How these 50 were chosen
These problems were selected using three criteria: (1) Frequency — reported in real interview question databases from Google, Meta, Amazon, Apple, and Microsoft in 2023–2025. (2) Pattern coverage — each problem is the cleanest representative of its pattern. Solving it gives you a transferable template, not just the answer to one problem. (3) Difficulty calibration — roughly 60% Medium, 30% Hard, 10% Easy warmups. If you solve all 50 correctly with no hints, you are interview-ready for any tier-1 tech company.
Arrays and hashing (problems 1–8)
1. Two Sum (LC 1) — the hash map lookup template. 2. Group Anagrams (LC 49) — sorted string as hash key. 3. Top K Frequent Elements (LC 347) — bucket sort trick beats heap. 4. Product of Array Except Self (LC 238) — prefix/suffix pass, no division. 5. Longest Consecutive Sequence (LC 128) — hash set, O(n). 6. Valid Sudoku (LC 36) — multi-constraint hash tracking. 7. Encode and Decode Strings (LC 271) — delimiter-free serialization. 8. Subarray Sum Equals K (LC 560) — prefix sum + hash map, count of subarrays.
Two pointers and sliding window (problems 9–16)
9. Container With Most Water (LC 11) — converging pointers, greedy step. 10. Trapping Rain Water (LC 42) — two pointers tracking max left/right. 11. 3Sum (LC 15) — sort + two pointers, skip duplicates pattern. 12. Minimum Window Substring (LC 76) — sliding window with character counts. 13. Longest Substring Without Repeating Characters (LC 3) — variable window + set. 14. Permutation in String (LC 567) — fixed window, frequency match. 15. Sliding Window Maximum (LC 239) — monotonic deque. 16. Longest Repeating Character Replacement (LC 424) — window with most-frequent count trick.
Binary search (problems 17–20)
17. Find Minimum in Rotated Sorted Array (LC 153) — binary search on rotated array invariant. 18. Search in Rotated Sorted Array (LC 33) — which half is sorted? 19. Koko Eating Bananas (LC 875) — binary search on answer, classic template. 20. Median of Two Sorted Arrays (LC 4) — binary search on partition, Hard, frequently asked at Google.
Trees and graphs (problems 21–34)
21. Invert Binary Tree (LC 226) — recursive swap, tree DFS warmup. 22. Maximum Depth of Binary Tree (LC 104) — DFS, base case matters. 23. Binary Tree Maximum Path Sum (LC 124) — postorder, global max trick. 24. Lowest Common Ancestor of a Binary Tree (LC 236) — recursive LCA pattern. 25. Serialize and Deserialize Binary Tree (LC 297) — BFS/preorder round-trip. 26. Construct Binary Tree from Preorder and Inorder (LC 105). 27. Number of Islands (LC 200) — DFS flood fill on grid. 28. Clone Graph (LC 133) — DFS + hash map for visited nodes. 29. Course Schedule (LC 207) — cycle detection, topological sort. 30. Word Ladder (LC 127) — BFS on word graph, very common at Google. 31. Pacific Atlantic Water Flow (LC 417) — multi-source BFS/DFS. 32. Rotting Oranges (LC 994) — multi-source BFS. 33. Alien Dictionary (LC 269) — topological sort from character ordering. 34. Network Delay Time (LC 743) — Dijkstra, weighted shortest path.
Dynamic programming (problems 35–44)
35. Climbing Stairs (LC 70) — Fibonacci DP, the entry point. 36. House Robber (LC 198) — skip/take recurrence. 37. Coin Change (LC 322) — unbounded knapsack template. 38. Longest Increasing Subsequence (LC 300) — 1D DP or patience sort. 39. Longest Common Subsequence (LC 1143) — 2D DP, string/sequence problems. 40. Edit Distance (LC 72) — 2D DP, very common at Google. 41. Partition Equal Subset Sum (LC 416) — 0/1 knapsack to boolean. 42. Word Break (LC 139) — string segmentation DP. 43. Best Time to Buy and Sell Stock III (LC 123) — state machine DP, at most 2 transactions. 44. Burst Balloons (LC 312) — interval DP, Hard, memorable pattern.
Heap, backtracking, and other patterns (problems 45–50)
45. Merge K Sorted Lists (LC 23) — min-heap merge, O(n log k). 46. Find Median from Data Stream (LC 295) — two heaps. 47. Combination Sum (LC 39) — backtracking template, reusable elements. 48. N-Queens (LC 51) — backtracking with pruning on diagonals. 49. LRU Cache (LC 146) — doubly linked list + hash map design. 50. Design Add and Search Words Data Structure (LC 211) — Trie with wildcard DFS. These final 6 span heap (45–46), backtracking (47–48), design (49), and Trie (50) — the four patterns most likely to appear as a second problem in a two-problem interview round.