LeetCode Solutions Repository

Curated, well-documented Python 3 solutions for a growing set of LeetCode problems. Every problem folder contains:

• README.md with a concise explanation and complexity notes.
• solution.py with a clean, runnable implementation.

Use this repo as a reusable algorithm notebook or a quick refresher before interviews.

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Repository structure

• ####-problem-name/ — individual problems; run python solution.py to execute the reference implementation.
• 0000-notes/ — reusable study notes and templates grouped by topic.
• LICENSE — repository license details.

Repository map

flowchart TD
    Repo[LeetCode Solutions Repo]
    Repo --> Problems["####-problem-name/"]
    Repo --> Notes["0000-notes/"]
    Repo --> License["LICENSE"]
    Problems --> ProblemReadme["README.md (approach + complexity)"]
    Problems --> ProblemSolution["solution.py (reference implementation)"]
    Notes --> NotesTopics["Topic folders & guides"]

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Notes library highlights

• How to think: reading prompts, choosing algorithms, and avoiding common mistakes.
• Core patterns: arrays, hashing, two pointers, sliding window, binary search, prefix sum, greedy, and dynamic programming.
• Practice helpers: quick references for frequency counting, bucket techniques, interval scheduling, and more.

Notes index

• 00-how-to-think: how to read problems, how to choose algorithms, common mistakes
• 01-array: array basics
• 02-hash-table: hashing basics, frequency count, bucket technique
• 03-two-pointers: two-pointers basics
• 04-sliding-window: fixed window, variable window
• 05-binary-search: binary search on array, binary search on answer
• 06-prefix-sum: prefix sum, difference array
• 07-greedy: greedy thinking, interval greedy
• 08-dynamic-programming: introduction, state design, dp on array, dp on strings, summary
• 09-graph: graph basics, BFS, union find, topological sort, shortest path (Dijkstra)
• 10-how-to-choose-algorithm: algorithm selection guide
• 11-pattern-library: core patterns

每篇笔记都有对应的中文版本，文件名以 -zh.md 结尾。

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Quick start

1. Clone: git clone <repo-url>
2. Navigate to a problem directory, e.g. cd 0001-two-sum.
3. Run the solution: python solution.py
4. Read the accompanying README.md for the approach, complexity, and edge cases.

Tip: The folder naming convention is <problem-id>-<kebab-case-title> so you can jump directly to the problem you want to review.

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Solution format

Each problem folder follows a consistent template so you can skim or reuse ideas quickly.

• README.md contains the problem summary, key insight, approach, complexity, and a short takeaway.
• solution.py is a runnable reference solution with type hints and inline notes for tricky steps.

If you want to add a new problem, start by copying any existing folder and updating the metadata.

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Quick checks

There is no centralized test runner yet, but you can sanity-check a solution with:

• python solution.py (runs the sample input if present)
• python -m py_compile solution.py (verifies syntax)

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Problem catalog

A topic-focused index of all solutions currently in the repository.

