LeetCode Solutions

This LeetCode Solutions column contains algorithm problem solutions, including clear problem analysis, multi-language implementations (Python/Go), and complexity analysis to help you efficiently prepare for algorithm interviews.

Problem Categories

Easy

Foundational algorithm problems for beginners to practice and reinforce core concepts.

#	Problem	#	Problem
12	Integer to Roman	26	Remove Duplicates from Sorted Array
28	Implement strStr()	34	Find First and Last Position of Element in Sorted Array
36	Valid Sudoku	50	Pow(x, n) - Implement pow function
60	Permutation Sequence	75	Sort Colors - Dutch National Flag
89	Gray Code	112	Path Sum
152	Maximum Product Subarray	162	Find Peak Element
165	Compare Version Numbers	168	Excel Sheet Column Title
172	Factorial Trailing Zeroes	173	Binary Search Tree Iterator
202	Happy Number	204	Count Primes
205	Isomorphic Strings	413	Arithmetic Slices
437	Path Sum III	451	Sort Characters By Frequency
503	Next Greater Element II	567	Permutation in String
647	Palindromic Substrings	859	Buddy Strings
969	Pancake Sorting	970	Powerful Integers
971	Flip Binary Tree To Match Preorder Traversal	977	Squares of a Sorted Array
985	Sum of Even Numbers After Queries	989	Add to Array-Form of Integer
997	Find the Town Judge	1002	Find Common Characters
1004	Max Consecutive Ones III	1018	Binary Prefix Divisible By 5
1078	Occurrences After Bigram	636	Exclusive Time of Functions
-	String to Integer (atoi)	-	String Reverse
-	Matrix Rotation

Medium

Classic interview questions worth revisiting and practicing systematically.

Category	Problem Link
Binary Tree	Construct Binary Tree Series
	Populating Next Right Pointers in Each Node
	Binary Tree and Linked List专题
	Validate Binary Search Tree
	Lowest Common Ancestor of a Binary Tree
	Convert Sorted List to Binary Search Tree
	Balanced Binary Tree
	Unique Binary Search Trees
	Unique Binary Search Trees II
	Flip Binary Tree To Match Preorder Traversal
Dynamic Programming	Decode Ways Series
	Word Break Series
	Best Time to Buy and Sell Stock
	Minimum Cost For Tickets
	Power of Series
	Unique Binary Search Trees
Backtracking	Combination Sum Series
	Palindrome Partitioning
	Letter Tile Possibilities
Linked List	LRU Cache
Binary Search	Search in Rotated Sorted Array
Bit Manipulation	Single Number Series
	Counting Bits
	Bitwise AND of Numbers Range
	Reverse Bits
	Sum of Two Integers
	Divide Two Integers
Sliding Window	Longest Substring Without Repeating Characters
	Minimum ASCII Delete Sum for Two Strings
Other Classics	3Sum
	Gas Station
	Candy
	Kth Largest Element in an Array
	Two City Scheduling
	Video Stitching
	Divisor Game
	Stone Game
	Longest Univalue Path
	Remove Sub-Folders from the Filesystem
	Queries on a Permutation With Key
	Capacity To Ship Packages Within D Days
	Maximum Sum of Two Non-Overlapping Subarrays
	Insufficient Nodes in Root to Leaf Paths
	Smallest Subsequence of Distinct Characters
	Word Ladder Series

Hard

Advanced algorithm challenges for deeper understanding and problem-solving skills.

Problem	Link
37	Sudoku Solver
952	Largest Component Size by Common Factor
-	Regular Expression Matching

General Techniques

Universal problem-solving methods and templates across different problem types.

Technique	Link
Common STL Operations	常用 STL 操作
Sliding Window专题	Sliding Window专题

Core Topics

Sliding Window - Two-pointer technique for efficient subarray/substring problems
Binary Search - Efficient search strategy for sorted data
Dynamic Programming - Optimal substructure problem decomposition
DFS/BFS - Tree and graph traversal methods
Backtracking - Exhaustive search for finding all solutions
Bit Manipulation - Optimize space and time complexity using bit operations

Each solution includes clear problem analysis, multi-language code implementations, and complexity analysis. Practice systematically by problem type for best results.