9) (MUST again)
234	Palindrome Linked List	Python, Java	O(n)	O(1)	Easy	linked list, `amazon`, `fb`	OK (4)
237	Delete Node in a Linked List	Python, Java	O(1)	O(1)	Easy	linked list, LintCode, apple	OK * (1) (but again)
328	Odd Even Linked List	Python, Java	O(n)	O(1)	Medium	linked list, `basic`	OK** (2)
369	Plus One Linked List	Python, Java	O(n)	O(1)	Medium	linked list, 🔒, `basic`, google	AGAIN****** (3)
445	Add Two Numbers II	Python, Java	O(m + n)	O(m + n)	Medium	linked list, `trick`, string, `good basic`, `amazon`, google	AGAIN*** (3)
725	Split Linked List in Parts	Python, Java	O(n + k)	O(1)	Medium	linked list, mod, split linked list, good trick, `amazon`	AGAIN************ (6) (again)
817	Linked List Components	Python, Java	O(m + n)	O(m)	Medium	linked list, set, array, google	AGAIN** (1)
430	Flatten a Multilevel Doubly Linked List	Python, Java	O(n)	O(n)	Medium	linked list, `good trick`, `doubly linked list`, `dfs`, iterative, stack, `fb`, google	AGAIN******** (6)
707	Design Linked List	Python, Java	get/addAtIndex/deleteAtIndex: O(n), addAtHead/addAtTail: O(1)	O(n)	Medium	linked list, linked list basic OP	AGAIN (1)
708	Insert into a Cyclic Sorted List	Python, Java	O(n)	O(1)	Medium	linked list, `AGAIN`, `cyclic linked list`, `good trick`, `google`, `amazon`, `fb`	AGAIN******** (4)
1836	Remove Duplicates From an Unsorted Linked List	Java	O(n)	O(n)	Medium	linked list, hashset of seen values, dummy head, `good basic`	AGAIN (not start)

Stack

#	Title	Solution	Time	Space	Difficulty	Note	Status
20	Valid Parentheses	Python, Java	O(n)	O(n)	Easy	stack, Curated Top 75, `good basic`, `fb`, `amazon`	OK** (6)
32	Longest Valid Parentheses	Python, Java	O(n)	O(n)	Hard	stack, brute force, dp, deque, LC top 100 likes, `amazon`, fb, m$	AGAIN********** (3)
71	Simplify Path	Python, Java	O(n)	O(n)	Medium	stack, `basic`, `amazon`, `fb`	AGAIN******** (5)
85	Maximal Rectangle	Python, Java	O(m n)*	O(n)	Hard	stack, top-100-like, brute force, dp, LC 84, google, amazon, apple	AGAIN (not start)
101	Symmetric Tree	Python, Java	O(n)	O(h)	Easy	stack, `good basic`, `bfs`, `dfs`, `linkedin`, `M$`, `amazon`, `fb`, google	AGAIN********** (6)
150	Evaluate Reverse Polish Notation	Python, Java	O(n)	O(n)	Medium	stack, good trick, `amazon`	AGAIN********* (5)
155	Min Stack	Python, Java	O(1) per op	O(n)	Medium	stack, basic, data structure, `amazon`	OK*********** (5)
173	Binary Search Tree Iterator	Python, Java	ctor: O(h), next: O(1) amortized, hasNext: O(1)	O(h)	Medium	stack, `good basic`, `tree, M$`, `linkedin`, `google`, `amazon`, `fb`	OK***** (5)
224	Basic Calculator	Python, Java	O(n)	O(n)	Hard	stack, `basic`, `trick`, LC 227, 772, amazon	AGAIN******* (3)
227	Basic Calculator II	Python, Java	O(n)	O(n)	Medium	stack, delay op, LC 224, 772, `good trick`, `airbnb`, `fb`, amazon	AGAIN************** (8) (MUST)
232	Implement Queue using Stacks	Python, Java	O(1), amortized	O(n)	Easy	stack, stack-queue, EPI, LintCode, `amazon`	AGAIN*** (3)(MUST)
255	Verify Preorder Sequence in Binary Search Tree	Python, Java	O(n)	O(n)	Medium	stack, 🔒	AGAIN (not start)
321	Create Maximum Number	Java	O(k^2 (m + n))*	O(m + n)	Hard	stack, dp, greedy, mono-stack, google	AGAIN (not start) (2)
331	Verify Preorder Serialization of a Binary Tree	Python, Java	O(n)	O(1)	Medium	stack, slot counting (no stack needed), preorder serialization, `good trick`	AGAIN (not start)
341	Flatten Nested List Iterator	Python, Java	O(n)	O(n)	Medium	stack, LC 284, 🔒 Iterator, generator, `good basic`, `amazon`, `fb***`	AGAIN********* (6)
385	Mini Parser	Python, Java	O(n)	O(n)	Medium	stack, parse nested list, recursion, `good basic`	AGAIN
394	Decode String	Python, Java	O(n)	O(n)	Medium	stack, good basic!!!, pre num string, LC 224, 227, `amazon`, `google`	AGAIN******************* (10) (MUST)
439	Ternary Expression Parser	Python, Java	O(n)	O(n)	Medium	stack, 🔒	AGAIN (not start)
456	132 Pattern	Python, Java	O(n)	O(n)	Medium	stack, binary search	AGAIN (not start)
496	Next Greater Element I	Python, Java	O(n)	O(n)	Easy	stack, good basic, mono stack, LC 739, LC 503, LC 406	OK*************** (6) (but again)
503	Next Greater Element II	Python, Java	O(n)	O(n)	Medium	stack, good basic, LC 739, LC 503, LC 406, next_big_val_idx	AGAIN************* (7)
591	Tag Validator	Java	O(n)	O(n)	Hard	stack, parse tags + CDATA, `complex`, string	AGAIN (not start)
636	Exclusive Time of Functions	Python, Java	O(n)	O(n)	Medium	stack, `trick`, `AGAIN`, `UBER`, `fb`	AGAIN********** (4)
682	Baseball Game	Python, Java	O(n)	O(n)	Easy	stack, good basic, `amazon`	OK* (3)
735	Asteroid Collision	Python, Java	O(n)	O(n)	Medium	stack, good basic, m$, GS, fb, `amazon`	AGAIN************** (5)
739	Daily Temperatures	Python, Java	O(n)	O(n)	Medium	stack, LC 739, LC 503, LC 406, LC 496, LC 42, Monotonic stack, good trick, `amazon`	AGAIN******************* (12) (MUST)
772	Basic Calculator III	Java	O(n)	O(n)	Hard	stack, check LC 224, 227	AGAIN (not start)
853	Car Fleet	Python, Java	O(nlogn)	O(n)	Medium	stack, good basic, google, amz	AGAIN********* (5)
856	Score of Parentheses	Python, Java	O(n)	O(n)	Medium	stack, stack of scores, or count depth on each `()`, `good trick`	AGAIN (not start)
872	Leaf-Similar Trees	Python, Java	O(n)	O(h)	Easy	stack, top down, bottom up dfs, good basic	AGAIN***** (MUST)
895	Maximum Frequency Stack	Python, Java	push: O(1), pop: O(1)	O(n)	Hard	stack, good basic, heap, design, amazon, apple, linkedin, m$, twitter	AGAIN********** (1) (not start)
901	Online Stock Span	Python, Java	O(n)	O(n)	Medium	stack, mono stack, good basic	AGAIN***** (2)
946	Validate Stack Sequences	Python, Java	O(n)	O(n)	Medium	stack, simulate push/pop with one stack, `good basic`, google	AGAIN
1028	Recover a Tree From Preorder Traversal	Java	O(n)	O(n)	Hard	stack, brute force, google	AGAIN (not start)
1047	Remove All Adjacent Duplicates in String	Python, Java	O(n)	O(n)	Easy	stack, good basic, LC 1209, fb, amazon, google	AGAIN************ (3) (MUST)
1081	Smallest Subsequence of Distinct Characters	Java	O(n)	O(n)	Medium	stack, LC 316	AGAIN (1)
1209	Remove All Adjacent Duplicates in String II	Python, Java	O(n)	O(n)	Medium	stack, LC 1047, good basic, two pointers, greedy, fb, amamzon, apple, spotify	AGAIN***************** (4) (MUST)
1544	Make The String Great	Java	O(n)	O(n)	Easy	stack, LC 1047, remove adjacent opposite-case pairs, `basic`	AGAIN(1)
1703	Minimum Adjacent Swaps for K Consecutive Ones	Python, Java	O(n)	O(n)	Hard	stack, heap, sliding window, amazon	AGAIN (not start)
1896	Minimum Cost to Change the Final Value of Expression	Python, Java	O(n)	O(n)	Hard	stack, complex, dp, dp+stack, dfs, `amazon`	AGAIN (not start)
2104	Sum of Subarray Ranges	Python, Java	O(n)	O(n)	Medium	stack, LC 907, good basic, monotonic stack, brute force, dp, `amazon`	AGAIN************ (5)
2289	Steps to Make Array Non-decreasing	Java	O(n)	O(n)	Medium	stack, mono stack + dp, good trick	AGAIN**** (1) (not start)

Tree

#	Title	Solution	Time	Space	Difficulty	Note	Status
94	Binary Tree Inorder Traversal	Python, Java	O(n)	O(h)	Medium	tree, iteration, recursion, `good basic`, `Morris Traversal`, `M$`, `fb`	AGAIN********* (4) (MUST)
124	Binary Tree Maximum Path Sum	Python, Java	O(n)	O(h)	Hard	tree, Curated Top 75, google, amazon, fb, m$, twitter	AGAIN*********** (6) (MUST)
144	Binary Tree Preorder Traversal	Python, Java	O(n)	O(h)	Medium	tree, `Morris Traversal`	OK
145	Binary Tree Postorder Traversal	Java	O(n)	O(h)	Medium	tree, `Morris Traversal`	OK
208	Implement Trie (Prefix Tree)	Python, Java	insert/search/startsWith: O(l)	O(t)	Medium	tree, Curated Top 75, dict tree, LC 211, trie, `amazon`, `fb`	AGAIN************** (7) (MUST)
211	Design Add and Search Words Data Structure	Python, Java	add: O(l), search: O(n l)* worst	O(t)	Medium	tree, Curated Top 75, `Trie`, recursive, node, hashMap, do `# 208 first`, `amazon`, `fb`	AGAIN*********** (9) (MUST)
226	Invert Binary Tree	Python, Java	O(n)	O(h)	Easy	tree, Curated Top 75, good basic	OK******* (4) (but again)
250	Count Univalue Subtrees	Python, Java	O(n)	O(h)	Medium	tree, LC 572	not start
297	Serialize and Deserialize Binary Tree	Python, Java	O(n)	O(n)	Hard	tree, Curated Top 75, LC 449, BFS, DFS, Binary Tree	again******* (4)
307	Range Sum Query - Mutable	Python, Java	ctor: O(n), update: O(logn), query: O(logn)	O(n)	Medium	tree, LintCode, DFS, Segment Tree, BIT, check `#303 Range Sum Query - Immutable`, google	AGAIN* (2)(not start)
427	Construct Quad Tree	Java	O(n^2 logn)*	O(logn)	Medium	tree, Quadtrees	again (not start)
536	Construct Binary Tree from String	Python, Java	O(n)	O(h)	Medium	tree, 🔒, recursive, str->tree, check LC 606, `amazon`	AGAIN******** (6)
538	Convert BST to Greater Tree	Python, Java	O(n)	O(h)	Medium	tree, good basic, dfs, BST, cum sum, `amazon`	OK************** (6)(MUST)
543	Diameter of Binary Tree	Python, Java	O(n)	O(h)	Easy	tree, dfs, `trick`, `google`, `amazon`, `fb`	AGAIN*********** (7)(MUST)
545	Boundary of Binary Tree	Python, Java	O(n)	O(h)	Medium	tree, recursive, dfs, tree boundary, 🔒, `good trick`, `amazon`	AGAIN********* (6)
548	Split Array with Equal Sum	Python, Java	O(n^2)	O(n)	Medium	tree, 🔒, `good trick`, `array`	AGAIN*** (2)
563	Binary Tree Tilt	Python, Java	O(n)	O(h)	Easy	tree, post-order dfs, subtree sum, `good basic`	AGAIN
572	Subtree of Another Tree	Python, Java	O(m n)*	O(h)	Easy	tree, Curated Top 75, LC 100, recursive call recursion func, good basic, dfs, bfs, `amazon`, `fb`	OK************* (10) (but again)
606	Construct String from Binary Tree	Python, Java	O(n)	O(h)	Easy	tree, good basic, tree->str, dfs, LC 536, `amazon`	OK************ (8) (again!)
617	Merge Two Binary Trees	Python, Java	O(n)	O(h)	Easy	tree, dfs, bfs, `good basic`, `amazon`	OK************* (7) (but again)
623	Add One Row to Tree	Python, Java	O(n)	O(h)	Medium	tree, `good basic`, `dfs`, `bfs`, re-connect tree	AGAIN***************** (5)(MUST)
637	Average of Levels in Binary Tree	Python, Java	O(n)	O(h)	Easy	tree, `bfs`, `dfs`, `good basic`, `fb`	OK*** (4)
652	Find Duplicate Subtrees	Python, Java	O(n)	O(n)	Medium	tree, good basic, serialization, path, bfs, dfs, Hashmap, `amazon`	AGAIN************** (9)(MUST)
653	Two Sum IV - Input is a BST	Python, Java	O(n)	O(h)	Easy	tree, Two Pointers, `2 sum`, `bfs`, `dfs`, `amazon`, `fb`	OK******* (4)
654	Maximum Binary Tree	Python, Java	O(n) ~ O(n^2)	O(n)	Medium	tree, LintCode, Descending Stack, `good basic`	AGAIN* (2)
655	Print Binary Tree	Python, Java	O(n)	O(n)	Medium	tree, matrix, dfs, bfs, good basic	AGAIN****** (3)
662	Maximum Width of Binary Tree	Python, Java	O(n)	O(n)	Medium	tree, tree width, bfs, dfs, `trick`, `amazon`	AGAIN************* (5)(MUST)
663	Equal Tree Partition	Python, Java	O(n)	O(n)	Medium	tree, `# LC 508`, AGAIN, 🔒, Hash, dfs, `trick`, `good trick`, `amazon`	OK******** (8)
677	Map Sum Pairs	Python, Java	insert: O(l), sum: O(p + t)	O(t)	Medium	tree, Trie, `trick`, `hard`	AGAIN* (2) (not start)
684	Redundant Connection	Python, Java	O(n)	O(n)	Medium	tree, dfs, graph, Union Find, `basic`	AGAIN****** (3) (not start)
687	Longest Univalue Path	Python, Java	O(n)	O(h)	Medium	tree, dfs, good basic, path, `google`	AGAIN********* (5)
814	Binary Tree Pruning	Python, Java	O(n)	O(h)	Medium	tree, prune, good basic, DFS, `basic`, LC 1325	AGAIN*********** (3)(MUST)
834	Sum of Distances in Tree	Java	O(n)	O(n)	Hard	tree, DP, re-rooting pattern	AGAIN****** (1) (not start)
863	All Nodes Distance K in Binary Tree	Python, Java	O(n)	O(n)	Medium	tree, LC 752, tree -> graph, radiate outward, DFS + BFS, good basic, `amazon`	AGAIN**************** (5)(MUST)
865	Smallest Subtree with all the Deepest Nodes	Python, Java	O(n)	O(h)	Medium	tree, LC 1123, LCA, dfs, `basic`	AGAIN***************** (6)(MUST)
889	Construct Binary Tree from Preorder and Postorder Traversal	Python, Java	O(n)	O(h)	Medium	tree, `DFS`, `stack`, `trick`, recursive, google	AGAIN** (2) (not start*)
897	Increasing Order Search Tree	Python, Java	O(n)	O(h)	Easy	tree, `DFS`, `good basic`, `inorder`	AGAIN** (2)
919	Complete Binary Tree Inserter	Python, Java	ctor: O(n) insert: O(1) get_root: O(1)	O(n)	Medium	tree, `bst`	AGAIN** (2)
938	Range Sum of BST	Python, Java	O(n)	O(h)	Easy	tree, DFS, check `# 108 Convert Sorted Array to Binary Search Tree`	OK* (2)
951	Flip Equivalent Binary Trees	Python, Java	O(n)	O(h)	Medium	tree, recursion, DFS, google	AGAIN***s (3)
958	Check Completeness of a Binary Tree	Python, Java	O(n)	O(w)	Medium	tree, `BFS`, `DFS`, `basic`	AGAIN** (2)
965	Univalued Binary Tree	Python, Java	O(n)	O(h)	Easy	tree, `DFS`, `BFS`, `good basic`	AGAIN** (2)
971	Flip Binary Tree To Match Preorder Traversal	Python, Java	O(n)	O(h)	Medium	tree, DFS, `trick`	AGAIN** (2) (not start)
979	Distribute Coins in Binary Tree	Python, Java	O(n)	O(h)	Medium	tree, DFS, `trick`	AGAIN*** (2)
508	Most Frequent Subtree Sum	Python, Java	O(n)	O(n)	Medium	tree, dfs, bfs, good basic, `amazon`	OK*********** (4) (but again MUST)
683	K Empty Slots	Python, Java	O(n)	O(n)	Hard	tree, BST, bucket, `google`	not start
701	Insert into a Binary Search Tree	Python, Java, Java-followup	O(h)	O(h)	Medium	tree, good basic, BST, BFS, DFS	OK***** (3)
1032	Stream of Characters	Java	ctor: O(t), query: O(l)	O(t)	Hard	tree, trie, node, google	AGAIN****** (1)
1157	Online Majority Element In Subarray	Java	ctor: O(n), query: O(klogn)	O(n)	Hard	tree, google	AGAIN (not start)
1325	Delete Leaves With a Given Value	Java, Python	O(n)	O(h)	Medium	tree, dfs, post-order, LC 450, 814	AGAIN********** (4)(MUST)
1339	Maximum Product of Splitted Binary Tree	Python, Java	O(n)	O(h)	Medium	tree, dfs, amazon	AGAIN*** (1) (not start)
1372	Longest ZigZag Path in a Binary Tree	Python, Java	O(n)	O(h)	Medium	tree, dfs, bfs, post order, good trick, amazon	AGAIN*** (1) (not start)
1448	Count Good Nodes in Binary Tree	Python, Java	O(n)	O(h)	Medium	tree, max in path, dfs, bfs, good trick, needcode, google, amz, m$	AGAIN************* (3) (MUST)
1522	Diameter of N-Ary Tree	Java	O(n)	O(h)	Medium	tree, LC 543	AGAIN (not start)
1666	Change the Root of a Binary Tree	Java	O(h)	O(h)	Medium	tree, LC 1448, good trick, google	AGAIN (1) (not start)
1740	Find Distance in a Binary Tree	Python, Java	O(n)	O(h)	Medium	tree, LCA, node dist, DFS, good basic	AGAIN*************** (5)(MUST)
2331	Evaluate Boolean Binary Tree	Java	O(n)	O(h)	Easy	tree, DFS, good basic	OK (1)****
2415	Reverse Odd Levels of Binary Tree	Java	O(n)	O(n)	Medium	tree, LC 226, dfs, bfs	AGAIN (not start)
2509	Cycle Length Queries in a Tree	Java	O(q logn)*	O(1)	Hard	tree, LC weekly	AGAIN (not start)
2641	Cousins in Binary Tree II	Java	O(n)	O(n)	Medium	tree, LC weekly, LC 993	AGAIN (1)
2707	Extra Characters in a String	Java	O(n^2)	O(n + t)	Medium	tree, dp, Trie	AGAIN (not start)
3093	Longest Common Suffix Queries	Java	O(t + q l)*	O(t)	Hard	tree, Trie	AGAIN (not start)

Heap

#	Title	Solution	Time	Space	Difficulty	Note	Status
264	Ugly Number II	Python, Java	O(n)	O(n)	Medium	heap, good trick, LC 263, LC 313, LintCode, DP, M$	AGAIN*********** (4) (MUST)
313	Super Ugly Number	Python, Java	O(n k)*	O(n + k)	Medium	heap, BST	AGAIN (not start*)
373	Find K Pairs with Smallest Sums	Python, Java	O(k log(min(n, m, k)))*	O(min(n, m, k))	Medium	heap, google, good basic	AGAIN******* (1)
378	Kth Smallest Element in a Sorted Matrix	Python, Java	O(klogn)	O(k)	Medium	heap, binary search, matrix, LintCode	AGAIN** (1)
502	IPO	Java	O(nlogn)	O(n)	Hard	heap, PQ	AGAIN (not start)
506	Relative Ranks	Java	O(nlogn)	O(n)	Easy	heap, PQ, sort, hashmap	AGAIN (not start)
846	Hand of Straights	Python, Java	O(nlogn)	O(n)	Medium	heap, LC 1296, good basic, PQ, Treemap, hashmap, google	OK******** (3)
855	Exam Room	Python, Java	seat: O(logn) leave: O(logn)	O(n)	Medium	heap, treeSet, BST, Hash, `trick`, LC 849, google	AGAIN* (3) (not start)
295	Find Median from Data Stream	Python, Java	addNum: O(logn), findMedian: O(1)	O(n)	Hard	heap, Curated Top 75, priority queue, trick, stream, `amazon`	AGAIN****** (6)
703	Kth Largest Element in a Stream	Python, Java	ctor: O(nlogk), add: O(logk)	O(k)	Easy	heap, priority queue, `amazon`	AGAIN******** (5)
1046	Last Stone Weight	Python, Java	O(nlogn)	O(n)	Easy	heap, Priority Queue	OK
1054	Distant Barcodes	Java	O(nlogn)	O(n)	Medium	heap, PQ, LC 767	Again (1)
1130	Minimum Cost Tree From Leaf Values	Python, Java	O(n)	O(n)	Medium	heap, LC 1167, monotonic stack (O(n)) or interval dp (O(n^3)), greedy	not start
1167	Minimum Cost to Connect Sticks	Python, Java	O(nlogn)	O(n)	Medium	heap, LC 1130, good basic, `amazon`	AGAIN***** (3)
1353	Maximum Number of Events That Can Be Attended	Python, Java	O(nlogn)	O(n)	Medium	heap, LC 252, 253, good trick, meeting room, PRIORITY QUEUE, fb, twitter, `amazon`	AGAIN********* (3) MUST
1383	Maximum Performance of a Team	Java	O(nlogn)	O(n)	Hard	heap, PQ	AGAIN***** (1)
1405	Longest Happy String	Python, Java	O(n)	O(1)	Medium	heap, PQ, most_freq, LC 767	AGAIN************* (2) (MUST)
1438	Longest Continuous Subarray With Absolute Diff Less Than or Equal to Limit	Java	O(n)	O(n)	Medium	heap, PQ, treemap, dequeue	AGAIN (not start)
1481	Least Number of Unique Integers after K Removals	Python, Java	O(nlogn)	O(n)	Medium	heap, good trick, greedy, Counter, dict, `amazon`	AGAIN******** (3)
1647	Minimum Deletions to Make Character Frequencies Unique	Java	O(nlogn)	O(n)	Medium	heap, greedy, PQ, good trick	AGAIN***********(2)
1675	Minimize Deviation in Array	Java	O(nlogn logM)*	O(n)	Hard	heap, PQ	AGAIN (not start)
1834	Single-Threaded CPU	Java	O(nlogn)	O(n)	Medium	heap, good trick, pq	AGAIN***** (1)
2406	Divide Intervals Into Minimum Number of Groups	Python, Java	O(nlogn)	O(n)	Medium	heap, LC 253, PQ, sort, scan line, good basic	AGAIN********** (2) (MUST)
2542	Maximum Subsequence Score	Java	O(nlogn)	O(n)	Medium	heap, pq with pair, good trick, sort	AGAIN (1)
2593	Find Score of an Array After Marking All Elements	Java	O(nlogn)	O(n)	Medium	heap, good trick, pq, sort, slide window, LC weekly	AGAIN***** (1)
3092	Most Frequent IDs	Python, Java	O(nlogn)	O(n)	Medium	heap, PQ, lazy delete, hashmap, LC weekly	AGAIN********** (MUST) (1)