ID	Problem	Topics
0001	Two Sum	Array; Hash Table
0002	Add Two Numbers	Linked List; Math
0003	Longest Substring Without Repeating Characters	Sliding Window; Hash Map
0004	Median of Two Sorted Arrays	Binary Search; Divide & Conquer
0005	Longest Palindromic Substring	String; Expand Around Center; Dynamic Programming
0006	Zigzag Conversion	String; Simulation
0007	Reverse Integer	Math; Overflow Handling
0008	String to Integer (atoi)	String; Parsing
0009	Palindrome Number	Math; Two Pointers
0010	Regular Expression Matching	Dynamic Programming; Pattern Matching
0011	Container With Most Water	Array; Two Pointers
0012	Integer to Roman	Math; Mapping
0013	Roman to Integer	Math; Mapping
0026	Remove Duplicates from Sorted Array	Array; Two Pointers
0027	Remove Element	Array; Two Pointers
0028	Find the Index of the First Occurrence in a String	String; Two Pointers
0049	Group Anagrams	Hash Table; Sorting
0066	Plus One	Array; Simulation
0085	Maximal Rectangle	Dynamic Programming; Stack; Monotonic Stack
0121	Best Time to Buy and Sell Stock	Array; Greedy
0164	Maximum Gap	Bucket Sort; Sorting
0220	Contains Duplicate III	Sliding Window; Bucket Sort
0242	Valid Anagram	Hash Table; Sorting
0347	Top K Frequent Elements	Heap; Hash Table
0451	Sort Characters by Frequency	Heap; Hash Table
0679	24 Game	Backtracking; DFS; Math
0692	Top K Frequent Words	Heap; Sorting
0712	Minimum ASCII Delete Sum for Two Strings	Dynamic Programming; String
0756	Pyramid Transition Matrix	Graph; DFS
0840	Magic Squares in Grid	Array; Enumeration
0865	Smallest Subtree with all the Deepest Nodes	Tree; Depth-First Search; Binary Tree; Lowest Common Ancestor
0912	Sort an Array	Sorting; Divide & Conquer
0944	Delete Columns to Make Sorted	Greedy; String
0955	Delete Columns to Make Sorted II	Greedy; String
0960	Delete Columns to Make Sorted III	Dynamic Programming; String
0961	N-Repeated Element in Size 2N Array	Hash Table; Counting
1161	Maximum Level of a Binary Tree	Tree; BFS
1266	Minimum Time Visiting All Points	Geometry; Greedy; Math
1339	Maximum Product of Splitted Binary Tree	Tree; Depth-First Search; Binary Tree; Math
1351	Count Negative Numbers in a Sorted Matrix	Matrix; Binary Search
1390	Four Divisors	Math; Number Theory
1411	Number of Ways to Paint N x 3 Grid	Dynamic Programming; Combinatorics
1458	Max Dot Product of Two Subsequences	Dynamic Programming; Array; Subsequence
1523	Count Odd Numbers in an Interval Range	Math
1590	Make Sum Divisible by P	Prefix Sum; Hash Map; Modular Arithmetic
1895	Largest Magic Square	Prefix Sum; Matrix; Enumeration
1925	Count Square Sum Triples	Math; Number Theory
1970	Last Day Where You Can Still Cross	Binary Search; BFS/Union Find
1975	Maximum Matrix Sum	Greedy; Matrix
2047	Find the Largest Area of Square Inside Two Rectangles	Geometry; Brute Force; Enumeration
2054	Two Best Non-Overlapping Events	Dynamic Programming; Sorting
2092	Find All People With Secret	Graph; Union Find
2110	Number of Smooth Descent Periods of a Stock	Sliding Window; Counting
2141	Maximum Running Time of N Computers	Binary Search; Greedy
2147	Number of Ways to Divide a Long Corridor	Dynamic Programming; Combinatorics
2211	Count Collisions on a Road	Simulation; Stack
2273	Find Resultant Array After Removing Anagrams	String; Stack
2343	Query Kth Smallest Trimmed Number	Sorting; Priority Queue
2402	Meeting Rooms III	Priority Queue; Scheduling
2435	Paths in Matrix Whose Sum Is Divisible by K	Dynamic Programming; Prefix Sum
2483	Minimum Penalty for a Shop	Prefix Sum; Enumeration
2872	Maximum Number of K-Divisible Components	Tree; DFS; Greedy; Graph
2943	Maximize Area of Square Hole in Grid	Greedy; Geometry; Sorting
2975	Maximum Square Area by Removing Fences From a Field	Geometry; Hash Set; Greedy
3074	Apple Redistribution into Boxes	Greedy; Sorting
3075	Maximize Happiness of Children	Greedy; Sorting
3381	Maximum Subarray Sum With Length Divisible by K	Prefix Sum; Hash Map
3432	Count Partitions with Even Sum Difference	Prefix Sum; Math
3433	Count Mentions Per User	Simulation; Hash Table
3453	Separate Squares I	Binary Search; Geometry; Prefix Area
3454	Separate Squares II	Sweep Line; Segment Tree; Geometry; Binary Search
3512	Minimum Operations to Make Array Sum Divisible by K	Math; Greedy; Modular Arithmetic
3531	Count Covered Buildings	Geometry; Prefix Min/Max
3562	Maximum Profit from Trading Stocks with Discounts	Tree DP; Knapsack
3573	Best Time to Buy and Sell Stock V	Dynamic Programming; State Machine
3577	Count the Number of Computer Unlocking Permutations	Sorting; Feasibility Check
3578	Count Partitions With Max-Min Difference at Most K	Sliding Window; Dynamic Programming
3583	Count Special Triplets	Hash Map; Prefix/Suffix Counting
3606	Coupon Code Validator	String; Simulation
3623	Count Number of Trapezoids I	Array; Geometry
3625	Count Number of Trapezoids II	Array; Geometry
3652	Best Time to Buy and Sell Stock using Strategy	Prefix Sum; Sliding Window

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Contributing

• Follow the existing folder naming pattern (####-title-in-kebab-case).
• Provide both README.md (approach & complexity) and solution.py (idiomatic Python).
• Keep explanations concise and prefer standard library solutions unless an external dependency is necessary.

Happy coding and studying! 🚀