"The journey of 1,000 problems begins with a single Array."
This repository is my living DSA notebook — a continuously updated log of optimized solutions to LeetCode problems. Every solution here is:
- ✅ Accepted on LeetCode (verified submission)
- ⚡ Optimized for both time and space complexity
- 📝 Documented with approach explanation and complexity analysis
- 🏷️ Tagged by topic, difficulty, and pattern
| # | Topic | Key Patterns |
|---|---|---|
| 1 | 📦 Arrays | Prefix Sum, Kadane's, Sorting |
| 2 | 🔤 Strings | Sliding Window, Two Pointers, KMP |
| 3 | 🔗 Linked Lists | Fast & Slow Pointers, Reversal |
| 4 | 🌳 Trees & BST | BFS, DFS, In/Pre/Post-order |
| 5 | 🕸️ Graphs | DFS, BFS, Union-Find, Dijkstra |
| 6 | 💡 Dynamic Programming | Top-Down, Bottom-Up, Memoization |
| 7 | 🔙 Backtracking | Permutations, Subsets, N-Queens |
| 8 | 🔍 Binary Search | Search Space Reduction, Rotated Arrays |
| 9 | 📊 Heaps & Priority Queues | Top-K, Merge K Sorted |
| 10 | 📚 Stacks & Queues | Monotonic Stack, BFS Queue |
| 11 | 🪟 Sliding Window | Fixed & Variable Window |
| 12 | 👆 Two Pointers | Opposite Ends, Same Direction |
| 13 | 💰 Greedy | Interval Scheduling, Huffman |
| 14 | 🔢 Math & Bit Manipulation | XOR, Bit Counting, Modular |
git clone https://github.com/Ayush-2703/LeetCode_Problems.git
cd LeetCode_ProblemsEvery solution in this repo is evaluated against three tiers before being committed:
Tier 1 — Correctness → Does it pass all LeetCode test cases?
Tier 2 — Complexity → Is Big-O time and space optimal or near-optimal?
Tier 3 — Readability → Is it clean enough to explain in a 45-minute interview?
📌 Click to expand — Pattern Cheatsheet
| Pattern | When to Use | Example Problems |
|---|---|---|
| Two Pointers | Sorted arrays, palindromes, pair sums | 3Sum, Container With Most Water |
| Sliding Window | Contiguous subarray/substring problems | Longest Substring, Min Window Substring |
| Fast & Slow Pointers | Cycle detection, middle of linked list | Linked List Cycle, Happy Number |
| Merge Intervals | Overlapping ranges | Merge Intervals, Insert Interval |
| Cyclic Sort | Problems involving 1→N range arrays | Missing Number, Find All Duplicates |
| In-place Reversal (LL) | Reversing without extra space | Reverse LL, Reverse Sublist |
| Tree BFS | Level-order traversal, shortest path | Level Order Traversal, Right Side View |
| Tree DFS | Path sums, max depth, subtree checks | Path Sum, Lowest Common Ancestor |
| Two Heaps | Median of stream, task scheduling | Find Median from Data Stream |
| Subsets / Backtracking | Generating combinations/permutations | Subsets, Permutations, Combination Sum |
| Binary Search | Sorted arrays, search space reduction | Search in Rotated Array, Koko Eating Bananas |
| Top K Elements | Kth largest/smallest, frequent elements | Top K Frequent, Kth Largest in Stream |
| K-way Merge | Merging K sorted arrays/lists | Merge K Sorted Lists, Smallest Range |
| Monotonic Stack | Next greater/smaller element | Daily Temperatures, Largest Rectangle |
| DP — Knapsack | Subset sums, capacity constraints | 0/1 Knapsack, Coin Change |
| DP — LCS/LIS | Subsequence problems | LCS, LIS, Edit Distance |
| Graph — Union Find | Connectivity, cycle detection | Number of Provinces, Redundant Connection |
| Graph — Topological Sort | Dependencies, ordering | Course Schedule, Alien Dictionary |
If this repo helped you, please ⭐ star it — it keeps me motivated to keep solving!
This repository is licensed under the MIT License.
Solutions are for learning and reference purposes only. Please attempt problems independently before viewing solutions.
Maintained by Ayush Kumar Singh