Bit Manipulation

#	Title	Solution	Time	Space	Difficulty	Note	Status
136	Single Number	Python, Java	O(n)	O(1)	Easy	bit manipulation, `airbnb`, `amazon`, `fb`	OK
137	Single Number II	Python, Java	O(n)	O(1)	Medium	bit manipulation, LC 136, count bits mod 3 / ones-twos state machine, `good trick`	OK
190	Reverse Bits	Python, Java	O(1)	O(1)	Easy	bit manipulation, Curated Top 75, bit, `apple`	AGAIN*** (2)
191	Number of 1 Bits	Python, Java	O(1)	O(1)	Easy	bit manipulation, Curated Top 75, apple	OK*
201	Bitwise AND of Numbers Range	Python, Java	O(1)	O(1)	Medium	bit manipulation, common binary prefix, shift right until m == n, `trick`	AGAIN (not start)
231	Power of Two	Python, Java	O(1)	O(1)	Easy	bit manipulation, good basic, `amazon`	OK*** (3)
260	Single Number III	Python, Java	O(n)	O(1)	Medium	bit manipulation, `xor`, `^=`, `^`	AGAIN*
268	Missing Number	Python, Java	O(n)	O(1)	Easy	bit manipulation, Curated Top 75, `xor`, `amazon`, `fb`	OK* (2)
318	Maximum Product of Word Lengths	Python, Java	O(n^2 + n l)*	O(n)	Medium	bit manipulation, Counting Sort, Pruning	AGAIN (not start)
342	Power of Four	Python, Java	O(1)	O(1)	Easy	bit manipulation, `power of num`	OK*
371	Sum of Two Integers	Python, Java	O(1)	O(1)	Medium	bit manipulation, Curated Top 75, `good basic`, `fb`	AGAIN********* (5) (bit op not start)
389	Find the Difference	Python, Java	O(n)	O(1)	Easy	bit manipulation, `amazon`	OK (2)
393	UTF-8 Validation	Python, Java	O(n)	O(1)	Medium	bit manipulation, `utf-8 encoding`, `google`, `fb`	AGAIN**** (not start*) (5)
401	Binary Watch	Python, Java	O(1)	O(1)	Easy	bit manipulation, enumerate the 12 * 60 times or use bit count, brute force, `basic`	OK*
421	Maximum XOR of Two Numbers in an Array	Python, Java	O(n)	O(n)	Medium	bit manipulation, Trie of bits or prefix hashset, greedy from the MSB, `good trick`	AGAIN (not start)
461	Hamming Distance	Python, Java	O(1)	O(1)	Easy	bit manipulation, `fb`	OK* (2)
462	Minimum Moves to Equal Array Elements II	Python, Java	O(n) on average	O(1)	Medium	bit manipulation, median minimizes the sum of absolute diffs, quick select, sort, `math`	AGAIN*
477	Total Hamming Distance	Python, Java	O(n)	O(1)	Medium	bit manipulation, `fb`	AGAIN****** (not start*) (4)
645	Set Mismatch	Python, Java	O(n)	O(1)	Easy	bit manipulation, `amazon`	OK* (3)
693	Binary Number with Alternating Bits	Python, Java	O(1)	O(1)	Easy	bit manipulation, `trick`	AGAIN*
762	Prime Number of Set Bits in Binary Representation	Python, Java	O(1)	O(1)	Easy	bit manipulation, `basic`, `prime number`, `amazon`	OK* (4)
868	Binary Gap	Python, Java	O(1)	O(1)	Easy	bit manipulation, `trick`, `linear scan`	AGAIN*
898	Bitwise ORs of Subarrays	Python, Java	O(n logM)*	O(logM)	Medium	bit manipulation, set of distinct ORs per index (at most logM), `good trick`	AGAIN (not start)
1573	Number of Ways to Split a String	Python, Java	O(n)	O(1)	Medium	bit manipulation, `amazon`, binary	AGAIN** (not start)
1915	Number of Wonderful Substrings	Python, Java	O(n)	O(1)	Medium	bit manipulation, bit mask, bit, prefix sum, dp, `amazon`	AGAIN*** (2) (not start)
2683	Neighboring Bitwise XOR	Java	O(n)	O(1)	Medium	bit manipulation, bit op, simulation LC weekly 345	AGAIN*** (1)
2871	Split Array Into Maximum Number of Subarrays	Java	O(n)	O(1)	Medium	bit manipulation, bit op, LC weekly	AGAIN*** (1)
3688	Bitwise OR of Even Numbers in an Array	Java	O(n)	O(1)	Easy	bit manipulation, bit mask	OK

String

#	Title	Solution	Time	Space	Difficulty	Note	Status
005	Longest Palindromic Substring	Python, Java	O(n^2)	O(1)	Medium	string, Curated Top 75, LC 647, good trick, 2 pointer, recursive, `Manacher's Algorithm`, LC 100 like, `DP`, `UBER`, `amazon`, `fb`	AGAIN******************* (8)(MUST)
006	ZigZag Conversion	Python, Java	O(n)	O(n)	Medium	string, simulate the rows, `basic`, `trick`	AGAIN (not start)
008	String to Integer (atoi)	Python, Java	O(n)	O(1)	Medium	string, string op, `regular expression`, `UBER`, `amazon`, `fb`	AGAIN***** (5)
14	Longest Common Prefix	Python, Java	O(n k)*	O(1)	Easy	string, `good basic`, LCP, prefix, binary search, scan	AGAIN !!! (5)
028	Implement strStr()	Python, Java	O(n + k)	O(k)	Easy	string, `KMP Algorithm`, `fb`	OK (3)
028	Find the Index of the First Occurrence in a String	Java	O(n + k)	O(k)	Easy	string, duplicate of LC 28, `KMP Algorithm`, `Rabin-Karp Algorithm`, two pointers	OK (3)
038	Count and Say	Python, Java	O(n 2^n)*	O(2^n)	Medium	string, `recursion`, `iteration`, basic, `fb`	AGAIN*********** (5)
43	Multiply Strings	Python, Java	O(m n)*	O(m + n)	Medium	string, `twitter`, `fb`	AGAIN*** (3)
58	Length of Last Word	Python, Java	O(n)	O(1)	Easy	string, scan from the end, `basic`	OK
067	Add Binary	Python, Java	O(n)	O(1)	Easy	string, bit op, `good basic`, `fb`, `amazon`	OK***** (7) (but again)
076	Minimum Window Substring	Python, Java	O(m + n)	O(k)	Hard	string, slide window, good pattern, `Curated Top 75`	AGAIN*********** (3)
125	Valid Palindrome	Python, Java	O(n)	O(1)	Easy	string, Curated Top 75, LC 680, `amazon`, `fb`	OK
151	Reverse Words in a String	Python, Java	O(n)	O(1)	Medium	string, `amazon`, `fb`	OK (2)
161	One Edit Distance	Python, Java	O(m + n)	O(1)	Medium	string, 🔒, DP, 2 pointers, `fb`	AGAIN ************ (6)(MUST)
165	Compare Version Numbers	Python, Java	O(n)	O(1)	Medium	string, good basic, `amazon`, `apple`	OK** (5)
186	Reverse Words in a String II	Python, Java	O(n)	O(1)	Medium	string, LC 151, LC 557, 🔒, `M$`, `UBER`, `amazon`	AGAIN* (3)
242	Valid Anagram	Python, Java	O(n)	O(1)	Easy	string, Curated Top 75, LintCode, `amazon`, `fb`	OK
271	Encode and Decode Strings	Python, Java	O(n)	O(1)	Medium	string, Curated Top 75, decode-encode, string op, check `# 394 Decode String`, 🔒, `google`	AGAIN******* (4)
306	Addictive Number	Python, Java	O(n^3)	O(n)	Medium	string, backtracking over the first two numbers, big-int add, `dfs`, LC 842	AGAIN (not start*)
340	Longest Substring with At Most K Distinct Characters	Python, Java	O(n)	O(k)	Hard	string, sliding window, `google`	not start
383	Ransom Note	Python, Java	O(n)	O(1)	Easy	string, EPI, apple	OK*
405	Convert a Number to Hexadecimal	Python, Java	O(n)	O(1)	Easy	string, `basics` `decimal -> Hexadecimal`	AGAIN
408	Valid Word Abbreviation	Python, Java	O(n)	O(1)	Easy	string, 🔒	AGAIN (not start)
415	Add Strings	Python, Java	O(n)	O(1)	Easy	string, `good basic`, `add xxx to sum`, `Airbnb`, `google`, `fb`	OK****** (4)
434	Number of Segments in a String	Python, Java	O(n)	O(1)	Easy	string, count word starts, split, `basic`	OK
443	String Compression	Python, Java	O(n)	O(1)	Easy	string, shoptify, `basics`	AGAIN**
459	Repeated Substring Pattern	Python, Java	O(n)	O(n)	Easy	string, good basic, `KMP Algorithm`, `amazon`	OK*** (6)
468	Validate IP Address	Python, Java	O(1)	O(1)	Medium	string, `ip basic`, `twitter`, `fb`, `amazon`	AGAIN****** (5)
482	License Key Formatting	Python, Java	O(n)	O(1)	Easy	string, basic, `google`	OK** (4) (again)
520	Detect Capital	Python, Java	O(l)	O(1)	Easy	string, three valid cases (all upper / all lower / leading capital), `basic`	OK
521	Longest Uncommon Subsequence I	Python, Java	O(min(a, b))	O(1)	Easy	string, if the two strings differ the answer is max(len), `trick`	OK*
522	Longest Uncommon Subsequence II	Python, Java	O(l n^2)*	O(1)	Medium	string, sort, compare all pairs, LC 521	AGAIN (not start)
524	Longest Word in Dictionary through Deleting	Python, Java	O(d l + d * logd)*	O(1)	Medium	string, good basic, sort, 2 pointers, subsequence check, `google`	AGAIN*********** (2)
539	Minimum Time Difference	Python, Java	O(nlogn)	O(n)	Medium	string, `basic`, `map & zip trick`, google	AGAIN**
541	Reverse String II	Python, Java	O(n)	O(1)	Easy	string, reverse every 2k block, `basic`	AGAIN**** (1)
551	Student Attendance Record I	Python, Java	O(n)	O(1)	Easy	string, count `A` and 3 consecutive `L`, `basic`	OK**
556	Next Greater Element III	Python, Java	O(n)	O(n)	Medium	string, LC 31 next permutation applied to digits, `good trick`	AGAIN (not start)
557	Reverse Words in a String III	Python, Java	O(n)	O(1)	Easy	string, split, reverse each word, `basic`	OK
616	Add Bold Tag in String	Python, Java	O(n d * l)*	O(n)	Medium	string, LC 758, 🔒, `google`	AGAIN (not start)
647	Palindromic Substrings	Python, Java	O(n^2)	O(1)	Medium	string, Curated Top 75, LC 005, greedy, 2D DP, expand from center, 2 pointers, `good basic`, `Manacher's Algorithm`, `Linkedin`, `amazon`, `fb`	OK************** (10) (MUST)
648	Replace Words	Python, Java	O(n)	O(t)	Medium	string, Trie, hashset, `good basic`	AGAIN (2)*******
657	Judge Route Circle	Python, Java	O(n)	O(1)	Easy	string, count moves per direction, LC 1041, `basic`	OK
678	Valid Parenthesis String	Python, Java	O(n)	O(1)	Medium	string, greedy low/high open-count range, dp, stack, `good trick`	AGAIN**
680	Valid Palindrome II	Python, Java	O(n)	O(1)	Easy	string, LC 125, two pointers, `good basic`, Palindrome, `fb`	AGAIN************* (4) (MUST)
681	Next Closest Time	Python, Java	O(1)	O(1)	Medium	string, `subset of set`, `<=`, `strptime`, `google`, `fb`	OK*** (5)
686	Repeated String Match	Python, Java	O(n + m)	O(1)	Medium	string, brute force, math, `Rabin-Karp Algorithm`, `google`	AGAIN**** (3)
696	Count Binary Substrings	Python, Java	O(n)	O(1)	Easy	string, good trick, linear scan, group sub-string, `amazon`	AGAIN******* (3) (MUST)
720	Longest Word in Dictionary	Python, Java	O(n l)*	O(t)	Medium	string, Trie, DFS, good trick	AGAIN**** (1)
722	Remove Comments	Python, Java	O(n)	O(k)	Medium	string, google	AGAIN (1)
734	Sentence Similarity	Python , Java	O(n + p)	O(p)	Easy	string, good basic, graph, `hash table`, `collections.defaultdict(set)`, `google`	OK***** (5)
751	IP to CIDR	Python, Java	O(n)	O(1)	Medium	string, 🔒, bit manipulation, largest CIDR block that fits, `ip`	AGAIN (not start*)
758	Bold Words in String	Python, Java	O(n l)*	O(t)	Easy	string, 🔒, LC 616, variant of Add Bold Tag in String, google	AGAIN (not start)
791	Custom Sort String	Python, Java	O(n)	O(1)	Medium	string, `good basic`, sort, counter, `amazon`, `fb`	AGAIN************ (7) (MUST)
796	Rotate String	Python, Java	O(n)	O(1)	Easy	string, good basic, `KMP Algorithm`, `Rabin-Karp Algorithm`, `amazon`	OK**** (2)(but again)
804	Unique Morse Code Words	Python, Java	O(n)	O(n)	Easy	string, map letters to morse, hashset of transformations, `basic`	OK*
806	Number of Lines To Write String	Python, Java	O(n)	O(1)	Easy	string, greedy line filling, `basic`	OK*
809	Expressive Words	Python, Java	O(n + s)	O(l + s)	Medium	string, 2 pointers, group len, good basic, google	AGAIN****** (2)
816	Ambiguous Coordinates	Python, Java	O(n^4)	O(n)	Medium	string, trick, google	AGAIN**** (1)
819	Most Common Word	Python, Java	O(m + n)	O(m + n)	Easy	string, `regular expression`, `amazon`	OK** (2)
820	Short Encoding of Words	Python, Java	O(n)	O(t)	Medium	string, Trie	AGAIN (not start)
824	Goat Latin	Python, Java	O(n + w^2)	O(l)	Easy	string, `string basic`, `fb`	OK
828	Count Unique Characters of All Substrings of a Given String	Python, Java	O(n)	O(1)	Hard	string, LC 1248, count each char’s contribution (prev/next same char), dp, `amazon`	AGAIN*** (2)
831	Masking Personal Information	Python, Java	O(1)	O(1)	Medium	string, `regular expression`	OK*
833	Find And Replace in String	Python, Java	O(n + m)	O(n)	Medium	string, good basic, string op, hashmap, google	AGAIN****** (3)
848	Shifting Letters	Python, Java	O(n)	O(1)	Medium	string, `remainder`, `basic`	AGAIN**
859	Buddy Strings	Python, Java	O(n)	O(1)	Easy	string, compare differing positions, duplicate-char check, `trick`	AGAIN*
880	Decoded String at Index	Python, Java	O(n)	O(1)	Medium	string, `trick`	AGAIN (not start**)
884	Uncommon Words from Two Sentences	Python, Java	O(m + n)	O(m + n)	Easy	string, `collections Counter`, `trick`	OK*
890	Find and Replace Pattern	Python, Java	O(n l)*	O(1)	Medium	string, hashmap, pattern, LC 49	AGAIN *** (3)
893	Groups of Special-Equivalent Strings	Python, Java	O(n l)*	O(n)	Easy	string, sort even- and odd-indexed chars separately, hashset, `trick`	AGAIN*
916	Word Subsets	Python, Java	O(m + n)	O(1)	Medium	string, max char count across all words in b, Counter, `good trick`	AGAIN (not start)
917	Reverse Only Letters	Python, Java	O(n)	O(1)	Easy	string, `good basic`, `stack`, `pointer`	AGAIN**** (1)
925	Long Pressed Name	Python, Java	O(n)	O(1)	Easy	string, `two pointer`, `basic`, `trick`	AGAIN*
929	Unique Email Addresses	Python, Java	O(n l)*	O(n l)*	Easy	string, normalize local part (drop dots, cut at +), hashset, `basic`	OK
939	Minimum Area Rectangle	Python, Java	O(n^2)	O(n)	Medium	string, hashmap, brute force, google	AGAIN !!! (2)
942	DI String Match	Python, Java	O(n)	O(1)	Easy	string, good basic, 2 pointers	AGAIN*** (1)
944	Delete Columns to Make Sorted	Python, Java	O(n l)*	O(1)	Easy	string, count columns that are not sorted, `basic`	AGAIN (not start)
953	Verifying an Alien Dictionary	Python, Java	O(n l)*	O(1)	Easy	string, map chars to the alien order then compare adjacent words, `good basic`	AGAIN********** (3) (MUST)
955	Delete Columns to Make Sorted II	Python, Java	O(n l)*	O(n)	Medium	string, LC 944, greedy column keeping, `good trick`	AGAIN (not start)
1119	Remove Vowels from a String	Python, Java	O(n)	O(n)	Easy	string, `amazon`	OK
4006	Count Valid Prefixes	Python, Java	O(n l)*	O(n)	Easy	string, prefix, good basic, LC weekly	OK

Queue

#	Title	Solution	Time	Space	Difficulty	Note	Status
281	Zigzag Iterator	Python, Java	O(n)	O(k)	Medium	queue, `good basic`, 🔒, `google`, `fb`	AGAIN******* (5)
346	Moving Average from Data Stream	Python, Java	O(1)	O(w)	Easy	queue, 🔒, `google`, `fb`	OK** (6)
933	Number of Recent Calls	Python, Java	O(1) on average	O(w)	Easy	queue, deque, drop pings older than t - 3000, `basic`	AGAIN**
622	Design Circular Queue	Python, Java	O(1) per op	O(k)	Medium	queue, `good basic`, array, linked list, design, `circular queue`, `amazon`, `fb`, apple, airbnb, m$	AGAIN************ (3) (MUST)
239	Sliding Window Maximum	Python, Java	O(n)	O(k)	Hard	queue, sliding window, heap, deque, good trick, `amazon`, `google`	AGAIN************ (6)

