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Hectal

Module 43

The Core 100

One hundred problems from across the course that cover every major pattern, in an order that builds from easy foundations to hard capstones.

Advanced 2 lessons 100 curated problems ~20 min of lessons

After learning the patterns module by module, you need a single list to practise and revise from. The Core 100 picks, for each important pattern, the problems that teach it best, and orders them so each set builds on the last: 25 easy foundations, 55 medium core problems, and 20 hard capstones.

Every problem lives in its own module, with full lessons, traces and Java solutions, so marking it complete there counts here too. Use the lessons below to plan a schedule and a revision routine.

Best after: Pattern Recognition Drills

Unlocks next: 44. Interview Method and Simulation

Part 1

Learn the ideas

  1. 43.1How the List Is Built and How to Use ItEvery major pattern appears at least once, the most common interview patterns several times. Work through the sets in order, roughly 10–15 problems a week.10 min
  2. 43.2Solving and Revising WellSolve, then re-solve from a blank file after 1, 3, 7 and 21 days. Track the pattern, the key insight and the mistake for every problem you missed.10 min

Set 1 · 25 problems

Foundation set (easy)

One clean example of each basic tool. Aim for 15–25 minutes each and a correct solution on the first run.

  1. 1.Two SumEasyModule 4Hash Map Lookup (Complement)
  2. 2.Valid AnagramEasyModule 4Frequency Counting
  3. 3.Contains DuplicateEasyModule 1Hash Map Lookup (Complement)
  4. 4.Best Time to Buy and Sell StockEasyModule 2Running State in One Pass
  5. 5.Majority ElementEasyModule 2Running State in One Pass
  6. 6.Missing NumberEasyModule 2Index as Hash (Cyclic Sort)
  7. 7.Valid PalindromeEasyModule 3Two Pointers: Opposite Ends
  8. 8.Move ZeroesEasyModule 6Two Pointers: Read and Write
  9. 9.Squares of a Sorted ArrayEasyModule 6Two Pointers: Opposite Ends
  10. 10.Find Pivot IndexEasyModule 5Prefix Sum
  11. 11.Maximum Average SubarrayEasyModule 7Sliding Window: Fixed Size
  12. 12.Reverse Linked ListEasyModule 9In-Place Linked List Reversal
  13. 13.Merge Two Sorted ListsEasyModule 9Dummy Head and Merging
  14. 14.Linked List CycleEasyModule 9Fast and Slow Pointers
  15. 15.Middle of the Linked ListEasyModule 9Fast and Slow Pointers
  16. 16.Valid ParenthesesEasyModule 10Stack for Matching and Undo
  17. 17.Binary SearchEasyModule 12Binary Search on an Index
  18. 18.Maximum Depth of Binary TreeEasyModule 17Tree DFS
  19. 19.Invert Binary TreeEasyModule 17Tree DFS
  20. 20.Diameter of Binary TreeEasyModule 17Tree DFS
  21. 21.Convert Sorted Array to BSTEasyModule 18BST Ordering
  22. 22.Climbing StairsEasyModule 291D Dynamic Programming
  23. 23.Find the Town JudgeEasyModule 21Graph BFS
  24. 24.Flood FillEasyModule 23Graph DFS and Flood Fill
  25. 25.Single NumberEasyModule 13Bit Manipulation

Set 2 · 55 problems

Core set (medium)

The patterns interviews ask most often. Aim for 30–40 minutes each; say the brute force first, then optimise.

