LeetCode Solutions

This repository contains my solutions to LeetCode problems, covering two distinct phases of my learning process.

• old/ - solutions from 2024, written mostly in JavaScript, organized by difficulty and study plan. Kept as an archive of earlier progress.
• new/ - current solutions, written in C#, organized by algorithmic pattern rather than difficulty. The goal here isn't just solving individual problems, but building a personal reference of recognizable patterns that transfer to new, unseen problems.

new/ - Pattern-based practice (2026, C#)

Each folder corresponds to one algorithmic pattern. Every solution file includes a short comment explaining when the pattern applies and its time/space complexity, so the folder works as a personal pattern reference, not just a solution dump.

Folder	Pattern	Example problems
arrays-hashmap	Storing state in a Dictionary/HashSet to avoid re-scanning	Two Sum, Contains Duplicate, Group Anagrams
two-pointers	Two pointers moving through a structure from different ends or speeds	3Sum, Container With Most Water
sliding-window	A window that expands/shrinks dynamically over a sequence	Longest Substring Without Repeating Characters
stack	LIFO structure for tracking "open" elements or reversing order	Valid Parentheses, Daily Temperatures
binary-search	Repeatedly halving the search space	Search in Rotated Sorted Array
linked-list	Pointer manipulation on linked structures	Reverse Linked List, Reorder List
trees	DFS/BFS traversal and recursion on trees	Validate BST, Level Order Traversal
graphs-bfs-dfs	Traversing graphs, detecting cycles, connectivity	Number of Islands, Course Schedule
backtracking	Exploring choices with the ability to undo them	Subsets, Permutations
dynamic-programming	Breaking a problem into overlapping subproblems	Climbing Stairs, Coin Change
priority-queue	Efficiently retrieving the highest- or lowest-priority element	Relative Ranks, Kth Largest Element in an Array

Example of the comment style used at the top of each new solution file:

// Pattern: Hashmap for tracking "what I've seen so far"
// When to use: for each element, I'm looking for something related to another element
// Complexity: O(n) time, O(n) space

// Solution:
public int[] TwoSum(int[] nums, int target) {
    // ... 
}

// Cases:
class Program
{
    public static void Main()
    {
        Solution solution = new Solution();
        
        // Case 1:
        var result1 = solution.TwoSum(nums, target);
        Console.WriteLine("Result for case 1: " + result1);
        // ...
    }
}

Progress in learning the basics of the most popular topics

• Arrays & Hashmap
• Two Pointers
• Sliding Window
• Stack
• Binary Search
• Linked List
• Trees
• Graphs (BFS/DFS)
• Backtracking
• Dynamic Programming
• Priority Queue

old/ - Archive (2024)

Solutions mostly in JavaScript/TypeScript, plus SQL, organized by difficulty and original LeetCode study plans.

Easy

• Standalone problems:
  • Two Sum (JavaScript version), (C# version)
  • Palindrome number (JavaScript version), (TypeScript version)
  • Score of a string (JavaScript version)
  • Search insert position (JavaScript version)
  • Longest common prefix (JavaScript version)
  • Roman to integer (JavaScript version)
• 30 Days of JavaScript - ~20 problems focused on core JavaScript concepts (closures, promises, array methods, function composition)
• LeetCode 75 – Easy - ~17 problems already grouped by topic: array/string, two pointers, sliding window, prefix sum, hash map/set, linked list, binary tree DFS, binary search tree, queue
• SQL 50 – Easy - ~32 SQL query problems (joins, aggregations, filtering)

Medium

• Standalone problems:
  • Add two numbers (JavaScript version)
• 30 Days of JavaScript – Medium - ~10 problems covering more advanced JS concepts (debounce, memoize, event emitter, async patterns)
• LeetCode 75 – Medium - ~31 problems grouped by topic: array/string, two pointers, sliding window, stack, hash map/set, linked list, binary tree BFS/DFS, binary search tree, graphs BFS/DFS, queue
• SQL 50 – Medium - ~17 SQL query problems (window functions, subqueries, more complex joins)

Hard

• Standalone problems:
  • Median of two sorted arrays (JavaScript version)
• SQL 50 – Hard - 1 problem (advanced ranking/window functions)

How to use

git clone https://github.com/wersosn/leetcode-problems.git

Contributions

This repository is mainly for personal use, but suggestions are welcome.

Links

LeetCode: wersosn