Math

#	Title	Solution	Time	Space	Difficulty	Note	Status
007	Reverse Integer	Python, Java	O(1)	O(1)	Medium	math, `0x7FFFFFFF`, amazon, apple	OK* (3)
009	Palindrome Number	Python, Java	O(1)	O(1)	Easy	math, amazon	OK
012	Integer to Roman	Python, Java	O(1)	O(1)	Medium	math, `math basic`, `twitter`, `fb`, amazon	AGAIN****** (4)
013	Roman to Integer	Python, Java	O(n)	O(1)	Easy	math, good trick, map, `UBER`, `amazon`, `fb`	AGAIN***** (6)
029	Divide Two Integers	Python, Java	O(1)	O(1)	Medium	math, `binary search`, `trick`, `fb`	AGAIN***** (3)
050	Pow(x, n)	Python, Java	O(1)	O(1)	Medium	math, `good trick`, `amazon`, `fb`, recursion	AGAIN****** (4)
060	Permutation Sequence	Python, Java	O(n^2)	O(n)	Medium	math, `Cantor Ordering`, trick, `amazon`	AGAIN*** (2)
089	Gray Code	Python, Java	O(2^n)	O(1)	Medium	math, bit op, `amazon`	AGAIN ********** (6)
166	Fraction to Recurring Decimal	Python, Java	O(n)	O(n)	Medium	math, hashmap remainder -> index to detect the repeating cycle, `good trick`	AGAIN (not start)
168	Excel Sheet Column Title	Python, Java	O(logn)	O(1)	Easy	math, trick, residual, AGAIN, `fb`	AGAIN****** (5)
149	Max Points on a Line	Java	O(n^2)	O(n)	Hard	math, hashmap	AGAIN(1)
171	Excel Sheet Column Number	Python, Java	O(n)	O(1)	Easy	math, base-26 accumulation, LC 168, `basic`	AGAIN (not start)
172	Factorial Trailing Zeroes	Python, Java	O(1)	O(1)	Easy	math, `basic`, `iteration`, `recursion`	AGAIN* (2)
223	Rectangle Area	Python, Java	O(1)	O(1)	Easy	math, area sum minus the overlap, `basic`	AGAIN
258	Add Digits	Python, Java	O(1)	O(1)	Easy	math, digital root: 1 + (n - 1) % 9, `trick`	AGAIN
263	Ugly Number	Python, Java	O(1)	O(1)	Easy	math, divide out 2, 3, 5, LC 264, `basic`	OK* (2)
273	Integer to English Words	Python, Java	O(logn)	O(1)	Hard	math, `amazon`, passed	not start
292	Nim Game	Python, Java	O(1)	O(1)	Easy	math, LintCode	OK*
319	Bulb Switcher	Python, Java	O(1)	O(1)	Medium	math, answer is floor(sqrt(n)) - only perfect squares have an odd divisor count, `good trick`	AGAIN
326	Power of Three	Python, Java	O(1)	O(1)	Easy	math, loop divide by 3, or 3^19 % n == 0, LC 231, LC 342, `basic`	OK
338	Counting Bits	Python, Java	O(n)	O(n)	Medium	math, Curated Top 75, dp, bit manipulation, `dp[i] = dp[i >> 1] + (i & 1)`, `good basic`	OK
343	Integer Break	Python, Java	O(n)	O(n)	Medium	math, max product, split into 3s, `trick`, `DP`	AGAIN********** (3)(MUST)
365	Water and Jug Problem	Python, Java	O(logn)	O(1)	Medium	math, `Bézout's identity`	AGAIN (not start)
372	Super Pow	Python, Java	O(n)	O(1)	Medium	math, modular fast power over the digit array	AGAIN
382	Linked List Random Node	Python, Java	ctor: O(1), getRandom: O(n)	O(1)	Medium	math, `Reservoir Sampling`	AGAIN (not start)
386	Lexicographical Numbers	Python, Java	O(n)	O(1) excluding output	Medium	math, pre-order DFS over the 10-ary number trie, iterative, `good trick`	AGAIN (not start)
390	Elimination Game	Python, Java	O(logn)	O(1)	Medium	math, track head / step / remaining count, `good trick`	AGAIN (not start)
398	Random Pick Index	Python, Java	ctor: O(1), pick: O(n)	O(1)	Medium	math, `Reservoir Sampling`, hashmap of value -> indices, `fb`	OK* (4)
400	Nth Digit	Python, Java	O(logn)	O(1)	Easy	math, count digits per length band then locate the number	AGAIN
413	Arithmetic Slices	Python, Java	O(n)	O(1)	Medium	math, dp, count consecutive runs with the same diff, `good basic`, `amazon`	OK*** (1) (but again)
423	Reconstruct Original Digits from English	Python, Java	O(n)	O(1)	Medium	math, GCJ2016 - Round 1B	AGAIN (not start)
441	Arranging Coins	Python, Java	O(logn)	O(1)	Easy	math, Binary Search on the row count, or the quadratic formula	OK
453	Minimum Moves to Equal Array Elements	Python, Java	O(n)	O(1)	Easy	math, incrementing n-1 elements equals decrementing 1, sum - n * min, `trick`	AGAIN
458	Poor Pigs	Python, Java	O(logn)	O(1)	Hard	math, information theory, (tests + 1) ^ pigs >= buckets, `good trick`	AGAIN
469	Convex Polygon	Python, Java	O(n)	O(1)	Medium	math, 🔒, cross-product sign must stay consistent, `geometry`	AGAIN (not start)
470	Implement Rand10() Using Rand7()	Python, Java	O(1)	O(1)	Medium	math, rejection sampling, LC 478, `good trick`	AGAIN (not start)
478	Generate Random Point in a Circle	Python, Java	O(1)	O(1)	Medium	math, rejection sampling or inverse transform with sqrt, `geometry`	AGAIN (not start)
497	Random Point in Non-overlapping Rectangles	Python, Java	ctor: O(n) pick: O(logn)	O(n)	Medium	math, prefix sum of rectangle areas + binary search, `good trick`	AGAIN (not start)
504	Base 7	Python, Java	O(logn)	O(logn)	Easy	math, base conversion, good basic, recursion	AGAIN******** (1)
517	Super Washing Machines	Python, Java	O(n)	O(1)	Hard	math, max of (max deficit, max single surplus), `greedy`, `amazon`	AGAIN (not start)
519	Random Flip Matrix	Python, Java	ctor: O(1) pick: O(1) reset: O(n)	O(n)	Medium	math, hashmap-based virtual Fisher-Yates swap, `good trick`	AGAIN (not start)
528	Random Pick with Weight	Python, Java	O(logn)	O(n)	Medium	math, good basic, binary search, prefix sum, google	AGAIN**** (1)
537	Complex Number Multiplication	Python, Java	O(1)	O(1)	Medium	math, `amazon`	OK (3)
553	Optimal Division	Python, Java	O(n)	O(1)	Medium	math, string, `amazon`	OK* (4)
573	Squirrel Simulation	Python, Java	O(n)	O(1)	Medium	math, 🔒	AGAIN (not start)
592	Fraction Addition and Subtraction	Python, Java	O(nlogx)	O(n)	Medium	math, parse signed fractions, reduce with gcd / lcm	AGAIN
593	Valid Square	Python, Java	O(1)	O(1)	Medium	math, google	AGAIN
598	Range Addition II	Python, Java	O(p)	O(1)	Easy	math, answer is min(all a) * min(all b), `trick`	AGAIN
625	Minimum Factorization	Python, Java	O(loga)	O(1)	Medium	math, 🔒	OK*
628	Maximum Product of Three Numbers	Python, Java	O(n)	O(1)	Easy	math, `amazon`	OK (2)
633	Sum of Square Numbers	Python, Java	O(sqrt© logc)*	O(1)	Easy	math, two pointers from 0 to sqrt©, or test each a, `basic`	OK*
634	Find the Derangement of An Array	Python, Java	O(n)	O(1)	Medium	math, 🔒	AGAIN (not start)
640	Solve the Equation	Python, Java	O(n)	O(n)	Medium	math, regular expression, `amazon`	AGAIN*** (3)
651	4 Keys Keyboard	Python, Java	O(n)	O(n)	Medium	math, 🔒, DP	AGAIN (not start)
672	Bulb Switcher II	Python, Java	O(1)	O(1)	Medium	math, only 6 distinct states after the first 3 lights, `trick`	AGAIN (not start)
728	Self Dividing Numbers	Python, Java	O(n)	O(1)	Easy	math, check that each digit divides the number, `basic`	AGAIN
754	Reach a Number	Python, Java	O(logn)	O(1)	Medium	math, walk to the first triangular number >= target then fix parity, `good trick`	OK*
775	Global and Local Inversions	Python, Java	O(n)	O(1)	Medium	math, trick, string, `amazon`	AGAIN** (not start) (3)
779	K-th Symbol in Grammar	Python, Java	O(1)	O(1)	Medium	math, brute force, binary, recursion, FB, amazon	AGAIN*** (1)
781	Rabbits in Forest	Python, Java	O(n)	O(n)	Medium	math, group rabbits by their answer, ceil division, `good trick`	AGAIN (not start)
789	Escape The Ghosts	Python, Java	O(n)	O(1)	Medium	math, compare Manhattan distances, `trick`	AGAIN
800	Similar RGB Color	Python, Java	O(1)	O(1)	Easy	math, 🔒	AGAIN (not start)
812	Largest Triangle Area	Python, Java	O(n^3)	O(1)	Easy	math, shoelace formula over all triples, `geometry`	AGAIN (not start)
829	Consecutive Numbers Sum	Python, Java	O(sqrt(n))	O(1)	Medium	math, count factorizations n = k * m + k(k - 1) / 2, `good trick`	AGAIN
836	Rectangle Overlap	Python, Java	O(1)	O(1)	Easy	math, similar as 223. Rectangle Area, `amazon`	AGAIN (2)
858	Mirror Reflection	Python, Java	O(1)	O(1)	Medium	math, LCM / parity reasoning, `good trick`	AGAIN (not start)
866	Prime Palindrome	Python, Java	O(n^(1/2) (logn + n^(1/2)))*	O(logn)	Medium	math, generate palindromes then test primality, even-length ones divide by 11	AGAIN***
867	Transpose Matrix	Java	O(m n)*	O(m n)*	Medium	math, duplicate of LC 868, `matrix`, `basic`	AGAIN*
883	Projection Area of 3D Shapes	Python, Java	O(n^2)	O(1)	Easy	math, xy = count of non-zero cells, yz = column max, zx = row max, `matrix`	AGAIN
891	Sum of Subsequence Widths	Python, Java	O(nlogn)	O(1)	Hard	math, sort, each element contributes as max/min weighted by powers of 2, `amazon`	AGAIN (not start)
907	Sum of Subarray Minimums	Python, Java	O(n)	O(n)	Medium	math, trick, LC 084, LC 2104, Ascending Stack, `amazon`	AGAIN**** (1)
908	Smallest Range I	Python, Java	O(n)	O(1)	Easy	math, max(0, max - min - 2k), `basic`	OK* (2)
910	Smallest Range II	Python, Java	O(nlogn)	O(1)	Medium	math, sort then try every split point, `greedy`, `good trick`	AGAIN (not start)
914	X of a Kind in a Deck of Cards	Python, Java	O(nlogn)	O(n)	Easy	math, gcd of all group counts must be >= 2, Counter	AGAIN (not start)
963	Minimum Area Rectangle II	Python, Java	O(n^2)	O(n)	Medium	math, hashmap keyed by (center, diagonal length), `geometry`	AGAIN (not start)
970	Powerful Integers	Python, Java	O((logn)^2)	O®	Easy	math, `basic`	AGAIN* (2)
1015	Smallest Integer Divisible by K	Java	O(k)	O(1)	Medium	math, track remainder of 1, 11, 111 … mod k, pigeonhole, google	AGAIN (not start)
1071	Greatest Common Divisor of Strings	Java	O(m + n)	O(1)	Easy	math, if s1 + s2 == s2 + s1 the answer is the prefix of length gcd(m, n), `good trick`	AGAIN (not start)
1342	Number of Steps to Reduce a Number to Zero	Python, Java	O(logn)	O(1)	Easy	math, `gra*`	ok
1492	The kth Factor of n	Python, Java	O(sqrt(n))	O(1)	Medium	math, `amazon`	AGAIN (not start)
1979	Find Greatest Common Divisor of Array	Python, Java	O(n + log(min))	O(1)	Easy	math, GCD	not start
2013	Detect Squares	Java	add: O(1), count: O(n)	O(n)	Medium	math, hashmap of point counts, iterate the diagonal point	not start
2507	Smallest Value After Replacing With Sum of Prime Factors	Java	O(logn sqrt(n))*	O(1)	Medium	math, LC weekly	again (1)
2614	Prime In Diagonal	Java	O(n sqrt(M))*	O(1)	Easy	math, diagonal matrix, good basic, LC weekly	again**** (1)
2807	Insert Greatest Common Divisors in Linked List	Java	O(n log(maxV))*	O(1)	Medium	math, walk the list and insert gcd nodes, `linked list`, `basic`	OK
2961	Double Modular Exponentiation	Java	O(q logE)*	O(1)	Medium	math, weekly375	AGAIN (1)
3091	Apply Operations to Make Sum of Array Greater Than or Equal to k	Java	O(1)	O(1)	Medium	math, LC weekly	AGAIN (1)
3190	Find Minimum Operations to Make All Elements Divisible by Three	Java	O(n)	O(1)	Easy	math, LC weekly	OK
3194	Minimum Average of Smallest and Largest Elements	Java	O(nlogn)	O(1)	Easy	math, LC weekly	OK
3993	Maximum Value of an Alternating Sequence	Python, Java	O(n)	O(1)	Medium	math, LC weekly, good trick	again**** (1)

Scan Line

#	Title	Solution	Time	Space	Difficulty	Note	Status
1943	Describe the Painting	Java	O(nlogn)	O(n)	Medium	scan line, treeMap, delta, good basic, google	AGAIN***** (1)(MUST)

Sort

#	Title	Solution	Time	Space	Difficulty	Note	Status
75	Sort Colors	Python, Java	O(n)	O(1)	Medium	sort, Tri Partition, `two pointers`, `M$`, `fb`	OK** (4)
88	Merge Sorted Array	Python, Java	O(m + n)	O(1)	Easy	sort, `good basic`, merge, `two pointers`, `M$`, `amazon`, `fb`	AGAIN************* (8) (MUST)
128	Longest Consecutive Sequence	Python, Java	O(n)	O(n)	Medium	sort, Curated Top 75, 2 pointers, sliding window, `O(N)`, `amazon`	AGAIN*********** (6) (MUST)
147	Insertion Sort List	Python, Java	O(n^2)	O(1)	Medium	sort, `trick`	AGAIN* (2)
148	Sort List	Python, Java	O(nlogn)	O(logn)	Medium	sort, `basic`, check `# 21 Merge Two Sorted Lists`	AGAIN** (2)
164	Maximum Gap	Java	O(n)	O(n)	Medium	sort, bucket sort / radix sort, pigeonhole on the gap	OK
179	Largest Number	Python, Java	O(nlogn)	O(n)	Medium	sort, good basic, sort with lambda, `amazon`	AGAIN************ (3)
252	Meeting Rooms	Python, Java	O(nlogn)	O(n)	Easy	sort, Curated Top 75, LC 554, `UBER`, `amazon`, `fb`	OK*** (4) (but again)
253	Meeting Rooms II	Python, Java	O(nlogn)	O(n)	Medium	sort, Curated Top 75, LC 2406, priority queue (min-heap), `scanning line`, `trick`, `booking.com`, `good`, `UBER`, `amazon`, `google`, `fb`	AGAIN************ (5) (MUST)
274	H-Index	Python, Java	O(n)	O(n)	Medium	sort, Counting Sort, `good trick`, `fb`	AGAIN***** (3)
280	Wiggle Sort	Python, Java	O(n)	O(1)	Medium	sort, 🔒, `google`, `fb`	OK*** (3)
324	Wiggle Sort II	Python, Java	O(n) on average	O(1)	Medium	sort, variant of Sort Colors, Tri Partition, `google`	AGAIN** (2)
327	Count of Range Sum	Java	O(nlogn)	O(n)	Hard	sort, merge sort with counting, BIT, segment tree	AGAIN (not start)
347	Top K Frequent Elements	Python, Java	O(n)	O(n)	Medium	sort, Curated Top 75, HashMap, Heap, `Yelp`, `amazon`, `fb`, GOOD basic	AGAIN******** (4)
406	Queue Reconstruction by Height	Python, Java	O(n^2)	O(n)	Medium	sort, `basic`, `trick`, `sort by key`, `insert`, `google`, next_big_val	AGAIN********* (5)
451	Sort Characters By Frequency	Python, Java	O(nlogn)	O(n)	Medium	sort, `collections.Counter(s).most_common`, `sorted(count_dict.items() with lambda`, good basic, `amazon`	OK* (4)
493	Reverse Pairs	Java	O(nlogn)	O(n)	Hard	sort, merge sort with counting, or BIT, `good trick`	AGAIN (not start)
692	Top K Frequent Words	Python, Java	O(n + klogk) on average	O(n)	Medium	sort, `good basic`, Quick Select, Heap, Bucket Sort, `heapq`, `yelp`, `UBER`, `amazon`, `fb`	OK***** (5)
937	Reorder Data in Log Files	Python, Java	O(nlogn l)*	O(l)	Medium	sort, good basic, `string`, `amazon`	OK**** (3)(again!!!)
969	Pancake Sorting	Python, Java	O(n^2)	O(1)	Medium	sort, `fb`	AGAIN********** (4)
973	K Closest Points to Origin	Python, Java	O(n) on average	O(1)	Medium	sort, good trick, sort with func, PQ, Heap, `amazon`	OK***** (3)(but again)
976	Largest Perimeter Triangle	Python, Java	O(nlogn)	O(1)	Easy	sort, sort descending then take the first valid triple, `greedy`, `basic`	OK* (2)
912	Sort an Array	Python, Java	O(nlogn)	O(n)	Medium	sort, merge sort, quick sort, apple, amazon, m$	AGAIN** (1)
1057	Campus Bike	Java	O(m nlog(m * n))*	O(m n)*	Medium	sort, sort all (dist, worker, bike) triples or bucket by distance, google	AGAIN
1066	Campus Bike II	Java	O(m 2^n)*	O(2^n)	Medium	sort, bitmask dp over bikes, min cost assignment, google	AGAIN
1152	Analyze User Website Visit Pattern	Python, Java	O(nlogn)	O(n)	Medium	sort, itertools.combinations, brute force, set, `amazon`	AGAIN***** (2) (not start)
1329	Sort the Matrix Diagonally	Java	O(m nlog(min(m, n)))*	O(m n)*	Medium	sort, group cells by (i - j), sort each diagonal, PQ, hashMap, google	AGAIN (not start)
1877	Minimize Maximum Pair Sum in Array	Java	O(nlogn)	O(1)	Medium	sort, good basic, google	AGAIN*** (1)
2021	Brightest Position on Street	Python, Java	O(nlogn)	O(n)	Medium	sort, LC 253 meeting room II, PQ, prefix, diff array, scanning line, trick, `amazon`	AGAIN*********** (3) (MUST)
2284	Sender With Largest Word Count	Java	O(n)	O(n)	Medium	sort, LC 347, hashmap, sort by (count, name), `good basic`	OK
2345	Finding the Number of Visible Mountains	Java	O(nlogn)	O(n)	Medium	sort, monotonic stack, hashmap, interval cover	AGAIN (not start)
2402	Meeting Rooms III	Java	O(nlogn)	O(n)	Hard	sort, LC 253, two heaps (free rooms / busy rooms), sort by start, `good trick`	AGAIN (not start)
2659	Make Array Empty	Java	O(nlogn)	O(n)	Hard	sort, BIT / sorted index scan, `good trick`, weekly_103	AGAIN (not start)

Two Pointers

#	Title	Solution	Time	Space	Difficulty	Note	Status
019	Remove Nth Node From End of List	Python, Java	O(n)	O(1)	Medium	two pointers, Curated Top 75, good basic, MUST, `linked list`, `fb`	AGAIN********** (4) (MUST)
042	Trapping Rain Water	Python, Java	O(n)	O(1)	Hard	two pointers, stack, prefix-suffix array, brute force, `amazon`, `apple`	AGAIN******** (3)
086	Partition List	Python, Java	O(n)	O(1)	Medium	two pointers, two dummy heads then splice, `linked list`, `good basic`	OK*
141	Linked List Cycle	Python, Java	O(n)	O(1)	Easy	two pointers, Curated Top 75, `basic`, `amazon`, `fb`, linked list, check #142	OK** (5)
142	Linked List Cycle II	Python, Java	O(n)	O(1)	Medium	two pointers, linked list, check #141	AGAIN****** (4) (again!)
143	Reorder List	Python, Java	O(n)	O(1)	Medium	two pointers, Curated Top 75, LC 021, 206, 876, `good trick`, `inverse linkedlist`, `merge linkedlist`, `fb`, amazon	AGAIN************** (6) (MUST)
167	Two Sum II - Input array is sorted	Python, Java	O(n)	O(1)	Medium	two pointers, dict, binary search, `good basic`, `amazon`	OK* (6)
214	Shortest Palindrome	Java	O(n)	O(n)	Hard	two pointers, KMP failure function on s + ‘#’ + reverse(s), `good trick`	AGAIN (not start)
259	3Sum Smaller	Python, Java	O(n^2)	O(1)	Medium	two pointers, 🔒, LintCode, `good trick`, `google`, `fb`, check `#015 3 Sum`, check `#001 two sum`	AGAIN******** (4)
283	Move Zeroes	Python, Java	O(n)	O(1)	Easy	two pointers, `basic`, `trick`, `Shopee`, `fb`	AGAIN********* (6)
344	Reverse String	Python, Java	O(n)	O(1)	Easy	two pointers, swap from both ends in place, `basic`	OK
345	Reverse Vowels of a String	Python, Java	O(n)	O(1)	Easy	two pointers, `google`	OK** (1)
349	Intersection of Two Arrays	Python, Java	O(m + n)	O(min(m, n))	Easy	two pointers, EPI, Hash, Binary Search, `fb`	OK
350	Intersection of Two Arrays II	Python, Java	O(m + n)	O(1)	Easy	two pointers, EPI, Hash, Binary Search, `good basic`, `fb`	OK** (3)
360	Sort Transformed Array	Python, Java	O(n)	O(1)	Medium	two pointers, `trick`, 🔒, `google`, `fb`	AGAIN*** (3)
457	Circular Array Loop	Python, Java	O(n)	O(1)	Medium	two pointers, fast/slow pointers per start, same-direction check, `good trick`	AGAIN (not start)
567	Permutation in String	Python, Java	O(n)	O(1)	Medium	two pointers, sliding window`, counter, hashmap,` good basic`,` M$`,` fb`	AGAIN********* (8)(MUST)
611	Valid Triangle Number	Python, Java	O(n^2)	O(1)	Medium	two pointers, sort then two pointers, count valid triples, `good trick`	AGAIN (not start)
777	Swap Adjacent in LR String	Python, Java	O(n)	O(1)	Medium	two pointers, L only moves left, R only moves right, relative order preserved, `good trick`, google	AGAIN (not start)
826	Most Profit Assigning Work	Python, Java	O(mlogm + nlogn)	O(n)	Medium	two pointers, `trick`, `good basic`, `zip+sorted`	OK*
844	Backspace String Compare	Python, Java	O(m + n)	O(1)	Easy	two pointers, `good basic`, `stack`, `google`, `fb`	OK* (4)
876	Middle of the Linked List	Python, Java	O(n)	O(1)	Easy	two pointers, basic, amazon, apple, google, linkedlist	OK* (2)
904	Fruit Into Baskets	Python, Java	O(n)	O(1)	Medium	two pointers, slide window, google	AGAIN*** (1)
930	Binary Subarrays With Sum	Python, Java	O(n)	O(1)	Medium	two pointers, `trick`	AGAIN* (2) (not start)
977	Squares of a Sorted Array	Python, Java	O(n)	O(1)	Easy	two pointers, two pointers from both ends, LC 88, `basic`	OK
986	Interval List Intersections	Python, Java	O(n)	O(1)	Medium	two pointers, scanning line, good basic, google	AGAIN********* (1) (MUST)
1023	Camelcase Matching	Java	O(n)	O(1)	Medium	two pointers, dp, good basic	AGAIN****** (1)
1055	Shortest Way to Form String	Python, Java	O(n m)*	O(1)	Medium	two pointers, `google`	again
1147	Longest Chunked Palindrome Decomposition	Java	O(n)	O(1)	Hard	two pointers, rolling hash, `google`	again (not start)
1151	Minimum Swaps to Group All 1’s Together	Python, Java	O(n)	O(1)	Medium	two pointers, good basic, Deque (Double-ended Queue), `amazon`	AGAIN***** (not start )
1229	Meeting Scheduler	Java	O(n)	O(1)	Medium	two pointers, sort `amazon`	AGAIN***** (not start )
1768	Merge Strings Alternately	Java	O(m + n)	O(m + n)	Easy	two pointers, merge two strings alternately then append the rest, `basic`, google	OK
1963	Minimum Number of Swaps to Make the String Balanced	Python, Java	O(n)	O(1)	Medium	two pointers, good basic, greedy, stack, amazon, fb, m$	AGAIN******** (2)
3191	Minimum Operations to Make Binary Array Elements Equal to One I	Java	O(n)	O(1)	Medium	two pointers, greedy flip a window of 3, sliding window, LC 995, `good basic`	Again* (2)
3192	Minimum Operations to Make Binary Array Elements Equal to One II	Java	O(n)	O(1)	Medium	two pointers, LC 3191, greedy prefix flip with a running parity counter, `good trick`	Again* (2)

Recursion

#	Title	Solution	Time	Space	Difficulty	Note	Status
10	Regular Expression Matching	Python, Java	O(m n)*	O(m n)*	Hard	recursion, dp, `amazon`	AGAIN (not start)
95	Unique Binary Search Trees II	Python, Java	O(4^n / n^(3/2))	O(4^n / n^(3/2))	Medium	recursion, DP, # BSTs = combinations of left × right, amazon, google, apple	AGAIN***************** (1)(MUST)
98	Validate Binary Search Tree	Python, Java	O(n)	O(h)	Medium	recursion, Curated Top 75, bfs, dfs, `BST`, `M$`, `amazon`, `fb`	AGAIN****************** (11)
100	Same Tree	Python, Java	O(n)	O(h)	Easy	recursion, Curated Top 75, dfs, bfs, `good basic`, `amazon`	OK****** (5) (but again)
104	Maximum Depth of Binary Tree	Python, Java	O(n)	O(h)	Easy	recursion, Curated Top 75, DFS, BFS, good basic, `amazon`, top down, bottom up, LC 111	AGAIN************ (5)(MUST)
105	Construct Binary Tree from Preorder and Inorder Traversal	Python, Java	O(n)	O(n)	Medium	recursion, Curated Top 75, `trick`, check # 106, `BST`, `M$`, `fb`	AGAIN********* (9)
106	Construct Binary Tree from Inorder and Postorder Traversal	Python, Java	O(n)	O(n)	Medium	recursion, `trick`, check # 105, `BST`, `M$`, `fb`	AGAIN******* (6)
108	Convert Sorted Array to Binary Search Tree	Python, Java	O(n)	O(logn)	Easy	recursion, `good basic`, BST, build tree from sorted array, DFS, LC 109	AGAIN************* (2) (MUST)
109	Convert Sorted List to Binary Search Tree	Python, Java	O(n)	O(logn)	Medium	recursion, `list`, `BST`, `good concept`, `fb`	AGAIN*** (3)
110	Balanced Binary Tree	Python, Java	O(n)	O(h)	Easy	recursion, `good trick`, LC 104, `dfs`, bfs, `amazon`, `fb`, google, top-down, bottom-up recursion	AGAIN************** (8)(MUST)
111	Minimum Depth of Binary Tree	Python, Java	O(n)	O(h)	Easy	recursion, `good basic`, `dfs`, `BST`, `amazon`, `fb`, LC 104	AGAIN******** (5)
114	Flatten Binary Tree to Linked List	Python, Java	O(n)	O(h)	Medium	recursion, path, `dfs`, `M$`, `fb`	AGAIN********* (4)(MUST)
116	Populating Next Right Pointers in Each Node	Python, Java	O(n)	O(1)	Medium	recursion, good basic, bfs, dfs, tree, AGAIN, `fb`, amazon	AGAIN************ (6)
117	Populating Next Right Pointers in Each Node II	Python, Java	O(n)	O(h)	Medium	recursion, good basic, prev node, `Populating Next Right Pointers in Each Node I`, bfs, linked list, `amazon`, `fb`, google	AGAIN************* (4)
129	Sum Root to Leaf Numbers	Python, Java	O(n)	O(h)	Medium	recursion, path sum, dfs, good basic, `fb`	AGAIN******* (5)
156	Binary Tree Upside Down	Python, Java	O(n)	O(1)	Medium	recursion, 🔒	AGAIN (not start)
241	Different Ways to Add Parentheses	Python, Java	O(n 4^n / n^(3/2))*	O(n 4^n / n^(3/2))*	Medium	recursion, complex, dp, Memoization, google	AGAIN (not start)
298	Binary Tree Longest Consecutive Sequence	Python, Java	O(n)	O(h)	Medium	recursion, LC 437, good basic, prev node, bfs, dfs, tree, 🔒, `google`	AGAIN *************** (6) (MUST)
333	Largest BST Subtree	Python, Java	O(n)	O(h)	Medium	recursion, 🔒, dfs, post-order, return (isBST, min, max, size), LC 98	AGAIN (not start)
337	House Robber III	Python, Java	O(n)	O(h)	Medium	recursion, good trick, dp + dfs, tree DP, rob/not-rob state, LC 198, amazon, google	AGAIN********** (3)(MUST)
395	Longest Substring with At Least K Repeating Characters	Python, Java	O(26 n)*	O(1)	Medium	recursion, slide window, `DIVIDE AND CONQUER`, `good trick`, `fb`, google	AGAIN************ (4)
404	Sum of Left Leaves	Python, Java	O(n)	O(h)	Easy	recursion, tree, bfs, dfs, amazon	AGAIN**** (4)
437	Path Sum III	Python, Java	O(n)	O(h)	Medium	recursion, LC 298, prefix sum, dfs, path sum, `good trick`, `UBER`, `AMAZON`, `fb`	AGAIN*********** (5) (MUST)
501	Find Mode in Binary Search Tree	Java	O(n)	O(h)	Medium	recursion, dfs, bfs, LC 98	OK (1)
544	Output Contest Matches	Python, Java	O(nlogn)	O(n)	Medium	recursion, 🔒, recursively pair first with last, string build, `good trick`	AGAIN
549	Binary Tree Longest Consecutive Sequence II	Python, Java	O(n)	O(h)	Medium	recursion, 🔒	AGAIN (not start)
669	Trim a Binary Search Tree	Python, Java	O(n)	O(h)	Medium	recursion, good basic, BST trim, dfs, `amazon`	AGAIN*********** (6)(MUST)
671	Second Minimum Node In a Binary Tree	Python, Java	O(n)	O(h)	Easy	recursion, dfs, tree	OK* (2)
753	Cracking the Safes	Java	O(k^n n)*	O(k^n n)*	Hard	recursion, de Bruijn sequence, Hierholzer / dfs over states, google	AGAIN (not start)
1145	Binary Tree Coloring Game	Java	O(n)	O(h)	Medium	recursion, tree, color, google	AGAIN***** (2)

