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Hand-written tutorials, ordered as a recommended learning path.

1. 161 Word Search & Boggle Solve word search with DFS backtracking, Boggle with Trie pruning, and word ladder with BFS for efficient string transformation problems. 11 min read →
2. 162 Validate BST The seductive wrong answer, the correct min/max bounds approach, and how to defend it in an interview. 5 min read →
3. 163 LCA of BST Why BST ordering collapses LCA into a one-line traversal, and the iterative version that needs zero extra space. 5 min read →
4. 164 Kth Smallest BST The inorder traversal trick, the iterative early-exit version, and the follow-up that haunts FAANG interviews. 5 min read →
5. 165 Serialize Tree The preorder-with-nulls encoding, the iterator-driven decoder, and why level-order is a worse choice than it looks. 6 min read →
6. 166 Path Sum The DFS-with-decrement trick, why the leaf condition is the whole problem, and the variants that build on it. 5 min read →
7. 167 Alien Dictionary Derive character ordering from sorted alien words using topological sort, with course schedule variants and prerequisite chain problems. 10 min read →
8. 168 Level Order The queue-with-size BFS pattern, why DFS still works, and how this template extends to zigzag and right-side view. 5 min read →
9. 169 Clone Graph Why a hash map from original to copy is the entire idea, plus the BFS variant for stack-limited environments. 5 min read →
10. 170 Pacific Atlantic Why reverse traversal beats per-cell flood fill, and how the intersection of two reach sets gives the answer. 7 min read →
11. 171 Word Ladder The wildcard-pattern adjacency trick, why BFS is mandatory, and the bidirectional speedup. 6 min read →
12. 172 LIS Longest Increasing Subsequence in detail — the O(n^2) DP, the O(n log n) patience-sorting trick with binary search, and when each one matters. 6 min read →
13. 173 Negative Cycles Learn how to detect negative cycles using Bellman-Ford's nth relaxation, SPFA algorithm, and apply it to arbitrage detection in currency exchange graphs. 14 min read →
14. 174 Edit Distance Edit Distance fully unpacked — the three-operation recurrence, the 2D table, and the rolling-array space optimization that makes interviewers nod. 6 min read →
15. 175 Unique Paths Unique Paths in two ways — the O(m * n) grid DP that interviewers expect and the binomial-coefficient closed form that surprises them. 5 min read →
16. 176 MST Applications Explore MST applications in network design, clustering, and competitive programming with second-best MST, critical edges, and minimum cost to connect points. 11 min read →
17. 177 Flood Fill Master flood fill, surrounded regions, number of enclaves, and Pacific Atlantic water flow using DFS and BFS grid traversal techniques in Python. 12 min read →
18. 178 Clone & Transform Learn to clone graphs with BFS and DFS, validate graph trees, find minimum height trees via centroid decomposition, and reconstruct itineraries with Hierholzer's algorithm. 12 min read →
19. 179 Graph Interview Guide Master the top 20 graph interview patterns with a BFS vs DFS decision flowchart, Union-Find strategies, grid vs adjacency list trade-offs, and template code. 14 min read →
20. 180 Palindrome Patterns Master palindrome problems — longest palindromic substring with expand-around-center and Manacher's algorithm, counting palindromic substrings, and palindrome partitioning. 11 min read →