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

1. 241 Container Water Solve Container With Most Water with the two-pointer technique. We prove correctness and contrast it with the quadratic brute force. 6 min read →
2. 242 Combination Sum Solve Combination Sum with a clean backtracking template. Pruning, duplicate avoidance, complexity discussion, and interview script. 5 min read →
3. 243 Course Schedule Detect cycles and produce a valid course order using DFS-based topological sort. Includes BFS Kahn alternative and interview script. 6 min read →
4. 244 Daily Temperatures Solve Daily Temperatures with a monotonic decreasing stack of indices. Includes brute force comparison, walkthrough, complexity, and interview tips. 6 min read →
5. 245 Group Anagrams Solve the Group Anagrams problem cleanly with a hash map keyed by sorted strings or character counts. Includes complexity analysis and interview talking points. 6 min read →
6. 246 Generate Parentheses Solve Generate Parentheses with counting-based backtracking. Clean invariant, walkthrough, complexity discussion, and interview tips. 4 min read →
7. 247 House Robber Solve House Robber with the pick-or-skip DP recurrence, then optimize to O(1) space. Includes interview script and related variants. 5 min read →
8. 248 Invert Tree Solve Invert Binary Tree with a four-line recursive swap and an iterative BFS alternative, including complexity and interview talking points. 5 min read →
9. 249 Phone Letters Solve Letter Combinations of a Phone Number with backtracking. Mapping setup, recursive enumeration, complexity, and interview walkthrough. 5 min read →
10. 250 Longest Consecutive Solve Longest Consecutive Sequence in O(n) using a hash set and a start-of-run check. Walkthrough, edge cases, and interview script. 5 min read →
11. 251 Longest Palindrome Walk through the Longest Palindromic Substring problem using the expand-around-center technique. Compare brute force, DP, and the optimal approach with examples. 7 min read →
12. 252 LRU Cache Design LRU Cache with a hash map and a doubly linked list to get O(1) get and put, with a clean implementation and interview tips. 6 min read →
13. 253 Longest Substring Solve Longest Substring Without Repeating Characters using a sliding window with a hash map. We go from brute force to a clean O(n) sweep. 5 min read →
14. 254 Max Depth Compute tree depth with a three-line recursive DFS and an iterative BFS alternative, with complexity analysis and interview tips. 5 min read →
15. 255 Max Subarray A clear walkthrough of Maximum Subarray. We build Kadane's algorithm from first principles and contrast it with the divide and conquer approach. 5 min read →
16. 256 Min Stack Design a stack that supports push, pop, top, and getMin in constant time. Walkthrough of the two-stack and pair-stack solutions with edge cases and interview tips. 5 min read →
17. 257 Num Islands Solve Number of Islands with grid DFS and BFS, including a union-find variant, edge-case handling, and clear interview talking points. 6 min read →
18. 258 Merge Two Lists Solve Merge Two Sorted Lists with the dummy-head pointer pattern, plus a recursive variant, edge cases, and interview explanation tips. 5 min read →
19. 259 Permutations Solve Permutations with backtracking and a used array. Compare swap-in-place vs used-array, walkthrough, complexity, and interview tips. 5 min read →
20. 260 Product Except Self Solve Product of Array Except Self in O(n) without division using prefix and suffix passes. Clean walkthrough plus interview script. 5 min read →