Binary Search

#	Title	Solution	Time	Space	Difficulty	Note	Status
33	Search in Rotated Sorted Array	Python, Java	O(logn)	O(1)	Medium	binary search, Curated Top 75, LC 153, 852, `good basic`, check `# 81 Search in Rotated Sorted Array II`, rotation array, `UBER`, `amazon`, `fb`	AGAIN************ (6) (MUST)
034	Find First and Last Position of Element in Sorted Array	Python, Java	O(logn)	O(1)	Medium	binary search, LC top 100 like, `amazon`, `fb`, google, apple, uber	AGAIN************** (4) (MUST)
034	Search for a Range	Python, Java	O(logn)	O(1)	Medium	binary search, duplicate of LC 34, lower bound + upper bound binary search	AGAIN*
35	Search Insert Position	Python, Java	O(logn)	O(1)	Medium	binary search, binary search on left, good basic	AGAIN************ (2) (MUST)
069	Sqrt(x)	Python, Java	O(logn)	O(1)	Medium	binary search, math, `amazon`, `fb`	OK* (4)
074	Search a 2D Matrix	Python, Java	O(logm + logn)	O(1)	Medium	binary search, binary search in 2D array, flatten matrix, `amazon`	OK**** (BUT AGAIN)(3)
081	Search in Rotated Sorted Array II	Python, Java	O(logn)	O(1)	Medium	binary search, check `# 33 Search in Rotated Sorted Array` first, `array`, `fb`, amazon, linkedin	OK***** (6)
153	Find Minimum in Rotated Sorted Array	Python, Java	O(logn)	O(1)	Medium	binary search, Curated Top 75, LC 33, ascending array, good basic, `amazon`	AGAIN**************** (10)(MUST)
154	Find Minimum in Rotated Sorted Array II	Java	O(logn)	O(1)	Medium	binary search, LC 153	AGAIN (not start)
162	Find Peak Element	Python, Java	O(logn)	O(1)	Medium	binary search, good trick, recursive, iterative binary search, `M$`, `google`, `fb`	AGAIN********** (6)
222	Count Complete Tree Nodes	Python, Java	O((logn)^2)	O(logn)	Easy	binary search, google, tree	OK (again)
275	H-Index II	Python, Java	O(logn)	O(1)	Medium	binary search, similar as `# 274 H-Index`, `fb`	AGAIN**** (3)
278	First Bad Version	Python, Java	O(logn)	O(1)	Easy	binary search, `good basic`, LintCode, `fb`	OK*** (5) (MUST)
300	Longest Increasing Subsequence	Python, Java	O(nlogn)	O(n)	Medium	binary search, Curated Top 75, Patience Sorting, `DP good basic`, LintCode, 1D DP, LIS, `amazon`, `fb`	AGAIN******************* (13) (MUST)
315	Count of Smaller Numbers After Self	Python, Java	O(nlogn)	O(n)	Hard	binary search, BST, BIT, `google`	again*
367	Valid Perfect Square	Python, Java	O(logn)	O(1)	Easy	binary search, good basic, similar as `# 69 Sqrt(x)`	OK* (3)
374	Guess Number Higher or Lower	Python, Java	O(logn)	O(1)	Easy	binary search, classic binary search on the guess API, `good basic`	OK*
410	Split Array Largest Sum	Python, Java	O(nlog(sum))	O(1)	Hard	binary search, google, LC 1231, 1552	AGAIN (not start)
436	Find Right Interval	Python, Java	O(nlogn)	O(n)	Medium	binary search, good basic	AGAIN****** (2) (MUST)
475	Heaters	Python, Java	O((m + n) logn)*	O(1)	Easy	binary search, sort houses and heaters, binary search the nearest heater, two pointers	AGAIN (not start)
540	Single Element in a Sorted Array	Python, Java	O(logn)	O(1)	Medium	binary search, binary search on even indices, pair-XOR check, `good trick`	OK*
564	Find the Closest Palindrome	Java	O(n)	O(n)	Hard	binary search, n = number of digits, build 5 palindrome candidates around the prefix, google	AGAIN(not start)
658	Find K Closest Elements	Python, Java	O(logn + k)	O(1)	Medium	binary search, `good trick`, PQ, binary search find window start point, `amazon`, `fb`, google	AGAIN ********************** (9) (MUST)
744	Find Smallest Letter Greater Than Target	Python, Java	O(logn)	O(1)	Easy	binary search, upper bound binary search with wraparound, `basic`	OK*
774	Minimize Max Distance to Gas Station	Python, Java	O(nlog(maxD / eps))	O(1)	Hard	binary search, binary search on a real-valued answer, PQ, google	AGAIN***** (1)
852	Peak Index in a Mountain Array	Python, Java	O(logn)	O(1)	Medium	binary search, LC 162, LC 33, `amazon`, google	AGAIN****** (2)
875	Koko Eating Bananas	Python, Java	O(nlogr)	O(1)	Medium	binary search, left boundary, good basic	AGAIN******* (4)(MUST)
894	All Possible Full Binary Trees	Python, Java	O(n 4^n / n^(3/2))*	O(n 4^n / n^(3/2))*	Medium	binary search, dfs + memoization, Catalan number, only odd n has a full binary tree	AGAIN (not start)
911	Online Election	Python, Java	ctor: O(n) query : O(logn)	O(n)	Medium	binary search, prefix leader array + binary search on the query time, `good basic`	AGAIN (not start)
981	Time Based Key-Value Store	Python, Java	set: O(1), get: O(logn)	O(n)	Medium	binary search, dict, treeMap, floorKey, sort, apple, M$, amz, google	AGAIN****** (2)
1011	Capacity To Ship Packages Within D Days	Python, Java	O(nlog(sum))	O(1)	Medium	binary search, good trick, amazon, apple, fb, m$, google	AGAIN****** (3)
1060	Missing Element in Sorted Array	Java	O(logn)	O(1)	Medium	binary search, brute force, good trick, google	AGAIN**** (2)
1095	Find in Mountain Array	Java	O(logn)	O(1)	Hard	binary search, LC 852 find the peak then binary search each side, MountainArray API, `good trick`	AGAIN (not start)s
1150	Check If a Number Is Majority Element in a Sorted Array	Java	O(logn)	O(1)	Easy	binary search, lower + upper bound, count occurrences	AGAIN**** (1)
1292	Maximum Side Length of a Square with Sum Less than or Equal to Threshold	Python, Java	O(m nlog(min(m, n)))*	O(m n)*	Medium	binary search, 2D prefix-sum, good trick, google	AGAIN******* (2)
1231	Divide Chocolate	Java	O(nlog(sum))	O(1)	Hard	binary search, google, LC 2616, Maximize Minimum, LC 410	AGAIN***** (not start)
1283	Find the Smallest Divisor Given a Threshold	Java	O(nlog(max))	O(1)	Medium	binary search, binary search on the divisor, LC 875, `good basic`	AGAIN***** (not start)
1482	Minimum Number of Days to Make m Bouquets	Java	O(nlog(max))	O(1)	Medium	binary search, binary search on the day, greedily count adjacent runs, `good basic`	AGAIN (not start)
1552	Magnetic Force Between Two Balls	Java	O(nlogn + nlog(max))	O(1)	Medium	binary search, LC 410, can distribute	AGAIN (1)
1760	Minimum Limit of Balls in a Bag	Java	O(nlog(max))	O(1)	Medium	binary search, binary search on the penalty, ceil(count / limit) - 1 operations, `good trick`	AGAIN (1)
1889	Minimum Space Wasted From Packaging	Python, Java	O((m + n) logn)*	O(n)	Hard	binary search, heap, prefix sum, amazon	AGAIN (not start)
1891	Cutting Ribbons	Java	O(nlog(max))	O(1)	Medium	binary search, 🔒, binary search on the ribbon length, `good basic`	AGAIN (not start)
2009	Minimum Number of Operations to Make Array Continuous	Python, Java	O(nlogn)	O(n)	Hard	binary search, sliding window, dequeue, bisect.bisect_right, amazon	AGAIN (not start)
2554	Maximum Number of Integers to Choose From a Range I	Java	O(n)	O(n)	Medium	binary search, hash set of banned values, greedily take the smallest, weekly 97	AGAIN (not start)
2594	Minimum Time to Repair Cars	Java	O(nlog(maxT))	O(1)	Medium	binary search, PQ, trick, weekly 100	AGAIN***** (1)
2616	Minimize the Maximum Difference of Pairs	Java	O(nlogn + nlog(max))	O(1)	Medium	binary search, LC weekly, good trick, Minimize the Maximum, LC 1231	AGAIN******* (1)
4008	Minimum Initial Strength to Defeat All Monsters	Python, Java	O(nlogM)	O(1)	Medium	binary search, LC weekly, good trick	AGAIN* (1)

Binary Search Tree

#	Title	Solution	Time	Space	Difficulty	Note	Status
99	Recover Binary Search Tree	Python, Java	O(n)	O(h)	Medium	BST, good trick, BST swap, DFS, inorder, Morris traversal	AGAIN******************** (5)(MUST)
220	Contains Duplicate III	Python, Java	O(nlogk)	O(k)	Medium	BST, LC 219, sliding window with TreeSet or bucketing, `good trick`	AGAIN (not start)
230	Kth Smallest Element in a BST	Python, Java	O(max(h, k))	O(min(h, k))	Medium	BST, Curated Top 75, good basic BST basic, DFS, BFS, STACK, `amazon`	OK********* (4)
235	Lowest Common Ancestor of a Binary Search Tree	Python, Java	O(h)	O(1)	Medium	BST, Curated Top 75, recursion, iteration, good basic, `LCA`, check `# 236 Lowest Common Ancestor of a Binary Tree`, `amazon`, `fb`	OK**************** (but again)(10)
270	Closest Binary Search Tree Value	Python, Java	O(h)	O(1)	Easy	BST, `good basic`, 🔒, `M$`, `google`, `fb`	OK***** (5)
272	Closest Binary Search Tree Value II	Java	O(h + k)	O(h + k)	Hard	BST, google, amazon, meta, closest val, binary search	AGAIN (1)
285	Inorder Successor in BST	Python, Java	O(h)	O(1)	Medium	BST, dfs, good basic, 🔒, `amazon`, `fb`	OK** (4)
449	Serialize and Deserialize BST	Python, Java	O(n)	O(h)	Medium	BST, LC 297, dfs, bfs, good trick, `serizlize deserizlize`, tree, `amazon`, `fb`	AGAIN************* (9) (MUST)
450	Delete Node in a BST	Python, Java	O(h)	O(h)	Medium	BST, good trick, tree, dfs, LC 1325	AGAIN***************** (9) (MUST !!!)
510	Inorder Successor in BST II	Java	O(h)	O(1)	Medium	BST, parent	AGAIN** (1) (not start)
530	Minimum Absolute Difference in BST	Python, Java	O(n)	O(h)	Easy	BST, LC 783	AGAIN*
700	Search in a Binary Search Tree	Python, Java	O(h)	O(h)	Easy	BST, good basic, recursion, iteration amazon	AGAIN*** (1)
776	Split BST	Python, Java	O(h)	O(h)	Medium	BST, 🔒, recursive, iterative, DFS, split, trick, AGAIN, `amazon`, google	AGAIN************ (9)(MUST)
783	Minimum Distance Between BST Nodes	Python, Java	O(n)	O(h)	Easy	BST, LC 530	OK*
426	Convert Binary Search Tree to Sorted Doubly Linked List	Python, Java	O(n)	O(h)	Medium	BST, recursion, tree, `linked list`, `good basic` `UBER`, `lyft`, `amazon`, `google`, `fb`	AGAIN******* (5) (not start)
968	Binary Tree Cameras	Python, Java	O(n)	O(h)	Hard	BST, node state, dfs, bottom up, post order, dp, greedy, m$, amz	AGAIN*********** (2)(MUST)
1022	Sum of Root To Leaf Binary Numbers	Python, Java	O(n)	O(h)	Easy	BST, good basic, LC 257, bst path, dfs, bfs, `amazon`	AGAIN******** (1) (MUST)
1382	Balance a Binary Search Tree	Python, Java	O(n)	O(n)	Medium	BST, LC 105, 106, inorder then rebuild, dfs, good trick, AVL tree	AGAIN******** (2)
1597	Build Binary Expression Tree From Infix Expression	Python, Java	O(n)	O(n)	Hard	BST, LC 224, recursive, iteration, stack, `amazon`, google	AGAIN*** (1)
1644	Lowest Common Ancestor of a Binary Search Tree 2	Java	O(h)	O(1)	Medium	BST, LC 235, 236, dfs	AGAIN (1)

Breadth-First Search

#	Title	Solution	Time	Space	Difficulty	Note	Status
102	Binary Tree Level Order Traversal	Python, Java	O(n)	O(n)	Medium	bfs, Curated Top 75, `GOOD basic`, `dfs`, `UBER`, `apple`, `amazon`, `fb`, LC 199	AGAIN******* (8) (must)
107	Binary Tree Level Order Traversal II	Python, Java	O(n)	O(n)	Medium	bfs, same as `# 102 Binary Tree Level Order Traversal`	OK* (3)
103	Binary Tree Zigzag Level Order Traversal	Python, Java	O(n)	O(n)	Medium	bfs, `amazon`, `fb`	OK
126	Word Ladder II	Python, Java	O(n l^2)*	O(n l)*	Hard	bfs, complex, trick, dfs, dfs+bfs, check `# 127 Word Ladder`, `amazon`	AGAIN*** (3) (not start)
127	Word Ladder	Python, Java	O(n d)*	O(d)	Hard	bfs, good basic, set, LC 126 Word Ladder II, `UBER`, `amazon`, `M$`, `fb`	AGAIN**************** (12)(MUST)
130	Surrounded Regions	Python, Java	O(m n)*	O(m n)*	Medium	bfs, `dfs`, `union find`, good basic, `amazon`	AGAIN*********** (5)
133	Clone Graph	Python, Java	O(n)	O(n)	Medium	bfs, Curated Top 75, good trick, LC 138, `graph`, `dfs`, `UBER`, `google`, `amazon`, `fb`	AGAIN**************** (11) (MUST)
207	Course Schedule	Python, Java	O(V + E)	O(E)	Medium	bfs, Curated Top 75, Topological Sort, LC 210, `good trick`, `dfs`, `amazon`, `fb`	AGAIN**************** (15) (MUST)
210	Course Schedule II	Python, Java	O(V + E)	O(E)	Medium	bfs, Topological Sort, LC 207, dfs, `amazon`, `fb`	AGAIN********* (10) (again)
261	Graph Valid Tree	Python, Java	O(V + E)	O(V + E)	Medium	bfs, Curated Top 75, `union find`, dfs, grpah, 🔒, `graph`, `google`, `amazon`, `fb`	AGAIN************* (12)(MUST)
286	Walls and Gates	Python, Java	O(m n)*	O(g)	Medium	bfs, 🔒, `dfs`, `good basic`, `google`, `amazon`, `FB`	AGAIN******** (8)
310	Minimum Height Trees	Python, Java	O(n)	O(n)	Medium	bfs, topo sort, Kahn algo, google	AGAIN**** (4)
317	Shortest Distance from All Buildings	Java	O(m^2 n^2)*	O(m n)*	Hard	bfs, multi-source BFS, total Path, cnt matrix, good trick, google	AGAIN*********** (1)(must)
407	Trapping Rain Water II	Java	O(m nlog(m * n))*	O(m n)*	Hard	bfs, PQ	AGAIN (not start)
429	N-ary Tree Level Order Traversal	Java	O(n)	O(n)	Medium	bfs, PQ, google	AGAIN*** (1)
433	Minimum Genetic Mutation	Python, Java	O(n b)*	O(b)	Medium	bfs, check `# 127 Word Ladder`, `good basic`	AGAIN*** (3)
444	Sequence Reconstruction	Python, Java	O(n + m)	O(n)	Medium	bfs, good trick, `Topological Sort`, `google`, `airbnb`	AGAIN******* (4) (not start)
499	The Maze III	Java	O(m nlog(m * n))*	O(m n)*	Medium	bfs, Dijkstra with lexicographically smallest path, roll until a wall, `good trick`, `google`	AGAIN (not start)
490	The Maze	Python, Java	O(max(r, c) w)*	O(w)	Medium	bfs, `basic`, `dfs`, `amazon`, `fb`	AGAIN**** (5)
505	The Maze II	Python, Java	O(max(r, c) wlogw)*	O(w)	Medium	bfs, Dijkstra, `google`, `fb`	AGAIN********* (7)(MUST)
542	01 Matrix	Python, Java	O(m n)*	O(m n)*	Medium	bfs, multi-source BFS, BFS get min dist, google, LC 994, 1765	AGAIN************* (MUST) (4)
666	Path Sum IV	Python, Java	O(n)	O(n)	Medium	bfs, 🔒, encode (depth, position) into a hashmap then dfs, tree	AGAIN* (3) (not start)
742	Closest Leaf in a Binary Tree	Python, Java	O(n)	O(n)	Medium	bfs, Graph, bfs+dfs, search, `good trick`, `amazon`	AGAIN********** (6)(MUST)
743	Network Delay Time	Python, Java	O(E logV)*	O(V + E)	Medium	bfs, Dijkstra, Bellman-Ford, shortest path from one source, `google`	AGAIN **** (3)
752	Open the Lock	Python, Java	O(k n^k + d)*	O(k n^k + d)*	Medium	bfs, LC 863, google	AGAIN (not start)
787	Cheapest Flights Within K Stops	Python, Java	O(E logV)*	O(E)	Medium	bfs, `Dijkstra's algorithm`, dfs, graph, priority queue, `amazon`, apple, google, airbnb	AGAIN****** (4)
815	Bus Routes	Python, Java	O(V + E)	O(V + E)	Hard	bfs, shortest route, graph, `amazon`, google, good trick	AGAIN****** (3)
886	Possible Bipartition	Python, Java	O(V + E)	O(V + E)	Medium	bfs, LC 785, `graph`, dfs, `union find`, `fb`	AGAIN********** (6)
934	Shortest Bridge	Python, Java	O(n^2)	O(n^2)	Medium	bfs, BFS+DFS, multi-src BFS, good trick, google	AGAIN************* (2)
967	Numbers With Same Consecutive Differences	Python, Java	O(2^n)	O(2^n)	Medium	bfs, `good trick`	AGAIN** (3)
675	Cut Off Trees for Golf Event	Python, Java	O((m n)^2)*	O(m n)*	Hard	bfs, good trick, tree, mult-source BFS, shortest path, `amazon`	AGAIN**************** (3)(must)
864	Shortest Path to Get All Keys	Python, Java	O(m n * 2^k)*	O(m n * 2^k)*	Hard	bfs, Dijkstra, Permutations, `amazon`	AGAIN (not start)
1091	Shortest Path in Binary Matrix	Java	O(n^2)	O(n^2)	Medium	bfs, `google`	AGAIN (not start)
994	Rotting Oranges	Python, Java	O(m n)*	O(m n)*	Medium	bfs, dp `amazon`, good trick, LC 542	AGAIN********* (MUST) (2)
1110	Delete Nodes And Return Forest	Python, Java	O(n)	O(n)	Medium	bfs, pre+post order DFS, 2 states, prune, tree, `google`	AGAIN************ (4)(MUST)
1162	As Far from Land as Possible	Python, Java	O(n^2)	O(n^2)	Medium	bfs, muti-source bfs, good basic, `amazon`	AGAIN********* (3)(MUST)
1197	Minimum Knight Moves	Java	O(n^2)	O(n^2)	Medium	bfs, `google`	OK (1)
1530	Number of Good Leaf Nodes Pairs	Python, Java	O(n d^2)*	O(n)	Medium	BFS, DFS, node pair, graph, good trick	AGAIN************ (2)(MUST)
1730	Shortest Path to Get Food	Python, Java	O(m n)*	O(m n)*	Medium	bfs, shortest path, `amazon`	OK (2)
1765	Map of Highest Peak	Java	O(m n)*	O(m n)*	Medium	bfs, dp, LC 542, google	again (1)(not start)
1905	Count Sub Islands	Python, Java	O(m n)*	O(m n)*	Medium	bfs, dfs	AGAIN (1)
2115	Find All Possible Recipes from Given Supplies	Java	O(n + e)	O(n + e)	Medium	bfs, dfs, topological sort, `google`	OK (2)
2684	Maximum Number of Moves in a Grid	Java	O(m n)*	O(m n)*	Medium	bfs, top-down dp, bottom-up dp, LC weekly 345	again (1)