  1. 1.Group AnagramsMediumModule 4Frequency Counting
  2. 2.Top K Frequent ElementsMediumModule 4Frequency Counting
  3. 3.Longest Consecutive SequenceMediumModule 4Hash Map Lookup (Complement)
  4. 4.Product of Array Except SelfMediumModule 5Prefix Sum
  5. 5.Subarray Sum Equals KMediumModule 5Prefix Sum + Hash Map
  6. 6.Maximum SubarrayMediumModule 2Kadane's Algorithm
  7. 7.3SumMediumModule 6Two Pointers: Opposite Ends
  8. 8.Container With Most WaterMediumModule 6Two Pointers: Opposite Ends
  9. 9.Longest Substring Without Repeating CharactersMediumModule 7Sliding Window: Variable Size
  10. 10.Longest Repeating Character ReplacementMediumModule 7Sliding Window: Variable Size
  11. 11.Remove Nth Node From EndMediumModule 9Dummy Head and Merging
  12. 12.Add Two NumbersMediumModule 9Dummy Head and Merging
  13. 13.Min StackMediumModule 10Combine Structures to Design
  14. 14.Daily TemperaturesMediumModule 10Monotonic Stack
  15. 15.Decode StringMediumModule 10Stack for Matching and Undo
  16. 16.Search in Rotated Sorted ArrayMediumModule 12Binary Search on an Index
  17. 17.Koko Eating BananasMediumModule 12Binary Search on the Answer
  18. 18.SubsetsMediumModule 14Backtracking
  19. 19.PermutationsMediumModule 14Backtracking
  20. 20.Combination SumMediumModule 14Backtracking
  21. 21.Word SearchMediumModule 14Backtracking
  22. 22.Kth Largest Element in an ArrayMediumModule 15Divide and Conquer
  23. 23.Merge IntervalsMediumModule 16Merge Intervals
  24. 24.Insert IntervalMediumModule 16Merge Intervals
  25. 25.Non-overlapping IntervalsMediumModule 16Greedy Choice
  26. 26.Meeting Rooms IIMediumModule 16Merge Intervals
  27. 27.Binary Tree Level Order TraversalMediumModule 17Tree BFS (Level Order)
  28. 28.Lowest Common Ancestor of a Binary TreeMediumModule 17Tree DFS
  29. 29.Construct Binary Tree from Preorder and InorderMediumModule 17Tree DFS
  30. 30.Validate Binary Search TreeMediumModule 18BST Ordering
  31. 31.Kth Smallest Element in a BSTMediumModule 18BST Ordering
  32. 32.K Closest Points to OriginMediumModule 19Top K with a Heap
  33. 33.Task SchedulerMediumModule 19Top K with a Heap
  34. 34.Implement Trie (Prefix Tree)MediumModule 20Trie (Prefix Tree)
  35. 35.Number of IslandsMediumModule 23Graph DFS and Flood Fill
  36. 36.Rotting OrangesMediumModule 22Graph BFS
  37. 37.Clone GraphMediumModule 23Graph DFS and Flood Fill
  38. 38.Course ScheduleMediumModule 24Topological Sort
  39. 39.Course Schedule IIMediumModule 25Topological Sort
  40. 40.Accounts MergeMediumModule 26Union-Find
  41. 41.Redundant ConnectionMediumModule 24Union-Find
  42. 42.Network Delay TimeMediumModule 27Dijkstra's Shortest Path
  43. 43.Cheapest Flights Within K StopsMediumModule 27Dijkstra's Shortest Path
  44. 44.Min Cost to Connect All PointsMediumModule 28Minimum Spanning Tree
  45. 45.House RobberMediumModule 291D Dynamic Programming
  46. 46.Word BreakMediumModule 291D Dynamic Programming
  47. 47.Unique PathsMediumModule 30Grid DP
  48. 48.Coin ChangeMediumModule 31Knapsack DP
  49. 49.Partition Equal Subset SumMediumModule 31Knapsack DP
  50. 50.Longest Increasing SubsequenceMediumModule 32Longest Increasing Subsequence
  51. 51.Longest Common SubsequenceMediumModule 32DP on Two Strings
  52. 52.Edit DistanceMediumModule 32DP on Two Strings
  53. 53.Jump GameMediumModule 34Greedy Choice
  54. 54.Gas StationMediumModule 34Greedy Choice
  55. 55.LRU CacheMediumModule 40Combine Structures to Design

Set 3 · 20 problems

Capstone set (hard)

Classics that combine two or more ideas. Give yourself up to an hour; it's normal to need a hint. Re-solve each one a week later.

  1. 1.Trapping Rain WaterHardModule 6Two Pointers: Opposite Ends
  2. 2.Minimum Window SubstringHardModule 7Sliding Window: Variable Size
  3. 3.Sliding Window MaximumHardModule 11Monotonic Deque
  4. 4.Largest Rectangle in HistogramHardModule 10Monotonic Stack
  5. 5.Merge K Sorted ListsHardModule 9K-Way Merge
  6. 6.Median of Two Sorted ArraysHardModule 12Binary Search on an Index
  7. 7.N-QueensHardModule 14Backtracking
  8. 8.Binary Tree Maximum Path SumHardModule 17Tree DFS
  9. 9.Find Median from Data StreamHardModule 19Two Heaps
  10. 10.Word Search IIHardModule 20Trie (Prefix Tree)
  11. 11.Word LadderHardModule 22Graph BFS
  12. 12.Alien DictionaryHardModule 25Topological Sort
  13. 13.Swim in Rising WaterHardModule 27Dijkstra's Shortest Path
  14. 14.Burst BalloonsHardModule 33Interval DP
  15. 15.Wildcard MatchingHardModule 32DP on Two Strings
  16. 16.Dungeon GameHardModule 30Grid DP
  17. 17.CandyHardModule 34Greedy Choice
  18. 18.LFU CacheHardModule 40Combine Structures to Design
  19. 19.Count of Smaller Numbers After SelfHardModule 36Range Queries (Fenwick & Segment Trees)
  20. 20.Serialize and Deserialize Binary TreeHardModule 17Tree DFS