Depth-First Search

#	Title	Solution	Time	Space	Difficulty	Note	Status
112	Path Sum	Python, Scala, Java	O(n)	O(h)	Easy	dfs, `good concept`, `amazon`, recursion	AGAIN** (2)
113	Path Sum II	Python, Java	O(n)	O(h)	Medium	dfs, backtrack, LC 112, `good basic`, `amazon`, `fb`	AGAIN************ (8)(MUST)
199	Binary Tree Right Side View	Python, Java	O(n)	O(h)	Medium	dfs, `good basic`, `bfs`, `apple`, `amazon`, `fb`, LC 102	OK*** (6)
200	Number of Islands	Python, Java	O(m n)*	O(m n)*	Medium	dfs, Curated Top 75, `bfs`, `good basic`, check `# 694, 711 Number of Distinct Islands`, `apple`, `Goldman Sachs`, `google`, `amazon`, `fb`, LC 305	OK****** (6)
236	Lowest Common Ancestor of a Binary Tree	Python, Java	O(n)	O(h)	Medium	dfs, `trick`, EPI, `LCA`, LC 1644, 235, `apple`, `amazon`, `linkedin`, `fb`	AGAIN************* (9)
247	Strobogrammatic Number II	Python, Java	O(n^2 5^(n/2))*	O(n)	Medium	dfs, 🔒, check `#246 Strobogrammatic Number`, `good trick`, `google`, `fb`	AGAIN******** (5)
257	Binary Tree Paths	Python, Java	O(n h)*	O(h)	Easy	dfs, good basic, `apple`, `google`, `amazon`, `fb`, top down, bottom up, LC 112, 113	AGAIN************ (MUST) (7)
332	Reconstruct Itinerary	Python, Java	O(E logE)*	O(E)	Hard	dfs, Hierholzer algo, `yelp`, `google`	AGAIN* (3) (not start)
339	Nested List Weight Sum	Python, Java	O(n)	O(h)	Easy	dfs, 🔒, `good basic`, `bfs`, `linkedin`, `fb`	AGAIN*** (3)
364	Nested List Weight Sum II	Python, Java	O(n)	O(h)	Medium	dfs, 🔒, `linkedin`	AGAIN*
366	Find Leaves of Binary Tree	Python, Java	O(n)	O(h)	Medium	dfs, 🔒, `linkedin`	AGAIN*
399	Evaluate Division	Python, Java	O(q (V + E))*	O(V + E)	Medium	dfs, `union find`, `good trick`, `union find` `google`, `fb`	AGAIN************** (9)(MUST)
417	Pacific Atlantic Water Flow	Python, Java	O(m n)*	O(m n)*	Medium	dfs, Curated Top 75, 2 direction dfs, matrix, `m$`, `google`	AGAIN** (3)
464	Can I Win	Python, Java	O(n 2^n)*	O(2^n)	Medium	dfs, `DP`, `linkedin`	AGAIN* (not start)
491	Increasing Subsequences	Python, Java	O(n 2^n)*	O(n 2^n)*	Medium	dfs, `trick`, `dfs good trick`, `dp`, `yahoo`, `fb`	AGAIN***** (2)
515	Find Largest Value in Each Tree Row	Python, Java	O(n)	O(h)	Medium	dfs, `good basic`, `linkedin`	AGAIN* (3)
529	Minesweeper	Python, Java	O(m n)*	O(m n)*	Medium	dfs, bfs, recursive update, `amazon`, google	AGAIN******** (6)
547	Friend Circles	Python, Java	O(n^2)	O(n)	Medium	dfs, graph, Union Find, bfs, check `# 733 Flood Fill`, `bloomberg`, `fb`	AGAIN*** (3) (not start)
547	Number of Provinces	Python, Java	O(n^2)	O(n)	Medium	dfs, bfs, graph, union find, good basic, apple, amazon	AGAIN************ (2) (MUST)
582	Kill Process	Python, Java	O(n)	O(n)	Medium	dfs, 🔒, BFS, `good basic`, `google`, `amazon`	AGAIN*********** (5) (MUST)
638	Shopping Offers	Python, Java	O(n 2^n)*	O(n)	Medium	dfs, `google`	AGAIN (not start*)
690	Employee Importance	Python, Java	O(n)	O(h)	Easy	dfs, BFS, `good basic`, `UBER`	OK** (3)
694	Number of Distinct Islands	Python, Java	O(m n)*	O(m n)*	Medium	dfs, LC200, LC711, path signature, good pattern, 🔒, `amazon`	AGAIN************* (7) (MUST)
695	Max Area of Island	Python, Java	O(m n)*	O(m n)*	Medium	dfs, primitives in java, `amazon`, `microsoft`, `linkedin`, `basic`	AGAIN* (3)
711	Number of Distinct Islands II	Python, Java	O(m n * logn)*	O(m n)*	Hard	dfs, complex, check `# 200, 694 Number of Distinct Islands`, `amazon`	OK*** (3)
721	Accounts Merge	Python , Java	O(nlogn)	O(n)	Medium	dfs, Disjoint Set Union (DSU), `Union Find`, `path compression`, `complex`, `fb`, google, amazon, m$, apple, twitter	AGAIN******* (4) (not start)
733	Flood Fill	Python, Java	O(m n)*	O(m n)*	Easy	dfs, `fb`, `amazon`, `good basic`	OK**** (5)
737	Sentence Similarity II	Python, Java	O(n + p)	O(p)	Medium	dfs, good basic, map, union find, dis-joint, `google`	AGAIN************* (4)(MUST)
756	Pyramid Transition Matrix	Python, Java	O(a^b)	O(a^b)	Medium	dfs, dfs over the allowed triples with memo, hashmap of (a, b) -> tops	AGAIN (3) (not start)
785	Is Graph Bipartite?	Python, Java	O(V + E)	O(V)	Medium	dfs, LC 886, color, `graph`, bfs, `fb`, `amazon`	AGAIN******************* (8)
797	All Paths From Source to Target	Python, Java	O(n 2^n)*	O(n 2^n)*	Medium	dfs, `graph`, `dfs classics`, `good basic`	AGAIN** (3)
802	Find Eventual Safe States	Python, Java	O(V + E)	O(V)	Medium	dfs, `Topological Sort`, graph, google	AGAIN********** (5)
827	Making A Large Island	Java	O(m n)*	O(m n)*	Hard	dfs, union find(DSU), 2 pass DFS/BFS, complex	AGAIN**** (1)
841	Keys and Rooms	Python, Java	O(n + e)	O(n)	Medium	dfs, `google`, `bfs`, `good basic`	AGAIN*** (3)
851	Loud and Rich	Python, Java	O(n + e)	O(n + e)	Medium	dfs, `amazon`, `defaultdict`, `good basic`	AGAIN*** (3)
987	Vertical Order Traversal of a Binary Tree	Java	O(nlogn)	O(n)	Hard	dfs, LC 314, bfs, tree map	AGAIN (not start)
988	Smallest String Starting From Leaf	Java	O(n h)*	O(h)	Medium	dfs, bfs	AGAIN (not start)
990	Satisfiability of Equality Equations	Python, Java	O(n a(n))*	O(1)	Medium	dfs, union find	AGAIN (not start)
1020	Number of Enclaves	Java	O(m n)*	O(m n)*	Medium	dfs, flood fill from the border then count what remains, LC 1254, `good basic`	AGAIN (1)
1087	Brace Explansion	Python, Java	O(n p)*	O(n p)*	Medium	DFS, bfs, prefix sum, backtrack, good trick, Cartesian product, google	AGAIN************** (3)(MUST)
1192	Critical Connections in a Network	Python, Java	O(n + e)	O(n + e)	Hard	dfs, Tarjan’s algorithm, graph, fb, amazon	AGAIN** (1) (not start)
1254	Number of Closed Islands	Java	O(m n)*	O(m n)*	Medium	dfs, bfs, flood, good trick, 2 pass approach	AGAIN (1)**********
1466	Reorder Routes to Make All Paths Lead to the City Zero	Java	O(n)	O(n)	Medium	dfs, bfs	AGAIN (1)
1644	Lowest Common Ancestor of a Binary Tree II	Java	O(n)	O(h)	Medium	dfs, LCA, LC 236, m$, fb	AGAIN (not start)
1650	Lowest Common Ancestor of a Binary Tree III	Python, Java	O(h)	O(h)	Medium	dfs, `parent node`, good trick, set, dict, LCA, LC 236, amazon, google, spotify m$, fb	AGAIN******** (2) (MUST)
1676	Lowest Common Ancestor of a Binary Tree IV	Python, Java	O(n)	O(h)	Medium	dfs, LCA, LC 236, a m$	AGAIN (not start)
2049	Count Nodes With the Highest Score	Python, Java	O(n)	O(n)	Medium	dfs, trick, graph, tree, google	AGAIN**** (not start)
2316	Count Unreachable Pairs of Nodes in an Undirected Graph	Python, Java	O(n + e)	O(n + e)	Medium	dfs, bfs, union find, good trick	AGAIN******** (1)
2658	Maximum Number of Fish in a Grid	Java	O(m n)*	O(m n)*	Medium	dfs, bfs, weekly_103	AGAIN (1)
2685	Count the Number of Complete Components	Java	O(n + e)	O(n + e)	Medium	dfs, good trick, bfs, union find, LC weekly 345	AGAIN*** (2)
2872	Maximum Number of K-Divisible Components	Java	O(n)	O(n)	Hard	dfs, BFS, topological sort, good trick, post order	AGAIN*** (2)
3319	K-th Largest Perfect Subtree Size in Binary Tree	Java	O(n)	O(n)	Medium	dfs, good trick, binary-tree, post order	AGAIN*** (2)
3965	Finish Time of Tasks I	Python, Java	O(nlogn)	O(n)	Medium	dfs, max_small_val, LC bi-weekly	AGAIN******** (4)(MUST)
3997	Count Dominant Nodes in a Binary Tree	Python, Java	O(n)	O(h)	Medium	dfs, LC weekly	AGAIN*** (1)
4015	Weighted Sum of a Tree	Python, Java	O(n)	O(h)	Medium	dfs, LC weekly	AGAIN*** (1)

Backtracking

#	Title	Solution	Time	Space	Difficulty	Note	Status
017	Letter Combinations of a Phone Number	Python, Java	O(n 4^n)*	O(n)	Medium	backtracking, dfs, `good trick`, `UBER`, `amazon`, `google`, `fb`	AGAIN************* (11) (MUST)
022	Generate Parentheses	Python, Java	O(4^n / n^(3/2))	O(n)	Medium	backtracking, LC 20, good basic, stack, backtrac, Divide and Conquer, `amazon`	OK**************** (5) (but again, MUST)
037	Sudoku Solver	Python, Java	O(9^m)	O(m)	Hard	backtracking, backtrack over the m empty cells, row/col/box bitsets, google, amazon, uber	AGAIN(not start)
039	Combination Sum	Python, Java	O(k n^k)*	O(k)	Medium	backtracking, `dfs`, `good basic`, `UBER`, `airbnb`, `amazon`, `fb`	OK********* (7) (but again)
040	Combination Sum II	Python, Java	O(k C(n, k))*	O(k)	Medium	backtracking, LC 39, avoud duplicated	AGAIN******* (4) (MUST)
046	Permutations	Python, Java	O(n n!)*	O(n)	Medium	backtracking, LC #77, 78, `good concept`, `M$`, `linkedin`, `amazon`, `fb`	OK*********** (4) (AGAIN, MUST)
047	Permutations II	Python, Java	O(n n!)*	O(n)	Medium	backtracking, LC 46, avoid same layer duplicated, apple, amazon, m$	AGAIN***** (MUST)
051	N-Queens	Java	O(n!)	O(n^2)	Hard	backtracking, matrix	AGAIN
052	N-Queens II	Python, Java	O(n!)	O(n)	Hard	backtracking, matrix	AGAIN
077	Combinations	Python, Java	O(k C(n, k))*	O(k)	Medium	backtracking, good trick	OK************* (5) (but again, MUST)
079	Word Search	Python, Java	O(m n * 4^l)*	O(l)	Medium	backtracking, LC 212, Curated Top 75, `good trick`, `dfs`, `bloomberg`, `M$`, `amazon`, `fb`	AGAIN*************** (10) (MUST)
093	Restore IP Addresses	Python, Java	O(1)	O(1)	Medium	backtracking, good trick, recursive, IP address, `amazon`	AGAIN****** (4)
078	Subsets	Python, Java	O(n 2^n)*	O(n)	Medium	backtracking, `good concept`, similar as, dfs, `#90 Subsets II` , `UBER`, `amazon`, `fb`	AGAIN**************** (7) (MUST again!!!)
090	Subsets II	Python, Java	O(n 2^n)*	O(n)	Medium	backtracking, good basic, check `# 078 Subsets`, dfs, `fb`	AGAIN********* (6)
131	Palindrome Partitioning	Python, Java	O(n 2^n)*	O(n^2)	Medium	backtracking, dfs, good trick, `amazon`	AGAIN*********** (7) (MUST)
139	Word Break	Python, Java	O(n^2 l)*	O(n)	Medium	backtracking, Curated Top 75, dfs, `dp`, `trick`, `UBER`, `yahoo`, `amazon`, `google`, `fb`	AGAIN********************** (9) (MUST)
140	Word Break II	Python, Java	O(n 2^n)*	O(n 2^n)*	Hard	backtracking, good basic, LC 078, dfs, `dp`, `trick`, `amazon`	AGAIN******** (2)
212	Word Search II	Python, Java	O(m n * 4^l)*	O(t)	Hard	backtracking, Curated Top 75, LC 208, LC 079, hashset, hashmap, trie, backtrack with trie, `amazon`, fb, apple, twitter, uber, google, indeed	AGAIN********** (3)
216	Combination Sum III	Python, Java	O(k C(n, k))*	O(k)	Medium	backtracking, LC 39, LC 40, pick k distinct numbers from 1…9	AGAIN*
254	Factor Combinations	Python, Java	O(sqrt(n) f)*	O(logn)	Medium	backtracking, 🔒, f = number of factor combinations, dfs from the smallest factor	AGAIN (not start)
267	Palindrome Permutation II	Python, Java	O(n n!)*	O(n)	Medium	backtracking, 🔒, LC 266, permute half then mirror	AGAIN (not start*)
294	Flip Game II	Python, Java	O(n 2^n)*	O(2^n)	Medium	backtracking, 🔒, DP, Hash	AGAIN*
320	Generalized Abbreviation	Python, Java	O(n 2^n)*	O(n)	Medium	backtracking, 🔒, abbreviate or keep each char, bit mask, dfs, `google`	AGAIN (not start*)
489	Robot Room Cleaner	Python, Java	O(m n)*	O(m n)*	Hard	backtracking, 🔒, google, amazon, fb, m$	AGAIN (not start)
526	Beautiful Arrangement	Python, Java	O(n!)	O(n)	Medium	backtracking, backtrack with divisibility pruning, bitmask dp, `good trick`	AGAIN (not start*)
676	Implement Magic Dictionary	Python, Java	build: O(t), search: O(26 l)*	O(t)	Medium	backtracking, Trie, DFS	AGAIN (not start)
698	Partition to K Equal Sum Subsets	Python, Java	O(n 2^n)*	O(2^n)	Medium	backtracking, DP, Memoization, similar as `#416 Partition Equal Subset Sum, #473 Matchsticks to Square`	AGAIN****** (2) (MUST)
718	Maximum Length of Repeated Subarray	Python, Java	O(m n)*	O(min(m, n))	Medium	backtracking, DP, Hash, Binary Search	AGAIN (not start*)
784	Letter Case Permutation	Python, Java	O(n 2^n)*	O(n)	Medium	backtracking, dfs, recursion, `good trick`, `fb`	AGAIN****** (3)
980	Unique Paths III	Python, Java	O(3^(m n))*	O(m n)*	Hard	backtracking, good trick, LC 62, 63, dfs, amazon, google, `fb`	AGAIN********** (3)
1219	Path with Maximum Gold	Python, Java	O(3^(m n))*	O(m n)*	Medium	backtracking, dfs, bfs, google	AGAIN**** (1)

Dynamic Programming

#	Title	Solution	Time	Space	Difficulty	Note	Status
53	Maximum Subarray	Python, Java	O(n)	O(1)	Medium	dp, MUST, Curated Top 75, brute force, Kadane algo, pre-sum array, `dp basic`, `amazon`, `fb`	AGAIN**************** (8)
62	Unique Paths	Python, Java	O(m n)*	O(n)	Medium	dp, Curated Top 75, `2D dp`, `basic`, fb, google, apple, amazon, m$	AGAIN********* (3)(MUST)
63	Unique Paths II	Python, Java	O(m n)*	O(n)	Medium	dp, `trick`, LC 64, dfs + dp, `amazon`	AGAIN*
64	Minimum Path Sum	Python, Java	O(m n)*	O(n)	Medium	dp, Sum of grid values, Dijkstra, LC 1631, `amazon`	AGAIN******* (3)(MUST)
70	Climbing Stairs	Python, Java	O(n)	O(1)	Easy	dp, Curated Top 75, Matrix Exponentiation, `recursion`, `apple`	OK* (2)
72	Edit Distance	Python, Java	O(m n)*	O(m n)*	Medium	dp, top 100 like, `2D DP`, dp state, `apple`, google, amazon	AGAIN(5)************ (MUST)
91	Decode Ways	Python, Java	O(n)	O(1)	Medium	dp, Curated Top 75, `good basic`, `M$`, `UBER`, `amazon`, `fb`	AGAIN*********************** (6) (MUST)
96	Unique Binary Search Trees	Python, Java	O(n^2)	O(n)	Medium	dp, Math, fb, amazon	AGAIN* (2) (not start)
115	Distinct Subsequences	Java	O(m n)*	O(n)	Hard	dp, 2D dp, good trick, needcode 150	AGAIN******** (1)
120	Triangle	Python, Java	O(m n)*	O(n)	Medium	dp, `amazon`	AGAIN* (not start) (2)
97	Interleaving String	Java, Python	O(m n)*	O(n)	Medium	dp, 1D, 2D DP, state machine, good trick	again************ (4)(MUST)
123	Best Time to Buy and Sell Stock III	Python, Java	O(n)	O(1)	Hard	dp, `amazon`, uber, apple	AGAIN* (not start) (1)
152	Maximum Product Subarray	Python, Java	O(n)	O(1)	Medium	dp, Kadane algo, brute force, Curated Top 75, google, amazon, linkedin	AGAIN***** (1)
198	House Robber	Python, Java	O(n)	O(1)	Easy	dp, Curated Top 75, dp basic, `amazon`	AGAIN*** (2)
213	House Robber II	Python, Java	O(n)	O(1)	Medium	dp, good trick, LC 198, circular dp, Curated Top 75, google, amazon	AGAIN*************** (4)(MUST)
221	Maximal Square	Python, Java	O(m n)*	O(n)	Medium	dp, EPI, `amazon`, `fb`, google	AGAIN** (3) (not start)
256	Paint House	Python, Java	O(n)	O(1)	Medium	dp, google, 🔒	AGAIN (2)
276	Paint Fence	Python, Java	O(n)	O(1)	Easy	dp, good basic, 🔒	AGAIN****(1)
279	Perfect Squares	Python, Java	O(n sqrt(n))*	O(n)	Medium	dp, bfs, GOOD trick, `google`	AGAIN************ (3)(MUST)
303	Range Sum Query - Immutable	Python, Java	ctor: O(n), lookup: O(1)	O(n)	Easy	dp, prefix sum, LC 304, `good basic`	OK
304	Range Sum Query 2D - Immutable	Python, Java	ctor: O(m n), lookup: O(1)*	O(m n)*	Medium	dp, prefix sum, NOTE: idx, `trick`, `fb`	AGAIN******** (6)
309	Best Time to Buy and Sell Stock with Cooldown	Python, Java	O(n)	O(1)	Medium	dp, 2D dp, n x m array, good basic	AGAIN********* (1)(MUST)
312	Burst Balloons	Java	O(n^3)	O(n^2)	Hard	dp, interval dp, think in reverse (which balloon bursts last), `good trick`, LC top 100 like	AGAIN (not start)
322	Coin Change	Python, Java	O(n k)*	O(k)	Medium	dp, Curated Top 75, bfs, dfs, grab, google, fb, apple, `amazon`	AGAIN****************** (7) (MUST)
329	Longest Increasing Path in a Matrix	Java	O(m n)*	O(m n)*	Hard	dp, neetcode 150, good trick, dfs+dp, 2D DP, topo sort	AGAIN************* (3)(MUST)
351	Android Unlock Patterns	Python, Java	O(9^2 2^9)*	O(9 2^9)*	Medium	dp, 🔒 Backtracking, `google`	AGAIN (not start)
357	Count Numbers with Unique Digits	Python, Java	O(n)	O(1)	Medium	dp, Backtracking, Math	AGAIN (not start)
361	Bomb Enemy	Python, Java	O(m n)*	O(m n)*	Medium	dp, 🔒, `google`	AGAIN* (2)
363	Max Sum of Rectangle No Larger Than K	Java	O(m^2 nlogn)*	O(n)	Medium	dp, pre-sum. `google`	AGAIN* (not start)
368	Largest Divisible Subset	Python, Java	O(n^2)	O(n)	Medium	dp, sort then LIS-style dp, backtrack the chain, `good trick`	AGAIN (not start)
375	Guess Number Higher or Lower II	Python, Java	O(n^3)	O(n^2)	Medium	dp, interval dp / minimax, pick the cost that minimizes the worst case, `good trick`	AGAIN (not start)
377	Combination Sum IV	Python, Java	O(n t)*	O(t)	Medium	dp, Curated Top 75, LC 518, dp basic, `fb`	AGAIN******** (3)
403	Frog Jump	Python, Java	O(n^2)	O(n^2)	Hard	dp, dfs, bfs, Memoization dfs, amazon, m$	AGAIN (not start)
416	Partition Equal Subset Sum	Python, Java	O(n s)*	O(s)	Medium	dp, good basic, 0/1 knapsack, backward, 1d, 2d dp, fb, google, apple, amazon	AGAIN******************* (6) (MUST)
418	Sentence Screen Fitting	Python, Java	O(r + n c)*	O(n)	Medium	dp, 🔒, `google`	AGAIN (not start)
467	Unique Substrings in Wraparound String	Python, Java	O(n)	O(1)	Medium	dp, longest consecutive substring ending at each letter, `good trick`	AGAIN (not start)
471	Encode String with Shortest Length	Python, Java	O(n^3) on average	O(n^2)	Medium	dp, 🔒, `google`	AGAIN (not start)
472	Concatenated Words	Python, Java	O(n l^2)*	O(n l)*	Hard	dp, `good trick`, trie+dfs, `dfs`, `amazon`	AGAIN******** (3)
474	Ones and Zeroes	Python, Java	O(s m * n)*	O(m n)*	Medium	dp, `google`	AGAIN (not start)
486	Predict the Winner	Python, Java	O(n^2)	O(n)	Medium	dp, `good trick`, 1d, 2d DP, `google`	AGAIN********* (again)(MUST)
509	Fibonacci Number	Python, Java	O(logn)	O(1)	Easy	dp, variant of Climbing Stairs, `Matrix Exponentiation`, Spotify	OK**
516	Longest Palindromic Subsequence	Python, Java	O(n^2)	O(n)	Medium	dp, good basic, 2D DP, backward+forward, forward+forward, `UBER`, `amazon`	AGAIN**************** (MUST)
562	Longest Line of Consecutive One in Matrix	Python, Java	O(m n)*	O(n)	Medium	dp, 🔒, `google`	AGAIN (not start)
552	Student Attendance Record II	Java	O(n)	O(1)	Medium	dp, `2d, 3d dp`, good trick, `google`	AGAIN*********** (3)
576	Out of Boundary Paths	Python, Java	O(N m * n)*	O(m n)*	Medium	dp, `bfs`	AGAIN (not start)
583	Delete Operation for Two Strings	Python, Java	O(m n)*	O(n)	Medium	dp, `google`	AGAIN (not start)
639	Decode Ways 2	Java	O(n)	O(1)	Hard	dp, LC 91, state dp with the `*` wildcard, mod 1e9+7	AGAIN (1) (not start)
650	2 Keys Keyboard	Python, Java	O(sqrt(n))	O(1)	Medium	dp, `dp basic`, `M$`	AGAIN*
673	Number of Longest Increasing Subsequence	Python, Java	O(n^2)	O(n)	Medium	dp, `good trick`, `fb`	AGAIN********** (3)
688	Knight Probability in Chessboard	Python, Java	O(k n^2)*	O(n^2)	Medium	dp, `dp basic`, `AGAIN`, `M$`, `Goldman Sachs`, `google`, `fb`, `amazon`	AGAIN********* (5)
712	Minimum ASCII Delete Sum for Two Strings	Python, Java	O(m n)*	O(n)	Medium	dp, LC 583, 2D dp on ASCII sums	AGAIN (not start)
714	Best Time to Buy and Sell Stock with Transaction Fee	Python, Java	O(n)	O(1)	Medium	dp, AGAIN, `good basic`, `greedy`, `fb`, google	AGAIN******** (3)
727	Minimum Window Subsequence	Java	O(m n)*	O(m n)*	Hard	dp, `google`	AGAIN (not start)
740	Delete and Earn	Python, Java	O(n + k)	O(k)	Medium	dp, LC 198	AGAIN********* (1)(MUST)
746	Min Cost Climbing Stairs	Python, Java	O(n)	O(1)	Easy	dp, `good dp basic`, `amazon`	AGAIN**** (2)
750	Number Of Corner Rectangles	Python, Java	O(n m^2)*	O(n m)*	Medium	dp, `fb`	AGAIN (3) (not start)
764	Largest Plus Sign	Python, Java	O(n^2)	O(n^2)	Medium	dp, complex, `fb`	AGAIN (not start) (2)
788	Rotated Digits	Python, Java	O(logn)	O(logn)	Easy	dp, `Brute Force`, Memoization	OK*
790	Domino and Tromino Tiling	Python, Java	O(logn)	O(1)	Medium	dp, Matrix Exponentiation, `google`	AGAIN (not start)
799	Champagne Tower	Python, Java	O(n^2)	O(n)	Medium	dp, simulate the poured volume flowing down each row, 2D dp	AGAIN
801	Minimum Swaps To Make Sequences Increasing	Python, Java	O(n)	O(1)	Medium	dp, `amazon`, `fb`	AGAIN***** (not start) (2)
808	Soup Servings	Python, Java	O(1)	O(1)	Medium	dp, memoized probability dp, converges to 1 for large n, `trick`	AGAIN (not start)
813	Largest Sum of Averages	Python, Java	O(k n^2)*	O(n)	Medium	dp, `google`	AGAIN (not start)
823	Binary Trees With Factors	Python, Java	O(n^2)	O(n)	Medium	dp, sort + hashmap dp, count trees keyed by root value	AGAIN (not start)
837	New 21 Game	Python, Java	O(n)	O(n)	Medium	dp, trick, math, `google`	AGAIN (2) (not start)
838	Push Dominoes	Python, Java	O(n)	O(n)	Medium	dp, `two pointers`, `google`	AGAIN (not start)
871	Minimum Number of Refueling Stops	Java	O(nlogn)	O(n)	Hard	dp, PQ, good trick, `google`	AGAIN***** (1)
877	Stone Game	Python, Java	O(n^2)	O(n)	Medium	dp, LC 486, math, game theory, variant of Predict the Winner	AGAIN*** (1)
926	Flip String to Monotone Increasing	Python, Java	O(n)	O(1)	Medium	dp, good trick, 1D dp, prefix sum, `google`, `amazon`	AGAIN********** (4)(MUST)
931	Minimum Falling Path Sum	Python, Java	O(n^2)	O(1)	Medium	dp, `google`, good basic	AGAIN******** (2)(MUST)
935	Knight Dialer	Python, Java	O(logn)	O(1)	Medium	dp, Matrix Exponentiation, `google`, `M$`, `fb`	AGAIN (not start)
983	Minimum Cost For Tickets	Java	O(n)	O(n)	Medium	dp, `google`	AGAIN (not start)
518	Coin Change 2	Python, Java	O(n amount)*	O(amount)	Medium	dp, `dp basic`, `good trick`, LC 322, 377, `google`, `fb`	AGAIN******** (4)
494	Target Sum	Python, Java	O(n s)*	O(s)	Medium	dp, 2D dp, math, `fb`	AGAIN****** (1)
1000	Minimum Cost to Merge Stones	Python, Java	O(n^3 / k)	O(n^2)	Hard	dp, amazon	AGAIN (not start)
1025	Divisor Game	Java	O(1)	O(1)	Easy	dp, n % 2 == 0 wins, `math`, `good trick`	AGAIN (not start)
1048	Longest String Chain	Java	O(n l^2)*	O(n)	Medium	dp, 1D dp, hashmap, string, good basic, google	AGAIN******** (1)(MUST)
1049	Last Stone Weight II	Python, Java	O(n s)*	O(s)	Medium	DP, 0/1 knapsack, minimize abs(S1 - S2), LC 416, 494	AGAIN (not start)
1137	N-th Tribonacci Number	Java	O(n)	O(1)	Easy	dp, good basic	AGAIN**
1140	Stone Game II	Java	O(n^3)	O(n^2)	Medium	dp, need_code_250	AGAIN**
1143	Longest Common Subsequence	Python, Java	O(m n)*	O(n)	Medium	dp, Curated Top 75, Memoization, 2D dp, amazon, karat	AGAIN******************** (6)(DP MUST)
1277	Count Square Submatrices with All Ones	Java	O(m n)*	O(1)	Medium	dp, google	AGAIN(1) (not start)
1312	Minimum Insertion Steps to Make a String Palindrome	Python, Java	O(n^2)	O(n)	Hard	dp, amazon	AGAIN (not start)
1335	Minimum Difficulty of a Job Schedule	Python, Java	O(d n^2)*	O(d n)*	Hard	dp, `amazon`	not start
1406	Stone Game III	Java	O(n)	O(n)	Medium	dp, `amazon`	AGAIN (not start)
1617	Count Subtrees With Max Distance Between Cities	Java	O(n^2 2^n)*	O(n^2)	Hard	dp, enumerate subsets with a bitmask, tree diameter per subset, `complex`	AGAIN (not start)
1799	Maximize Score After N Operations	Python, Java	O(n^2 2^n)*	O(2^n)	Medium	dp, `amazon`	not start
2560	House Robber IV	Java	O(nlog(max))	O(1)	Medium	dp, binary search	AGAIN (not start)
2646	Minimize the Total Price of the Trips	Java	O(m n)*	O(n)	Hard	dp, dfs, LC weekly	not start AGAIN (1)
2765	Longest Alternating Subarray	Java	O(n)	O(1)	Easy	dp, `2 pointers`, good trick	AGAIN(1) *****
2767	Partition String Into Minimum Beautiful Substrings	Java	O(n^2)	O(n)	Medium	dp, dp over prefixes, powers of 5 written in binary	AGAIN(1)
3122	Minimum Number of Operations to Satisfy Conditions	Java	O(m n * 100)*	O(n 10)*	Medium	dp, LC weekly, PQ	Again (1)
3196	Maximize Total Cost of Alternating Subarrays	Java	O(n)	O(1)	Medium	dp, LC weekly	Again

Greedy

#	Title	Solution	Time	Space	Difficulty	Note	Status
011	Container With Most Water	Python, Java	O(n)	O(1)	Medium	greedy, Curated Top 75, good basics, `two pointers`, `fb`, amazon	OK*** (3) (but again !!)
045	Jump Game II	Python, Java	O(n)	O(1)	Medium	greedy, google, apple, fb, tesla, `amazon`	AGAIN******** (4) (MUST)
055	Jump Game	Python, Java	O(n)	O(1)	Medium	greedy, Curated Top 75, good trick, backtrack, DP, `amazon`	AGAIN*********** (9)(MUST)
84	Largest Rectangle in Histogram	Python, Java	O(n)	O(n)	Hard	greedy, top 100 like, brute force, good trick, `mono stack`, LC 085, amazon, fb	AGAIN******** (3)
122	Best Time to Buy and Sell Stock II	Python, Java	O(n)	O(1)	Easy	greedy, compare with `#309 Best Time to Buy and Sell Stock with Cooldown` , `#714 Best Time to Buy and Sell Stock with Transaction Fee`, `amazon`	again* (2)
134	Gas Station	Python, Java	O(n)	O(1)	Medium	greedy, trick, `amazon`	AGAIN****** (5)
135	Candy	Python, Java	O(n)	O(n)	Hard	greedy, LC 123, boundary, array, `amazon`	AGAIN (2)
218	The Skyline Problem	Python, Java	O(nlogn)	O(n)	Hard	greedy, brute force, swipe line, Priority Queue, Union Find, Divide-and-Conquer, apple, microsoft	AGAIN (not start)
316	Remove Duplicate Letters	Python, Java	O(n)	O(1)	Medium	greedy, good trick, MONO stack, char, visited, LC 1081	AGAIN********* (4)(MUST)
330	Patching Array	Python, Java	O(n)	O(1)	Hard	greedy, array, `amazon`	AGAIN (not start)
358	Rearrange String k Distance Apart	Python, Java	O(n)	O(k)	Hard	greedy, PQ, check LC 767, cooldown queue, good trick	AGAIN*** (1)
376	Wiggle Subsequence	Python, Java	O(n)	O(1)	Medium	greedy, count direction changes, greedy or dp, `good basic`	AGAIN*
392	Is Subsequence	Python, Java	O(n)	O(1)	Easy	greedy, `basics`, 2 pointers, dp	AGAIN********* (1)(MUST)
397	Integer Replacement	Python, Java	O(logn)	O(1)	Medium	greedy, `Recursion basics`, Math	OK*
402	Remove K Digits	Python, Java	O(n)	O(n)	Medium	greedy, LintCode, mono stack, stack, good trick	AGAIN********** (3)(MUST)
435	Non-overlapping Intervals	Python, Java	O(nlogn)	O(1)	Medium	greedy, Curated Top 75, good trick, inverval, LC 452, Line Sweep, `google`	AGAIN**************** (10)(must)
452	Minimum Number of Arrows to Burst Balloons	Python, Java	O(nlogn)	O(1)	Medium	greedy, sort, compare i-1 element, overlap, good basic	AGAIN***** (3)
455	Assign Cookies	Python, Java	O(nlogn + mlogm)	O(1)	Easy	greedy, sort both arrays then two pointers, `basic`	OK*
621	Task Scheduler	Python, Java	O(n)	O(1)	Medium	greedy, `trick`, PQ+queue, cool down, `fb`, `amazon`	AGAIN********* (8)(MUST)
646	Maximum Length of Pair Chain	Python, Java	O(nlogn)	O(1)	Medium	greedy, good trick, similar as `#435 Non-overlapping Intervals`, Line Sweep, `amazon`	OK********** (5) (but again)
649	Dota2 Senate	Python, Java	O(n)	O(n)	Medium	greedy, complex question	AGAIN (not start*)
659	Split Array into Consecutive Subsequences	Python, Java	O(n)	O(n)	Medium	greedy, map, pq, dp, `google`	AGAIN***** (1)
738	Monotone Increasing Digits	Python, Java	O(logn)	O(logn)	Medium	greedy, good trick, string, trick, `Amazon`	AGAIN********* (4)
763	Partition Labels	Python, Java	O(n)	O(1)	Medium	greedy, good trick, dict, sliding window, `amazon`	AGAIN******* (6) (AGAIN)
765	Couples Holding Hands	Java	O(n)	O(n)	Hard	greedy, hashmap, union find	AGAIN (not start)
767	Reorganize String	Python, Java	O(n)	O(1)	Medium	greedy, LC 1054, PQ, hashmap, `good trick`, `amazon`, `fb`, google	AGAIN****************** (9) (MUST)
861	Score After Flipping Matrix	Python, Java	O(r c)*	O(1)	Medium	greedy, greedy: make column 0 all ones, then maximize each column, `good trick`	AGAIN (not start*)
870	Advantage Shuffle	Python, Java	O(nlogn)	O(n)	Medium	greedy, sort both then greedily assign the smallest winning card (Tian Ji horse racing), `good trick`	OK*
881	Boats to Save People	Python, Java	O(nlogn)	O(n)	Medium	greedy, `good trick`	AGAIN* (2)
948	Bag of Tokens	Python, Java	O(nlogn)	O(1)	Medium	greedy, Two Pointers	AGAIN*** (1) (not start)
962	Maximum Width Ramp	Python, Java	O(n)	O(n)	Medium	greedy, Descending Stack	AGAIN (not start)
978	Longest Turbulent Subarray	Java	O(n)	O(1)	Medium	greedy, sliding window	AGAIN (not start)
984	String Without AAA or BBB	Java, Python	O(n)	O(n)	Medium	greedy, good basic, while, string builder, endsWith, PQ	AGAIN***** (2)
1024	Video Stitching	Python, Java	O(nlogn)	O(1)	Medium	greedy, Spotify	AGAIN (not start)
1102	Path With Maximum Minimum Value	Python, Java	O(m nlog(m * n))*	O(m n)*	Medium	greedy, dfs, dp, `amazon`	not start
319	Bulb Switcher	Python, Java	O(1)	O(1)	Medium	greedy, duplicate of the Math entry, answer is floor(sqrt(n)), `good trick`	not start
672	Bulb Switcher II	Python, Java	O(1)	O(1)	Medium	greedy, duplicate of the Math entry, only 6 reachable states, `trick`	not start
1375	Bulb Switcher III	Python, Java	O(n)	O(1)	Medium	greedy, queue, array, `gra*`	AGAIN*** (1)
1380	Lucky Numbers in a Matrix	Java	O(m n)*	O(m + n)	Easy	greedy, intersect the row minimums with the column maximums, `matrix`, `basic`	AGAIN (1)
1546	Maximum Number of Non-Overlapping Subarrays With Sum Equals Target	Python, Java	O(n)	O(n)	Medium	greedy, prefix + greedy, good trick	AGAIN*** (1)
1648	Sell Diminishing-Valued Colored Balls	Python, Java	O(nlogn)	O(1)	Medium	greedy, trick, sorting, heapq, binary search, `amazon`	AGAIN*** (2) (not start)
1710	Maximum Units on a Truck	Python, Java	O(nlogn)	O(1)	Easy	greedy, sort, `amazon`	AGAIN******** (3) (MUST)
1791	Find Center of Star Graph	Java	O(1)	O(1)	Easy	greedy, degree cnt	AGAIN (1)
1851	Minimum Interval to Include Each Query	Java	O(nlogn + qlogq)	O(n + q)	Hard	greedy, needcode 150, PQ, interval, sort, scan line, Segment Tree, `google`	AGAIN************** (1)
1871	Jump Game VII	Java	O(n)	O(n)	Medium	greedy, bfs	AGAIN (not start)
1899	Merge Triplets to Form Target Triplet	Java	O(n)	O(1)	Medium	greedy, sort, `amazon`	AGAIN(1) (not srart)
2126	Destroying Asteroids	Java	O(nlogn)	O(1)	Medium	greedy, sort, google	AGAIN(1) (not srart)
2279	Maximum Bags With Full Capacity of Rocks	Java	O(nlogn)	O(n)	Medium	greedy, sort	AGAIN (1)
2591	Distribute Money to Maximum Children	Java	O(1)	O(1)	Easy	greedy, LC weekly	AGAIN (1)
2643	Row With Maximum Ones	Java	O(m n)*	O(1)	Easy	greedy, count ones per row, `matrix`, `basic`	AGAIN(1) (not srart)
2645	Minimum Additions to Make Valid String	Java	O(n)	O(1)	Medium	greedy, string	AGAIN(1) (not srart)
2656	Maximum Sum With Exactly K Elements	Java	O(n)	O(1)	Easy	greedy, weekly_103	AGAIN(1) (not srart)
2960	Count Tested Devices After Test Operations	Java	O(n)	O(1)	Easy	greedy, running count of tested devices acts as the decrement, `good trick`	AGAIN(1) (not srart)
3120	Count the Number of Special Characters I	Java	O(n)	O(1)	Easy	greedy, hashset of seen chars, require both cases, LC 3121, `basic`	AGAIN(1) (not srart)
3689	Maximum Total Subarray Value I	Java	O(n)	O(1)	Medium	greedy, math, brute force	OK
3690	Split and Merge Array Transformation	Java	O(n! n^2)*	O(n!)	Medium	greedy, bfs dfs	AGAIN (not start)
3691	Maximum Total Subarray Value II	Java	O(n^2)	O(n)	Hard	greedy, brute force, tree	AGAIN
3994	Transform Binary String Using Subsequence Sort	Python, Java	O(n)	O(1)	Medium	greedy, brute force, LC weekly	AGAIN*****(1)
3998	Transform Binary String Using Subsequence Sort	Python, Java	O(n)	O(1)	Medium	greedy, brute force, prefix, LC weekly	AGAIN

Graph

#	Title	Solution	Time	Space	Difficulty	Note	Status
269	Alien Dictionary	Python, Java	O(n l)*	O(1)	Hard	graph, Curated Top 75, good trick, dfs, bfs, Topologicals sort, `fb`, google, m$, airbnb, uber, amazon	AGAIN********** (6)
305	Number of Islands II	Java	O(k a(m * n))*	O(m n)*	Hard	graph, union find, LC 200	AGAIN (1)
323	Number of Connected Components in an Undirected Graph	Python , Java	O(n + e)	O(n)	Medium	graph, Curated Top 75, LC 547, 🔒, bfs, dfs, Union Find, `linkedin`, `amazon`, fb, google, m$	AGAIN************** (6) (MUST)
778	Swim in Rising Water	Java	O(n^2 logn)*	O(n^2)	Medium	graph, Dijkstra	AGAIN1 (1) (not start)
959	Regions Cut By Slashes	Python, Java	O(n^2)	O(n^2)	Medium	graph, Union Find	again
997	Find the Town Judge	Java	O(n + e)	O(n)	Easy	graph, in-degree minus out-degree equals n - 1, `good basic`	again
1135	Connecting Cities With Minimum Cost	Python, Java	O(e loge)*	O(n)	Medium	graph, union find, Kruskal, prime, amazon, google	AGAIN (not start)
1462	Course Schedule IV	Java	O(n^3)	O(n^2)	Medium	graph, Floyd-Warshall reachability, or topological sort with reach sets, LC 207	AGAIN (not start)
1489	Find Critical and Pseudo-Critical Edges in Minimum Spanning Tree	Java	O(e^2 a(n))*	O(n + e)	Hard	graph, dfs, bfs, Dijkstra	AGAIN (not start)
1514	Path with Maximum Probability	Python, Java	O(e logn)*	O(n + e)	Medium	graph, Dijkstra	AGAIN (not start)
1584	Min Cost to Connect All Points	Java	O(n^2 logn)*	O(n^2)	Medium	graph, Kruskal algo, Prim algo, advanced graph	AGAIN (not start)
1631	Path With Minimum Effort	Python, Java	O(m nlog(m * n))*	O(m n)*	Medium	graph, `Max of step differences`, dfs, bfs+PQ, LC 64, Dijkstra	AGAIN********** (2)(MUST)
1761	Minimum Degree of a Connected Trio in a Graph	Java	O(n^3)	O(n^2)	Hard	graph, DP	AGAIN (not start)
1971	Find if Path Exists in Graph	Java	O(n + e)	O(n + e)	Easy	graph, bfs, dfs, union find	AGAIN (1)
2290	Minimum Obstacle Removal to Reach Corner	Java	O(m nlog(m * n))*	O(m n)*	Hard	graph, bfs, PQ, Dijkstra classics	AGAIN****** (2)(MUST)
2392	Build a Matrix With Conditions	Java	O(k^2)	O(k^2)	Hard	graph, dfs, bfs, Dijkstra	AGAIN (not start)
2508	Add Edges to Make Degrees of All Nodes Even	Java	O(n + e)	O(n + e)	Hard	graph, LC weekly	AGAIN (not start)
2617	Minimum Number of Visited Cells in a Grid	Java	O(m nlog(m * n))*	O(m n)*	Hard	graph, PQ, stack, LC weekly	AGAIN***** (1) (not start)
2642	Design Graph With Shortest Path Calculator	Java	ctor: O(n^3), addEdge: O(n^2), shortestPath: O(1)	O(n^2)	Medium	graph, Dijkstra, floyd warshall, LC weekly 102	AGAIN******** (2)
2709	Greatest Common Divisor Traversal	Java	O(n sqrt(maxV) * a(n))*	O(n + maxV)	Hard	graph, union find over shared prime factors, sieve, `complex`	AGAIN (not start)
3123	Find Edges in Shortest Paths	Java	O(e logn)*	O(n + e)	Hard	graph, Dijkstra from both ends, keep edges lying on some shortest path, `good trick`	AGAIN (1) (not start)

Geometry

#	Title	Solution	Time	Space	Difficulty	Note	Status
892	Surface Area of 3D Shapes	Java	O(n^2)	O(1)	Easy	geometry, count 6 faces per cube then subtract hidden overlaps, `matrix`, `math`	AGAIN
587	Erect the Fence	Python, Java	O(nlogn)	O(n)	Hard	geometry, complex, Convex Hull, `Monotone Chain`, `amazon`	AGAIN (not start)

Design

#	Title	Solution	Time	Space	Difficulty	Note	Status
146	LRU Cache	Python, Java	O(1)	O(k)	Medium	design, good basic, LRU, dequeue, `Double Linkedlist`, LinkedHashMap, OrderedDict, `amazon`	AGAIN********** (5)
225	Implement Stack using Queues	Python, Java	push: O(n), pop: O(1), top: O(1)	O(n)	Easy	design, two queues, or one queue rotated after each push, LC 232, `basic`	AGAIN** (1)(MUST)
284	Peeking Iterator	C++ Python, Java	O(1)	O(1)	Medium	design, good basic, `google`, apple	not start*** (1)
348	Design Tic-Tac-Toe	Python, Java	O(1)	O(n^2)	Medium	design, matrix, array, 🔒, google, `fb`, `amazon`	AGAIN****** (5)
353	Design Snake Game	Python, Java	O(1)	O(s)	Medium	design, good trick, Deque, 🔒, amazon, google, square, m$	AGAIN*** (1)
355	Design Twitter	Python, Java	O(klogu)	O(t + f)	Medium	design, good trick, data structure, heapq, defaultdict, `amazon`	OK******** (6)
362	Design Hit Counter	Java	O(1), amortized	O(k)	Medium	design, 🔒, deque, sliding window count, google	Again (1)
379	Design Phone Directory	Java	O(1)	O(n)	Medium	design, 🔒, queue, free-slot pool, google	AGAIN(1)
380	Insert Delete GetRandom O(1)	Python, Java	O(1)	O(n)	Medium	design, set, list, dict, `amazon`, `fb`, google	OK* (5)
0381	Insert Delete GetRandom O(1) - Duplicates allowed	C++ Python, Java	O(1)	O(n)	Hard	design, LC 380, hashmap of value -> index set + swap-with-last, `good trick`
432	All O`one Data Structure	Java	O(1) per op	O(n)	Hard	design, doubly linked list of count buckets + hashmap, LC 146, LC 460, `complex`
460	LFU Cache	Python, Java	O(1)	O(k)	Hard	design, Least Frequently Used (LFU) cache, complex, `amazon`	AGAIN*** (3)
535	Encode and Decode TinyURL	Python, Java	O(1)	O(n)	Medium	design, `hash map`, “duplicated”, “hash map collision”, `UBER`, `amazon`, `google`, `fb`	OK**** (5)
588	Design In-Memory File System	Python, Java	ls: O(l + klogk) mkdir: O(l) addContentToFile: O(l + c) readContentFromFile: O(l + c)	O(n + s)	Hard	design, dict, a bit complex basic, 🔒, shoptify, amazon	AGAIN******* (3)
0604	Design Compressed String Iterator	C++ Python, Java	ctor: O(n), next/hasNext: O(1)	O(n)	Easy	design, 🔒, decode (char, count) pairs lazily, iterator
631	Design Excel Sum Formula	C++ Python, Java	set: O((r c)^2)* get: O(1) sum: O((r c)^2)*	O(r c)*	Hard	design, 🔒
635	Design Log Storage System	C++ Python, Java	put: O(1) retrieve: O(n + dlogd)	O(n)	Medium	design, 🔒
641	Design Circular Deque	Python, Java	O(1) per op	O(k)	Medium	design, 🔒, circular buffer or doubly linked list, LC 622
642	Design Search Autocomplete System	Python, Java	O(p^2)	O(p t + s)*	Hard	design, 🔒, `amazon`	AGAIN**** (not start) (5)
705	Design HashSet	Java	O(1) average	O(n)	Easy	design, bucket array + chaining, LC 706, `good basic`
706	Design HashMap	Java	O(1) average	O(n)	Easy	design, bucket array + chaining, LC 705, `good basic`
715	Range Module	C++ Python, Java	add: O(n) remove: O(n) query: O(logn)	O(n)	Hard	design, sorted interval list with binary search, LC 352, TreeMap, segment tree
716	Max Stack	Python, Java	push: O(logn) pop: O(logn) popMax: O(logn) top: O(1) peekMax: O(1)	O(n)	Hard	design, basic, stack, `amazon`	again*
745	Prefix and Suffix Search	Python, Java	ctor: O(w l^2)* search : O(p + s)	O(t)	Hard	design, Trie, `fb`	Not start* (1) (good basic)
900	RLE Iterator	C++ Python, Java	O(n)	O(1)	Medium	design, google, treemap, array	AGAIN (not start) (1)
1146	Snapshot Array	Java	set: O(1) get: O(logn)	O(n)	Medium	design, treeMap, binary search, google	AGAIN*** (1)
1166	Design File System	C++ Python, Java	create: O(n) get: O(n)	O(n)	Medium	design, 🔒, google, airbnb, amazon	AGAIN***** (1)
1172	Dinner Plate Stacks	C++ Python, Java	push: O(logn) pop: O(1), amortized popAtStack: (logn)	O(n c)*	Hard	design, PQ of indices of non-full stacks, `good trick`
1188	Design Bounded Blocking Queue	Java	O(1) per op	O(k)	Medium	design, google, Semaphore	AGAIN (not start)
1206	Design Skiplist	C++ Python, Java	O(logn), on average	O(n)	Hard	design, probabilistic multi-level linked list, `complex`
1236	Web Crawler	C++ Python	O(V + E)	O(V)	Medium	design, 🔒, BFS, DFS
1244	Design A Leaderboard	Python, Java	ctor: O(1) add: O(1) top: O(n) reset: O(1)	O(n)	Medium	design, google, hashmap	AGAIN (not start)
1268	Search Suggestions System	Python, Java	ctor: O(n l)* suggest: O(l^2)	O(t)	Medium	design, good basic, array, heap, sort, Trie, `amazon`	AGAIN******** (3)
1286	Iterator for Combination	Python	ctor: O(1), next: O(k)	O(k)	Medium	design, Stack
1429	First Unique Number	Python, Java	O(1) amortized per op	O(n)	Medium	design, good trick, heap, `amazon`	AGAIN*** (not start)
1472	Design Browser History	Python, Java	visit: O(1), back/forward: O(steps)	O(n)	Medium	design, two stacks, or a list with a cursor, `good basic`	AGAIN
1381	Design a Stack With Increment Operation	Java	O(1) per op	O(n)	Medium	design, array, stack, lazy increment (`good trick`), linked list	AGAIN (1)
1500	Design a File Sharing System	Python, Java	join/leave: O(logn), request: O(n)	O(n)	Medium	design, 🔒, min-heap of free ids + hashmap of owners, google	AGAIN*** (not start)
1603	Design Parking System	Python, Java	O(1)	O(1)	Easy	design, amazon, paypal	OK
1628	Design an Expression Tree With Evaluate Function	Python, Java	build: O(n), evaluate: O(n)	O(n)	Medium	design, complex, `amazon`	AGAIN*** (not start)
1656	Design an Ordered Stream	Python, Java	insert: O(1) amortized	O(n)	Easy	design, pointer + bucket array, return the contiguous chunk, `basic`
1670	Design Front Middle Back Queue	Python, Java	O(1) per op	O(n)	Medium	design, good basic, `amazon`	AGAIN** (1) (not start)
1756	Design Recent Used Queue	Python, Java	fetch: O(k)	O(n)	Medium	design, 🔒, move-to-front list or BIT, LC 146	AGAIN
1797	Design Authentication Manager	Python, Java	O(1) amortized	O(n)	Medium	design, hashmap	AGAIN
1976	Number of Ways to Arrive at Destination	Python, Java	O(e logn)*	O(n + e)	Medium	design, Dijkstra + count shortest paths, dp, mod 1e9+7, `graph`

Prefix Sum

#	Title	Solution	Time	Space	Difficulty	Note	Status
1124	Longest Well-Performing Interval	Python, Java	O(n)	O(n)	Medium	prefix sum, hashmap, first idx, Monotonic Stack, good basic, google	AGAIN************* (3)(MUST)

Simulation

#	Title	Solution	Time	Space	Difficulty	Note	Status
874	Walking Robot Simulation	C++ Python	O(n + k)	O(k)	Easy	simulation, hashset of obstacles, direction array, LC 1041, `good basic`

Concurrency

#	Title	Solution	Time	Space	Difficulty	Note
1114	Print in Order	C++ Python	O(n)	O(1)	Easy	concurrency, two semaphores (or CountDownLatch), enforce sequential ordering
1115	Print FooBar Alternately	C++ Python	O(n)	O(1)	Medium	concurrency, two semaphores alternating foo/bar
1116	Print Zero Even Odd	C++ Python	O(n)	O(1)	Medium	concurrency, three semaphores: zero releases even or odd
1117	Building H2O	C++ Python	O(n)	O(1)	Hard	concurrency, two semaphores sized 2 (H) and 1 (O), barrier per molecule
1188	Design Bounded Blocking Queue	C++ Python	O(n)	O(1)	Medium	concurrency, 🔒, BlockingQueue with a Condition, or two counting semaphores, producer-consumer
1195	Fizz Buzz Multithreaded	C++ Python	O(n)	O(1)	Medium	concurrency, four semaphores, one per printer, LC 412
1226	The Dining Philosophers	C++ Python	O(n)	O(1)	Medium	concurrency, resource ordering, or allow at most 4 diners, deadlock avoidance
1242	Web Crawler Multithreaded	C++ Python	O(\|V\| + \|E\|)	O(\|V\|)	Medium	concurrency, 🔒, BFS, DFS, thread pool
1279	Traffic Light Controlled Intersection	C++ Python	O(n)	O(1)	Easy	concurrency, 🔒, single lock guarding the shared green-light state

SQL

#	Title	Solution	Time	Space	Difficulty	Note	Status
0175	Combine Two Tables	MySQL	O(m + n)	O(m + n)	Easy	sql, left join on two tables, `good basic`	OK (4)
0176	Second Highest Salary	MySQL	O(n)	O(1)	Easy	sql, `amazon`, `fb`	OK (4)
0177	Nth Highest Salary	MySQL	O(n^2)	O(n)	Medium	sql, `MySQL PROCEDURE`, `SQL FAQ`	AGAIN*** (3)
0178	Rank Scores	MySQL	O(n^2)	O(n)	Medium	sql, `trick`, `SQL var`, ank, select same table, rank	AGAIN****** (7)
0180	Consecutive Numbers	MySQL	O(n)	O(n)	Medium	sql, `SQL var`, join same table multi times	OK* (3)
0181	Employees Earning More Than Their Managers	MySQL	O(n^2)	O(1)	Easy	sql, self join on manager id, `good basic`	OK
0182	Duplicate Emails	MySQL	O(n^2)	O(n)	Easy	sql, group by + having count > 1, `good basic`	OK
0183	Customers Who Never Order	MySQL	O(n^2)	O(1)	Easy	sql, left join	OK** (3) (but again)
0184	Department Highest Salary	MySQL	O(n^2)	O(n)	Medium	sql, `trick`, where, good basic	OK***** (but again) (6)
0185	Department Top Three Salaries	MySQL	O(n^2)	O(n)	Hard	sql, similiar as `# 0177 Nth Highest Salary`, good trick, top N in group	AGAIN******** (5)
0196	Delete Duplicate Emails	MySQL	O(n^2)	O(n)	Easy	sql, DELETE with self join, good basic, `SQL FAQ`	AGAIN (2)
0197	Rising Temperature	MySQL	O(n^2)	O(n)	Easy	sql, self join on date - 1, or LAG window function	OK
0262	Trips and Users	MySQL	O((t u) + tlogt)*	O(t)	Hard	sql, case condisions	OK* (2)
0511	Game Play Analysis I	MySQL	O(n)	O(n)	Easy	sql, 🔒	OK
0512	Game Play Analysis II	MySQL	O(n)	O(n)	Easy	sql, 🔒, `2 col where` `good trick`	AGAIN**(4)
0534	Game Play Analysis III	MySQL	O(nlogn)	O(n)	Medium	sql, 🔒, `SQL var`, `tricky`, window func	AGAIN******** (6)
0550	Game Play Analysis IV	MySQL	O(n)	O(n)	Medium	sql, case, fraction, join, good basic, where min condition	OK*********** (7) (but again)
569	Median Employee Salary	MySQL	O(n)	O(n)	Medium	sql, medium number	AGAIN* (not start)
571	Find Median Given Frequency of Numbers	MySQL	O(n)	O(n)	Medium	sql, medium number	AGAIN** (not start) (2)
577	Employee Bonus	MySQL	O(n)	O(n)	Easy	sql, left join + where is null or condition, `good basic`	OK
578	Get Highest Answer Rate Question	MySQL	O(m + n)	O(n)	Medium	sql, `fb`, case condition	OK* (3)
579	Find Cumulative Salary of an Employee	MySQL	O(n^2)	O(n)	Hard	sql, window function, `amazon`, good trick	OK***** (5)(but again)
580	Count Student Number in Departments	MySQL	O(m + n)	O(n)	Medium	sql, 🔒, left join + group by, count students per department	OK*** (2) (but again)
595	Big Countries	MySQL	O(n)	O(n)	Easy	sql, simple where with an OR condition, `basic`	OK
597	Friend Requests I: Overall Acceptance Rate	MySQL	O(n)	O(n)	Medium	sql, select distinct, ifnull, `fb`	OK*** (7)
601	Human Traffic of Stadium	MySQL	O(n)	O(n)	Hard	sql, consecutive nums	OK* (1)
602	Friend Requests II: Who Has Most Friend?	MySQL	O(n)	O(n)	Medium	sql, union all, `fb`	OK*** (4)
603	Consecutive Available Seats	MySQL	O(n^2)	O(n)	Medium	sql, consecutive nums, good basic, cross join	OK***** (3)(but again)
618	Students Report By Geography	MySQL	O(nlogn)	O(n)	Hard	sql, sql var	AGAIN (not start)
620	Not Boring Movies	MySQL	O(n)	O(n)	Easy	sql, simple where + order by, `basic`	OK
626	Exchange Seats	MySQL	O(n)	O(n)	Medium	sql, CASE on id parity, count total rows, good trick	AGAIN (not start)
627	Swap Salary	MySQL	O(n)	O(n)	Easy	sql, UPDATE with CASE, good basic	OK* (1) (but again)
1045	Customers Who Bought All Products	MySQL	O(n + k)	O(n + k)	Medium	sql, 🔒, where with count	OK*** (4)
1050	Actors and Directors Who Cooperated At Least Three Times	MySQL	O(n)	O(n)	Easy	sql, 🔒	OK
1068	Product Sales Analysis I	MySQL	O(m + n)	O(m + n)	Easy	sql, 🔒	OK
1069	Product Sales Analysis II	MySQL	O(n)	O(n)	Easy	sql, 🔒	OK
1070	Product Sales Analysis III	MySQL	O(n)	O(n)	Medium	sql, 🔒	OK
1075	Project Employees I	MySQL	O(m + n)	O(m + n)	Easy	sql, 🔒	OK
1076	Project Employees II	MySQL	O(n)	O(n)	Easy	sql, 🔒	OK (2)
1077	Project Employees III	MySQL	O((m + n)^2)	O(m + n)	Medium	sql, 🔒, `tricky where`	OK***** (2) (but again)
1082	Sales Analysis I	MySQL	O(n)	O(n)	Easy	sql, 🔒, group by only 1 col, good basic	OK***** (5) (again)
1083	Sales Analysis II	MySQL	O(m + n)	O(m + n)	Easy	sql, 🔒, not in	OK*** (3) (again)
1084	Sales Analysis III	MySQL	O(m + n)	O(m + n)	Easy	sql, 🔒	OK** (2)
1097	Game Play Analysis V	MySQL	O(n^2)	O(n)	Hard	sql, 🔒, `good basic`, `retention`, case condition, data_add, left join	AGAIN******* (8)
1098	Unpopular Books	MySQL	O(m + n)	O(n)	Medium	sql, 🔒, where not in	OK* (2)
1107	New Users Daily Count	MySQL	O(n)	O(n)	Medium	sql, 🔒	OK*
1112	Highest Grade For Each Student	MySQL	O(nlogn)	O(n)	Medium	sql, 🔒, good basic	OK** (4)
1113	Reported Posts	MySQL	O(n)	O(n)	Easy	sql, 🔒	OK
1126	Active Businesses	MySQL	O(n)	O(n)	Medium	sql, 🔒, `good basic`	AGAIN*** (3)
1127	User Purchase Platform	MySQL	O(n)	O(n)	Hard	sql, 🔒, `complex`, union all, case	OK**** (6) (but again)
1132	Reported Posts II	MySQL	O(m + n)	O(n)	Medium	sql, 🔒	OK*** (4)
1141	User Activity for the Past 30 Days I	MySQL	O(n)	O(n)	Easy	sql, 🔒	AGAIN* (2)
1142	User Activity for the Past 30 Days II	MySQL	O(n)	O(n)	Easy	sql, 🔒	AGAIN* (1)
1148	Article Views I	MySQL	O(nlogn)	O(n)	Easy	sql, 🔒	OK
1149	Article Views II	MySQL	O(nlogn)	O(n)	Medium	sql, 🔒	OK*
1158	Market Analysis I	MySQL	O(m + n)	O(m + n)	Medium	sql, 🔒, `case when ... is not null`	AGAIN**** (5)
1159	Market Analysis II	MySQL	O(m + n)	O(m + n)	Hard	sql, 🔒, good trick	AGAIN** (3)
1164	Product Price at a Given Date	MySQL	O(mlogn)	O(m)	Medium	sql, 🔒, good trick, union, case, not in a period	AGAIN******** (2)
1173	Immediate Food Delivery I	MySQL	O(n)	O(1)	Easy	sql, 🔒	OK
1174	Immediate Food Delivery II	MySQL	O(n)	O(m)	Medium	sql, 🔒	OK**** (3)
1179	Reformat Department Table	MySQL	O(n)	O(n)	Easy	sql, 🔒	OK
1193	Monthly Transactions I	MySQL	O(n)	O(n)	Medium	sql, 🔒	OK (2)
1194	Tournament Winners	MySQL	O(m + n + nlogn)	O(m + n)	Hard	sql, 🔒	AGAIN*
1204	Last Person to Fit in the Elevator	MySQL	O(nlogn)	O(n)	Medium	sql, 🔒, cumsum, window func, good basic	OK****** (3)
1205	Monthly Transactions II	MySQL	O(n)	O(n)	Medium	sql, 🔒, good trick, union all, case conditions	OK*** (3) (but again)
1211	Queries Quality and Percentage	MySQL	O(n)	O(n)	Easy	sql, avg with CASE, round, `good basic`	OK
1212	Team Scores in Football Tournament	MySQL	O(nlogn)	O(n)	Medium	sql, good trick	OK** (2)
1225	Report Contiguous Dates	MySQL	O(nlogn)	O(n)	Hard	sql, 🔒	AGAIN* (2) (not start)
1241	Number of Comments per Post	MySQL	O(n)	O(n)	Easy	sql, 🔒, good basic	OK**** (5) (but again)
1251	Average Selling Price	MySQL	O(n)	O(n)	Easy	sql, 🔒, good basic	OK*** (but again) (3)
1264	Page Recommendations	MySQL	O(m + n)	O(m)	Medium	sql, 🔒	OK** (2)
1270	All People Report to the Given Manager	MySQL	O(n)	O(n)	Medium	sql, 🔒, good basic	OK* (3)
1280	Students and Examinations	MySQL	O((m n) * log(m * n))*	O(m n)*	Easy	sql, 🔒	OK (1)
1285	Find the Start and End Number of Continuous Ranges	MySQL	O(n)	O(n)	Medium	sql, 🔒, variable	AGAIN (not start) (1)
1294	Weather Type in Each Country	MySQL	O(m + n)	O(n)	Easy	sql, 🔒	OK (1)
1303	Find the Team Size	MySQL	O(n)	O(n)	Easy	sql, 🔒	ok (1)
1308	Running Total for Different Genders	MySQL	O(nlogn)	O(n)	Medium	sql, 🔒, good basic, union all, window func	OK**** (3)
1321	Restaurant Growth	MySQL	O(n^2)	O(n)	Medium	sql, 🔒, good basic, trick	AGAIN***** (3)
1322	Ads Performance	MySQL	O(nlogn)	O(n)	Easy	sql, 🔒, good basic	OK******* (3) (but again)
1327	List the Products Ordered in a Period	MySQL	O(n)	O(n)	Easy	sql, 🔒	OK
1336	Number of Transactions per Visit	MySQL	O(m + n)	O(m + n)	Medium	sql, 🔒, complex, variable	AGAIN (not start)
1341	Movie Rating	MySQL	O(nlogn)	O(n)	Medium	sql, 🔒	AGAIN (not start)
1350	Students With Invalid Departments	MySQL	O(n)	O(n)	Easy	sql, 🔒	OK
1355	Activity Participants	MySQL	O(n)	O(n)	Medium	sql, 🔒	OK
1364	Number of Trusted Contacts of a Customer	MySQL	O(n + m + l + nlogn)	O(n + m + l)	Medium	sql, 🔒	OK
1369	Get the Second Most Recent Activity	MySQL	O(nlogn)	O(n)	Hard	sql, 🔒, union, having	OK** (2) (but again)
1378	Replace Employee ID With The Unique Identifier	MySQL	O(n)	O(n)	Easy	sql, 🔒	OK
1384	Total Sales Amount by Year	MySQL	O(nlogn)	O(n)	Hard	sql, 🔒, complex	OK*
1393	Capital Gain/Loss	MySQL	O(n)	O(n)	Medium	sql, 🔒	OK
1398	Customers Who Bought Products A and B but Not C	MySQL	O(m + n)	O(m + n)	Medium	sql, 🔒	OK
1407	Top Travellers	MySQL	O(m + nlogn)	O(m + n)	Easy	sql, 🔒, ifnull	OK*
1412	Find the Quiet Students in All Exams	MySQL	O(m + nlogn)	O(m + n)	Hard	sql, 🔒, good trick, not in, inner join on multiple cols	OK**** (3) (but again)
1421	NPV Queries	MySQL	O(n)	O(n)	Medium	sql, 🔒	OK
1435	Create a Session Bar Chart	MySQL	O(n)	O(1)	Easy	sql, 🔒	OK
1440	Evaluate Boolean Expression	MySQL	O(n)	O(n)	Medium	sql, 🔒	OK
1445	Apples & Oranges	MySQL	O(n)	O(n)	Medium	sql, 🔒	OK
1454	Active Users	MySQL	O(nlogn)	O(n)	Medium	sql, 🔒, good basic, window func	AGAIN*** (2)
1459	Rectangles Area	MySQL	O(n^2)	O(n^2)	Medium	sql, 🔒	OK*
1468	Calculate Salaries	MySQL	O(m + n)	O(m + n)	Easy	sql, 🔒	OK*
1479	Sales by Day of the Week	MySQL	O(m + n)	O(n)	Hard	sql, 🔒, case, DAYOFWEEK	OK
1484	Group Sold Products By The Date	MySQL	O(nlogn)	O(n)	Easy	sql, 🔒, `GROUP_CONCAT`	OK*
1495	Friendly Movies Streamed Last Month	MySQL	O(n)	O(n)	Easy	sql, 🔒	OK
1501	Countries You Can Safely Invest In	MySQL	O(n)	O(n)	Medium	sql, 🔒, good trick, group avg, global avg, SUBSTRING, inner join on `or conditions`	OK****** (4)
1511	Customer Order Frequency	MySQL	O(n)	O(n)	Easy	sql, 🔒, having, like	OK* (1)
1517	Find Users With Valid E-Mails	MySQL	O(n)	O(n)	Easy	sql, 🔒, regular expression	AGAIN
1527	Patients With a Condition	MySQL	O(n)	O(n)	Easy	sql, 🔒	OK
1532	The Most Recent Three Orders	MySQL	O(nlogn)	O(n)	Medium	sql, 🔒, window func, good basic	OK** (2) (but again)
1543	Fix Product Name Format	MySQL	O(nlogn)	O(n)	Easy	sql, 🔒, trim	OK
1549	The Most Recent Orders for Each Product	MySQL	O(nlogn)	O(n)	Medium	sql, 🔒, max day per id, good basic	OK* (again)
1555	Bank Account Summary	MySQL	O(m + n)	O(m + n)	Medium	sql, 🔒, union all, good trick	OK**** (2)(but again)
1565	Unique Orders and Customers Per Month	MySQL	O(n)	O(n)	Easy	sql, 🔒	OK
1571	Warehouse Manager	MySQL	O(n)	O(n)	Medium	sql, 🔒	OK* (1)
1581	Customer Who Visited but Did Not Make Any Transactions	MySQL	O(n)	O(n)	Easy	sql, 🔒, left join, good basic	OK** (2)(but again)
1587	Bank Account Summary II	MySQL	O(m + n)	O(m + n)	Easy	sql, 🔒	OK
1596	The Most Frequently Ordered Products for Each Customer	MySQL	O(n)	O(n)	Medium	sql, 🔒, good trick, `rank() ORDER BY count`, get max in each group	OK********* (3)(but again)
1607	Sellers With No Sales	MySQL	O(nlogm)	O(n + m)	Medium	sql, 🔒	OK
1613	Find the Missing IDs	MySQL	O(n^2)	O(n)	Medium	sql, 🔒, `RECURSIVE CTE`, good trick	AGAIN*** (2)
1623	All Valid Triplets That Can Represent a Country	MySQL	O(n^3)	O(n^3)	Easy	sql, 🔒, `<>`	AGAIN* (not start)
1633	Percentage of Users Attended a Contest	MySQL	O(m + nlogn)	O(n)	Easy	sql, 🔒	OK
1635	Hopper Company Queries I	MySQL	O(d + r + tlogt)	O(d + r + t)	Hard	sql, 🔒, complex	AGAIN (not start)
1645	Hopper Company Queries II	MySQL	O(d + r + tlogt)	O(d + r + t)	Hard	sql, 🔒, complex	AGAIN (not start)
1651	Hopper Company Queries III	MySQL	O(d + r + tlogt)	O(d + r + t)	Hard	sql, 🔒, complex	AGAIN (not start)
1661	Average Time of Process per Machine	MySQL	O(n)	O(n)	Easy	sql, 🔒, good basic	OK* (1)
1667	Fix Names in a Table	MySQL	O(nlogn)	O(n)	Easy	sql, 🔒, concat, sub-string	OK (1)
1677	Product’s Worth Over Invoices	MySQL	O(nlogn)	O(n)	Easy	sql, 🔒	OK
1683	Invalid Tweets	MySQL	O(n)	O(n)	Easy	sql, 🔒	OK
1693	Daily Leads and Partners	MySQL	O(n)	O(n)	Easy	sql, 🔒	OK
1699	Number of Calls Between Two Persons	MySQL	O(n)	O(n)	Medium	sql, 🔒, cte, union all	OK* (1)(but again)
1709	Biggest Window Between Visits	MySQL	O(nlogn)	O(n)	Medium	sql, 🔒, window func, datediff, coalesce, good trick	AGAIN***** (2)
1715	Count Apples and Oranges	MySQL	O(n)	O(n)	Medium	sql, 🔒, ifnull	OK (2)
1729	Find Followers Count	MySQL	O(nlogn)	O(n)	Easy	sql, 🔒	OK
1731	The Number of Employees Which Report to Each Employee	MySQL	O(nlogn)	O(n)	Easy	sql, 🔒	OK
1741	Find Total Time Spent by Each Employee	MySQL	O(nlogn)	O(n)	Easy	sql, 🔒	OK
1747	Leetflex Banned Accounts	MySQL	O(n^2)	O(n)	Medium	sql, 🔒	OK (2) (but again)
1757	Recyclable and Low Fat Products	MySQL	O(n)	O(n)	Easy	sql, 🔒	OK
1767	Find the Subtasks That Did Not Execute	MySQL	O(n c)*	O(n c)*	Hard	sql, 🔒, `RECURSIVE CTE`	AGAIN (not start)
1777	Product’s Price for Each Store	MySQL	O(n)	O(n)	Easy	sql, 🔒	OK (2)
1783	Grand Slam Titles	MySQL	O(n)	O(n)	Medium	sql, 🔒, union all, good basic	OK* (1) (but again)
1789	Primary Department for Each Employee	MySQL	O(n)	O(n)	Easy	sql, 🔒, where id in	OK (1) (but again)
1795	Rearrange Products Table	MySQL	O(n)	O(n)	Easy	sql, 🔒, union all, good basic	OK****** (2)
1809	Ad-Free Sessions	MySQL	O(n)	O(n)	Easy	sql, 🔒, left join where col is null	OK**** (3)(but again)
1811	Find Interview Candidates	MySQL	O(nlogn)	O(n)	Medium	sql, 🔒, union all, cte, where, `good basic`	AGAIN* (2)
1821	Find Customers With Positive Revenue this Year	MySQL	O(n)	O(n)	Easy	sql, 🔒	OK
1831	Maximum Transaction Each Day	MySQL	O(nlogn)	O(n)	Medium	sql, 🔒, rank(), where on 2 attr, good basic	AGAIN**** (2)
1841	League Statistics	MySQL	O(nlogn)	O(n)	Medium	sql, 🔒, case, complex	AGAIN* (2)
1843	Suspicious Bank Accounts	MySQL	O(nlogn)	O(n)	Medium	sql, 🔒, good trick, window func	OK** (2) (but AGAIN)
1853	Convert Date Format	MySQL	O(n)	O(n)	Easy	sql, 🔒	OK
1867	Orders With Maximum Quantity Above Average	MySQL	O(n)	O(n)	Easy	sql, 🔒	OK (1)
1873	Calculate Special Bonus	MySQL	O(n)	O(n)	Easy	sql, 🔒, left, odd num	OK (2)
1875	Group Employees of the Same Salary	MySQL	O(nlogn)	O(n)	Medium	sql, 🔒, complex, good basic	AGAIN* (1)
1890	The Latest Login in 2020	MySQL	O(n)	O(n)	Easy	sql, 🔒	OK
1892	Page Recommendations II	MySQL	O(n m)*	O(n m)*	Hard	sql, 🔒, join on friendship + page likes, `not in`	AGAIN*** (1) (not start)
1907	Count Salary Categories	MySQL	O(n)	O(n)	Medium	sql, 🔒, union all	OK* (1)
1917	Leetcodify Friends Recommendations	MySQL	O(n^2)	O(n^2)	Hard	sql, 🔒, `NOT EXISTS`	AGAIN*** (2)
1919	Leetcodify Similar Friends	MySQL	O(n l)*	O(n l)*	Hard	sql, 🔒	AGAIN* (not start)
1934	Confirmation Rate	MySQL	O(n + m)	O(n + m)	Medium	sql, 🔒, good basic	OK* (1) (but again)
1939	Users That Actively Request Confirmation Messages	MySQL	O(nlogn)	O(n)	Easy	sql, 🔒, TIMESTAMPDIFF, good basic	OK*** (2) (but again)
1949	Strong Friendship	MySQL	O(n^3)	O(n^2)	Medium	sql, 🔒, good basic, self join, where on 2 cols	OK* (2) (but again)
1951	All the Pairs With the Maximum Number of Common Followers	MySQL	O(n^3)	O(n^2)	Medium	sql, 🔒, good trick	AGAIN* (2)
1965	Employees With Missing Information	MySQL	O(nlogn)	O(n)	Easy	sql, 🔒, good basic	OK* (1) (but again)
1972	First and Last Call On the Same Day	MySQL	O(n)	O(n)	Hard	sql, 🔒, rank, union all, amazon	AGAIN** (2)
1978	Employees Whose Manager Left the Company	MySQL	O(nlogn)	O(n)	Easy	sql, 🔒	OK (1)
1988	Find Cutoff Score for Each School	MySQL	O(n m)*	O(n m)*	Medium	sql, 🔒, no ref	AGAIN (not start)
1990	Count the Number of Experiments	MySQL	O(n)	O(n)	Easy	sql, 🔒, cross join	AGAIN* (1)
2004	The Number of Seniors and Juniors to Join the Company	MySQL	O(nlogn)	O(n)	Hard	sql, 🔒, not ref	AGAIN (not start)

Shell Script

#	Title	Solution	Time	Space	Difficulty	Note
0192	Word Frequency	Shell	O(n)	O(k)	Medium	shell, awk word count then sort by frequency, `good basic`
0193	Valid Phone Numbers	Shell	O(n)	O(1)	Easy	shell, grep with a regular expression, `basic`
0194	Transpose File	Shell	O(n^2)	O(n^2)	Medium	shell, awk transpose with a 2D array, `good trick`
0195	Tenth Line	Shell	O(n)	O(1)	Easy	shell, sed / awk NR == 10, `basic`

Newly Added (kamyu104 gap)

Problems imported from the coverage audit against kamyu104/LeetCode-Solutions.

These solutions were generated from the problem statements and have not been run against LeetCode’s judge. Treat them as drafts to review, not as verified-accepted answers.

Ranges covered so far: 0001-1000 — 127 problems.

Array

#	Title	Solution	Time	Space	Difficulty	Note
0158	Read N Characters Given read4 II - Call Multiple Times	Python, Java	O(n) per read() call	O(1)	Hard	array, 🔒, LC 157, carry the read4 buffer across calls, `good trick`, google, fb, new
0296	Best Meeting Point	Python, Java	O(m n)*	O(m n)*	Hard	array, 🔒, median of the row and column coordinates, Manhattan distance, `math`, `good trick`, new
0485	Max Consecutive Ones	Python, Java	O(n)	O(1)	Easy	array, running count of consecutive 1s, LC 487, LC 1004, `basic`, new
0487	Max Consecutive Ones II	Python, Java	O(n)	O(1)	Medium	array, 🔒, sliding window allowing one flip, LC 1004, `good basic`, new
0495	Teemo Attacking	Python, Java	O(n)	O(1)	Easy	array, sum of min(duration, gap), interval merge, `basic`, new
0644	Maximum Average Subarray II	Python, Java	O(n log((max - min) / eps))*	O(1)	Hard	array, 🔒, binary search on the real-valued average + prefix sum, LC 643, `good trick`, new
0732	My Calendar III	Python, Java	O(n log n) per book call	O(n)	Hard	array, LC 729, LC 731, sweep line over a TreeMap of deltas, segment tree, `good trick`, new
0803	Bricks Falling When Hit	Python, Java	O(m n * alpha + h * alpha)*	O(m n)*	Hard	array, reverse thinking, union find with a virtual roof node, LC 200, `complex`, new
0927	Three Equal Parts	Python, Java	O(n)	O(1)	Hard	array, count the ones then match the suffix blocks, `good trick`, new
0952	Largest Component Size by Common Factor	Python, Java	O(n sqrt(M) * a(M))*	O(M)	Hard	array, union find over prime factors, sieve, `complex`, new
0985	Sum of Even Numbers After Queries	Python, Java	O(n + m)	O(1) excluding the output list	Medium	array, maintain a running even-sum and apply deltas, `good basic`, new
0999	Available Captures for Rook	Python, Java	O(m n)*	O(1)	Easy	array, scan the 4 directions from the rook, `matrix`, `basic`, new

Backtracking

#	Title	Solution	Time	Space	Difficulty	Note
0291	Word Pattern II	Python, Java	O(n^m)	O(m + n)	Medium	backtracking, 🔒, LC 290, backtracking with a bijection map, `good trick`, new
0488	Zuma Game	Python, Java	O(states len(hand) * len(board) * len(board))*	O(states (len(board) + len(hand)))*	Hard	backtracking, backtracking + memo over (board, hand), `complex`, new
0679	24 Game	Python, Java	O(1) – bounded search: at most 12 4 * 6 * 4 * 2 * 4 states for 4 cards*	O(1)	Hard	backtracking, backtrack over every pair and operator, float epsilon compare, `good trick`, new
0996	Number of Squareful Arrays	Python, Java	O(n!) worst case	O(n)	Hard	backtracking, permutations with perfect-square adjacency pruning, dedupe same-layer, new

Binary Heap

#	Title	Solution	Time	Space	Difficulty	Note
0480	Sliding Window Median	Python, Java	O(n k)*	O(k)	Hard	heap, two heaps with lazy deletion, or SortedList, LC 295, LC 239, `good trick`, new
0632	Smallest Range Covering Elements from K Lists	Python, Java	O(n log(k))*	O(k)	Hard	heap, min-heap over k list pointers while tracking the current max, LC 23, `good trick`, new
0857	Minimum Cost to Hire K Workers	Python, Java	O(n log(n))*	O(n)	Hard	heap, sort by wage/quality ratio + max-heap of quality, `greedy`, `good trick`, new

Binary Search

#	Title	Solution	Time	Space	Difficulty	Note
0302	Smallest Rectangle Enclosing Black Pixels	Python, Java	O(m log n + n * log m)*	O(1)	Hard	binary search, 🔒, binary search each border of the black region, LC 200, `good trick`, new
0354	Russian Doll Envelopes	Python, Java	O(n log n)	O(n)	Hard	binary search, sort by width asc / height desc then LIS, LC 300, `good trick`, new
0363	Max Sum of Rectangle No Larger Than K	Python, Java	O(m^2 n * log n)*	O(n)	Hard	binary search, prefix sum per row band + sorted set binary search, LC 304, new
0668	Kth Smallest Number in Multiplication Table	Python, Java	O(m log(m * n))*	O(1)	Hard	binary search, binary search on the answer, count values <= x, `good trick`, new
0702	Search in a Sorted Array of Unknown Size	Python, Java	O(log M)	O(1)	Medium	binary search, 🔒, exponential search then binary search, ArrayReader API, new
0719	Find K-th Smallest Pair Distance	Python, Java	O(n log n + n log W)	O(1) (ignoring the sort)	Hard	binary search, binary search on the distance + two pointers to count pairs, `good trick`, new
0786	K-th Smallest Prime Fraction	Python, Java	O(n + k log(n))*	O(n)	Medium	binary search, binary search on the fraction value, or a heap of k candidates, new
0793	Preimage Size of Factorial Zeroes Function	Python, Java	O(log(k) log(k))*	O(1)	Hard	binary search, binary search on the trailing-zero count, LC 172, new
0878	Nth Magical Number	Python, Java	O(log(n min(a, b)))*	O(1)	Hard	binary search, binary search + inclusion-exclusion with lcm(a, b), `math`, new

Binary Search Tree

#	Title	Solution	Time	Space	Difficulty	Note	Status
0352	Data Stream as Disjoint Intervals	Python, Java	O(log n) search + O(n) list shift per addNum	O(n)	Hard	BST, sorted interval list with binary search insert, TreeMap, LC 228, LC 715, new

Bit Manipulation

#	Title	Solution	Time	Space	Difficulty	Note	Status
0411	Minimum Unique Word Abbreviation	Python, Java	O(2^m n)*	O(n)	Hard	bit manipulation, 🔒, bit-mask each word then enumerate abbreviation masks, LC 320, LC 408, new
0476	Number Complement	Python, Java	O(1)	O(1)	Easy	bit manipulation, flip the bits below the highest set bit, LC 190, `basic`, new

Breadth-First Search

#	Title	Solution	Time	Space	Difficulty	Note
0499	The Maze III	Python, Java	O(m nlog(m * n))*	O(m n)*	Hard	bfs, duplicate of the BFS entry, Dijkstra with the lexicographically smallest path, new
0773	Sliding Puzzle	Python, Java	O(6! 6) -> effectively O(1)*	O(6!)	Hard	bfs, BFS over board states encoded as strings, LC 752, `good trick`, new
0854	K-Similar Strings	Python, Java	O(n n! / …) in theory*	O(number of visited states)	Hard	bfs, BFS over swap states with pruning, `good trick`, new
0882	Reachable Nodes In Subdivided Graph	Python, Java	O(E log(E))*	O(N + E)	Hard	bfs, Dijkstra on the original graph then count the subdivided nodes, `good trick`, new
0913	Cat and Mouse	Python, Java	O(n^3)	O(n^2)	Hard	bfs, minimax BFS over (mouse, cat, turn) states, game theory, `complex`, new

Depth-First Search

#	Title	Solution	Time	Space	Difficulty	Note	Status
0440	K-th Smallest in Lexicographical Order	Python, Java	O(log(n)^2)	O(1)	Hard	dfs, count nodes in each 10-ary prefix subtree, LC 386, `good trick`, new
0749	Contain Virus	Python, Java	O((mn)^2) — at most O(mn) rounds	O(mn)*	Hard	dfs, simulate rounds, dfs each region, count walls, `complex`, new

Design

#	Title	Solution	Time	Space	Difficulty	Note
0381	Insert Delete GetRandom O(1) - Duplicates allowed	Python, Java	O(1) average for insert / remove / getRandom	O(n)	Hard	design, LC 380, hashmap of value -> index set + swap-with-last, `good trick`, new
0604	Design Compressed String Iterator	Python, Java	O(n) for init* (n = len(compressedString))*	O(n)	Easy	design, 🔒, decode (char, count) pairs lazily, iterator, LC 900, new
0631	Design Excel Sum Formula	Python, Java	O(1) for set	O(height width)*	Hard	design, 🔒, formula dependency graph, recompute on set, `complex`, new
0641	Design Circular Deque	Python, Java	O(1) per operation	O(k)	Medium	design, circular buffer or doubly linked list, LC 622, `good basic`, new
0715	Range Module	Python, Java	O(n) per addRange / removeRange (list splice)	O(n)	Hard	design, sorted interval list, LC 352, TreeMap, segment tree, new

Dynamic Programming

#	Title	Solution	Time	Space	Difficulty	Note
0087	Scramble String	Python, Java	O(n^4)	O(n^3)	Hard	dp, 3D interval dp / memoized recursion on (i, j, len), new
0174	Dungeon Game	Python, Java	O(m n)*	O(m n)*	Hard	dp, 2D dp filled from the bottom-right, track the min health needed, `good trick`, new
0188	Best Time to Buy and Sell Stock IV	Python, Java	O(n k)*	O(k)	Hard	dp, LC 122, LC 123, k-transaction stock dp, state machine, new
0446	Arithmetic Slices II - Subsequence	Python, Java	O(n^2)	O(n^2)	Hard	dp, dp with a hashmap of (index, diff), LC 413, `good trick`, new
0465	Optimal Account Balancing	Python, Java	O(m!)	O(m)	Hard	dp, 🔒, net balances then backtracking over settlements, bitmask dp, new
0466	Count The Repetitions	Python, Java	O(len(s1) len(s2) + n1)*	O(len(s2))	Hard	dp, find the repeating pattern of s2 matches inside s1, `good trick`, new
0514	Freedom Trail	Python, Java	O(m n^2)*	O(n)	Hard	dp, dp over (ring position, key index), precompute char positions, new
0546	Remove Boxes	Python, Java	O(n^4)	O(n^3)	Hard	dp, 3D interval dp on (l, r, k attached same-colour boxes), new
0568	Maximum Vacation Days	Python, Java	O(k n^2)*	O(n)	Hard	dp, 🔒, dp over (week, city) with a flight matrix, new
0600	Non-negative Integers without Consecutive Ones	Python, Java	O(32) = O(1)	O(32) = O(1)	Hard	dp, digit dp with Fibonacci counts, bit manipulation, `good trick`, new
0629	K Inverse Pairs Array	Python, Java	O(n k)*	O(k)	Hard	dp, dp on (n, k) with a sliding-window prefix sum, mod 1e9+7, new
0656	Coin Path	Python, Java	O(n maxJump)*	O(n)	Hard	dp, 🔒, dp from the back, lexicographically smallest path, new
0664	Strange Printer	Python, Java	O(n^3)	O(n^2)	Hard	dp, interval dp, print the leftmost char across the whole range, new
0689	Maximum Sum of 3 Non-Overlapping Subarrays	Python, Java	O(n)	O(n)	Hard	dp, prefix sums + best-left / best-right arrays, LC 643, new
0691	Stickers to Spell Word	Python, Java	O(2^L n * L)*	O(2^L)	Hard	dp, bitmask dp over target letters, BFS alternative, new
0730	Count Different Palindromic Subsequences	Python, Java	O(n^2 4) (the lo/hi scan is bounded by the 4-letter alphabet in practice)*	O(n^2)	Hard	dp, interval dp counting distinct palindromic subsequences, mod, new
0741	Cherry Pickup	Python, Java	O(n^3)	O(n^2)	Hard	dp, two walkers moving simultaneously, 3D dp, LC 1463, new
0805	Split Array With Same Average	Python, Java	O(2^(n/2))	O(2^(n/2))	Hard	dp, meet in the middle over subset sums, `good trick`, new
0818	Race Car	Python, Java	O(target log(target))*	O(target)	Hard	dp, BFS over (position, speed) or dp, `good trick`, new
0847	Shortest Path Visiting All Nodes	Python, Java	O(n^2 2^n)*	O(n 2^n)*	Hard	dp, bitmask BFS/dp over visited sets, TSP-style, `good trick`, new
0879	Profitable Schemes	Python, Java	O(m n * minProfit)*	O(n minProfit)*	Hard	dp, 3D knapsack dp on (members, profit), mod, new
0903	Valid Permutations for DI Sequence	Python, Java	O(n^2)	O(n)	Hard	dp, dp on (index, number of smaller values used), prefix sums, new
0920	Number of Music Playlists	Python, Java	O(goal n)*	O(goal n)*	Hard	dp, combinatorial dp on (songs used, distinct songs), mod, new
0940	Distinct Subsequences II	Python, Java	O(26 n)*	O(26)	Hard	dp, dp with last-occurrence subtraction per letter, mod, new
0943	Find the Shortest Superstring	Python, Java	O(n^2 2^n + n^2 * L)*	O(n 2^n)*	Hard	dp, bitmask dp (TSP) over pairwise overlap, reconstruct the path, new
0956	Tallest Billboard	Python, Java	O(n S)*	O(S)	Hard	dp, dp over the height difference between the two supports, new
0960	Delete Columns to Make Sorted III	Python, Java	O(m^2 n)*	O(m)	Hard	dp, LIS over columns, LC 300, LC 944, LC 955, new
0964	Least Operators to Express Number	Python, Java	O(log_x(target) ^ 2) states	O(number of memoized states)	Hard	dp, memoized recursion over base-x digits, `complex`, new
0975	Odd Even Jump	Python, Java	O(n log n)	O(n)	Hard	dp, monotonic stack / TreeMap for the next greater-or-equal, dp from the back, new

Graph

#	Title	Solution	Time	Space	Difficulty	Note
0924	Minimize Malware Spread	Python, Java	O(n^2 alpha(n))*	O(n)	Hard	graph, union find, pick the node in the largest single-infected component, new
0928	Minimize Malware Spread II	Python, Java	O(n^2 alpha(n))*	O(n^2)	Hard	graph, union find over clean nodes, count each infected node’s exclusive reach, new
0959	Regions Cut By Slashes	Python, Java	O(n^2 a(n^2))*	O(n^2)	Medium	graph, split each cell into 4 triangles then union find, LC 200, new

Greedy

#	Title	Solution	Time	Space	Difficulty	Note
0484	Find Permutation	Python, Java	O(n)	O(n)	Medium	greedy, 🔒, reverse the D runs of the identity permutation, `good trick`, new
0630	Course Schedule III	Python, Java	O(n log(n))*	O(n)	Hard	greedy, sort by deadline + max-heap of durations, LC 1353, `good trick`, new
0757	Set Intersection Size At Least Two	Python, Java	O(n log n)	O(1) (ignoring the sort)	Hard	greedy, sort by end then greedily add the last two points, new
0798	Smallest Rotation with Highest Score	Python, Java	O(n)	O(n)	Hard	greedy, difference array over rotation scores, `good trick`, new
0843	Guess the Word	Python, Java	O(g n^2 * L)*	O(n)	Hard	greedy, minimax guess against the worst-case bucket, interactive, new
0936	Stamping The Sequence	Python, Java	O(n (n - m + 1))*	O(n (n - m + 1))*	Hard	greedy, reverse thinking, un-stamp back to ‘???’, `good trick`, new
0991	Broken Calculator	Python, Java	O(log(target))	O(1)	Medium	greedy, work backwards from the target, halve when even, `good trick`, new
0995	Minimum Number of K Consecutive Bit Flips	Python, Java	O(n)	O(n)	Hard	greedy, sliding window with a flip-difference array, `good trick`, new

Hash Table

#	Title	Solution	Time	Space	Difficulty	Note	Status
0159	Longest Substring with At Most Two Distinct Characters	Python, Java	O(n)	O(1)	Medium	hash table, 🔒, sliding window with at most 2 distinct chars, LC 340, `good basic`, new
0982	Triples with Bitwise AND Equal To Zero	Python, Java	O(n^2 + n 2^16)*	O(2^16)	Hard	hash table, precompute pairwise ANDs into a counter over 2^16 masks, new

Math

#	Title	Solution	Time	Space	Difficulty	Note
0065	Valid Number	Python, Java	O(n)	O(n)	Hard	math, finite state machine, or careful manual parsing, `complex`, new
0233	Number of Digit One	Python, Java	O(log n)	O(1)	Hard	math, count the 1s per digit position, digit dp, `good trick`, new
0248	Strobogrammatic Number III	Python, Java	O(5^(L/2) L)*	O(5^(L/2) L)*	Hard	math, 🔒, LC 246, LC 247, build strobogrammatic numbers per length and compare bounds, new
0391	Perfect Rectangle	Python, Java	O(n)	O(n)	Hard	math, corner-count parity + total area check, hashset, `good trick`, new
0479	Largest Palindrome Product	Python, Java	O(10^n 10^n) worst case*	O(1)	Hard	math, build palindromes downward and test their factors, new
0483	Smallest Good Base	Python, Java	O(log(num)^2 log(num))*	O(1)	Hard	math, enumerate the digit count then binary search the base, new
0492	Construct the Rectangle	Python, Java	O(sqrt(area))	O(1)	Easy	math, walk down from sqrt(area) to the first divisor, `basic`, new
0660	Remove 9	Python, Java	O(log(n))	O(log(n))	Hard	math, 🔒, treat n as a base-9 number, `good trick`, new
0780	Reaching Points	Python, Java	O(log(max(tx, ty)))	O(1)	Hard	math, work backwards with modulo instead of subtraction, `good trick`, new
0782	Transform to Chessboard	Python, Java	O(n^2)	O(1)	Hard	math, check the row patterns then count the minimum swaps, bit manipulation, new
0810	Chalkboard XOR Game	Python, Java	O(n)	O(1)	Hard	math, game theory: the first player wins if xor != 0 or n is even, new
0887	Super Egg Drop	Python, Java	O(k moves)*	O(k)	Hard	math, dp on (eggs, moves) -> max floors, binary search, `good trick`, new
0899	Orderly Queue	Python, Java	O(n^2)	O(n)	Hard	math, k == 1 means try every rotation, otherwise just sort, `trick`, new
0902	Numbers At Most N Given Digit Set	Python, Java	O(m d)*	O(1)	Hard	math, digit dp counting numbers <= n built from a digit set, new
0906	Super Palindromes	Python, Java	O(M^(1/4) log(M))*	O(1)	Hard	math, enumerate palindrome roots and test their squares, new
0972	Equal Rational Numbers	Python, Java	O(L)	O(1)	Hard	math, parse the repeating decimal into an exact fraction, new

Recursion

#	Title	Solution	Time	Space	Difficulty	Note	Status
0761	Special Binary String	Python, Java	O(n^2 log n)	O(n^2) (recursive slices)	Hard	recursion, recursively sort the balanced special substrings, `good trick`, new

Simulation

#	Title	Solution	Time	Space	Difficulty	Note	Status
0874	Walking Robot Simulation	Python, Java	O(sum(commands) + len(obstacles))	O(len(obstacles))	Medium	simulation, duplicate of the Simulation entry, hashset of obstacles, direction array, new

Sort

#	Title	Solution	Time	Space	Difficulty	Note	Status
0937	Reorder Data in Log Files	Python, Java	O(n L * log(n))*	O(n L)*	Medium	sort, duplicate of the Sort entry, custom comparator, letter-logs before digit-logs, new

Stack

#	Title	Solution	Time	Space	Difficulty	Note
0726	Number of Atoms	Python, Java	O(n^2) worst case (n = len(formula)) — merging a popped group is O(distinct atoms)	O(n)	Hard	stack, stack of counters, parse nested parentheses, sort the atoms, new
0736	Parse Lisp Expression	Python, Java	O(n^2) worst case (n = len(expression)) — each level re-slices its body	O(n)	Hard	stack, recursive evaluation with a scope stack, `complex`, new
0770	Basic Calculator IV	Python, Java	O(n T^2)*	O(n + T)	Hard	stack, parse the expression into polynomial terms, `complex`, new

String

#	Title	Solution	Time	Space	Difficulty	Note
0068	Text Justification	Python, Java	O(n maxWidth)*	O(maxWidth) extra (excluding output)	Hard	string, greedy line packing then distribute spaces, `complex`, google, new
0420	Strong Password Checker	Python, Java	O(n)	O(n)	Hard	string, case analysis on length, greedy replace/insert/delete, `complex`, new
0481	Magical String	Python, Java	O(n)	O(n)	Medium	string, generate the magical string with two pointers, `good trick`, new
0500	Keyboard Row	Python, Java	O(L)	O(1)	Easy	string, map each letter to its keyboard row, hashset, `basic`, new
0527	Word Abbreviation	Python, Java	O(n log(n) * L)*	O(n L)*	Hard	string, 🔒, group by (length, first, last) and resolve collisions with a longer prefix, Trie, new
0555	Split Concatenated Strings	Python, Java	O(L (L + n))*	O(L)	Medium	string, try each string reversed or not, then cut at the best position, `good trick`, new
0709	To Lower Case	Python, Java	O(n)	O(n)	Easy	string, shift by 32 or use a lookup table, bit manipulation, `basic`, new
0839	Similar String Groups	Python, Java	O(n^2 m)*	O(n)	Hard	string, union find over similar pairs, LC 200, `good trick`, new

Tree

#	Title	Solution	Time	Space	Difficulty	Note
0428	Serialize and Deserialize N-ary Tree	Python, Java	O(n)	O(n)	Hard	tree, 🔒, LC 297, serialize with child counts or sentinels, dfs / bfs, new
0431	Encode N-ary Tree to Binary Tree	Python, Java	O(n)	O(n)	Hard	tree, 🔒, first-child / next-sibling encoding, `good trick`, new
0558	Logical OR of Two Binary Grids Represented as Quad-Trees	Python, Java	O(n1 + n2)	O(log N)	Medium	tree, recursive quad-tree merge, LC 427, new
0559	Maximum Depth of N-ary Tree	Python, Java	O(n)	O(h)	Easy	tree, LC 104 for n-ary trees, dfs, bfs, `good basic`, new
0589	N-ary Tree Preorder Traversal	Python, Java	O(n)	O(h)	Easy	tree, LC 144 for n-ary trees, iterative with a stack, new
0590	N-ary Tree Postorder Traversal	Python, Java	O(n)	O(h)	Medium	tree, LC 145 for n-ary trees, reverse-preorder trick, new
0685	Redundant Connection II	Python, Java	O(n alpha(n))*	O(n)	Hard	tree, LC 684, union find with the two-parent case, `good trick`, new
0699	Falling Squares	Python, Java	O(n^2)	O(n)	Hard	tree, coordinate compression + segment tree, or an O(n^2) interval scan, new
0850	Rectangle Area II	Python, Java	O(n^2)	O(n)	Hard	tree, sweep line + coordinate compression, segment tree, LC 218, new
0993	Cousins in Binary Tree	Python, Java	O(n)	O(h)	Easy	tree, LC 2641, dfs / bfs tracking depth and parent, `good basic`, new
0998	Maximum Binary Tree II	Python, Java	O(h)	O(h)	Medium	tree, LC 654, insert along the right spine, `good trick`, new

Two Pointers

#	Title	Solution	Time	Space	Difficulty	Note	Status
0862	Shortest Subarray with Sum at Least K	Python, Java	O(n)	O(n)	Hard	two pointers, prefix sum + monotonic deque, LC 209, `good trick`, new