Introduction

This comprehensive guide contains 500 curated LeetCode problems organized for efficient interview preparation. Each problem includes:

  • ✅ Interactive checkboxes for progress tracking
  • 🔥 Priority markers for must-solve problems
  • 🟢🟡🔴 Difficulty indicators (Easy/Medium/Hard)
  • Pattern tags for systematic learning

Study Recommendations

Beginner Path (3-4 months):

  • Start with Easy problems in Array & Hash Table
  • Master Two Pointers and Sliding Window patterns
  • Build foundation with Trees and Linked Lists
  • Practice 2-3 problems daily

Intermediate Path (2-3 months):

  • Focus on Medium difficulty across all patterns
  • Deep dive into Dynamic Programming
  • Master Graph algorithms (DFS/BFS)
  • Solve 3-4 problems daily

Advanced Path (1-2 months):

  • Tackle Hard problems and system design
  • Practice under timed conditions
  • Review company-specific patterns
  • Aim for 4-5 problems daily

Table of Contents

Problems by Number

Pattern-Based Study Guide

Problems List

1–50

  1. Two Sum - Medium 🟡 🔥 Two Pointers
  2. Add Two Numbers - Medium 🟡 Two Pointers
  3. Longest Substring Without Repeating Characters - Medium 🟡 Array
  4. Median Of Two Sorted Arrays - Medium 🟡 Two Pointers
  5. Longest Palindromic Substring - Medium 🟡 Array
  6. Zigzag Conversion - Medium 🟡 Array
  7. Reverse Integer - Medium 🟡 Array
  8. String To Integer atoi - Medium 🟡 Array
  9. Palindrome Number - Medium 🟡 Array
  10. Regular Expression Matching - Medium 🟡 Array
  11. Container With Most Water - Medium 🟡 🔥 Array
  12. Integer To Roman - Medium 🟡 Array
  13. Roman To Integer - Medium 🟡 Array
  14. Longest Common Prefix - Medium 🟡 Array
  15. 3sum - Medium 🟡 🔥 Array
  16. 3sum Closest - Medium 🟡 Array
  17. Letter Combinations Of A Phone Number - Medium 🟡 Backtracking
  18. 4sum - Medium 🟡 Array
  19. Remove Nth Node From End Of List - Medium 🟡 Linked List
  20. Valid Parentheses - Medium 🟡 🔥 Array
  21. Merge Two Sorted Lists - Medium 🟡 Linked List Two Pointers
  22. Generate Parentheses - Medium 🟡 Array
  23. Merge K Sorted Lists - Medium 🟡 🔥 Linked List
  24. Swap Nodes In Pairs - Medium 🟡 Array
  25. Reverse Nodes In K Group - Medium 🟡 Array
  26. Remove Duplicates From Sorted Array - Medium 🟡 Array
  27. Remove Element - Medium 🟡 Array
  28. Find The Index Of The First Occurrence In A String - Medium 🟡 Array
  29. Divide Two Integers - Medium 🟡 Two Pointers
  30. Substring With Concatenation Of All Words - Medium 🟡 Array
  31. Next Permutation - Medium 🟡 Backtracking
  32. Longest Valid Parentheses - Medium 🟡 Array
  33. Search In Rotated Sorted Array - Medium 🟡 🔥 Binary Search
  34. Find First And Last Position Of Element In Sorted Array - Medium 🟡 Array
  35. Search Insert Position - Medium 🟡 Binary Search
  36. Valid Sudoku - Medium 🟡 Array
  37. Sudoku Solver - Medium 🟡 Array
  38. Count And Say - Medium 🟡 Array
  39. Combination Sum - Medium 🟡 🔥 Backtracking
  40. Combination Sum II - Medium 🟡 Backtracking
  41. First Missing Positive - Medium 🟡 Array
  42. Trapping Rain Water - Medium 🟡 🔥 Array
  43. Multiply Strings - Medium 🟡 Array
  44. Wildcard Matching - Medium 🟡 Array
  45. Jump Game II - Medium 🟡 Array
  46. Permutations - Medium 🟡 🔥 Backtracking
  47. Permutations II - Medium 🟡 Backtracking
  48. Rotate Image - Medium 🟡 Array
  49. Group Anagrams - Medium 🟡 🔥 Array
  50. Powx N - Medium 🟡 Array

51–100

  1. Maximum Subarray - Medium 🟡 🔥 Array
  2. Spiral Matrix - Medium 🟡 Array
  3. Jump Game - Medium 🟡 🔥 Array
  4. Merge Intervals - Medium 🟡 🔥 Array
  5. Insert Interval - Medium 🟡 Array
  6. Length Of Last Word - Medium 🟡 Array
  7. Spiral Matrix II - Medium 🟡 Array
  8. Unique Paths - Medium 🟡 🔥 DP
  9. Unique Paths II - Medium 🟡 DP
  10. Minimum Path Sum - Medium 🟡 DP
  11. Climbing Stairs - Medium 🟡 🔥 DP
  12. Edit Distance - Medium 🟡 🔥 DP
  13. Set Matrix Zeroes - Medium 🟡 Array
  14. Search A 2D Matrix - Medium 🟡 Binary Search
  15. Sort Colors - Medium 🟡 Array
  16. Minimum Window Substring - Medium 🟡 🔥 Sliding Window
  17. Combinations - Medium 🟡 Backtracking
  18. Subsets - Medium 🟡 Array
  19. Word Search - Medium 🟡 🔥 Binary Search
  20. Largest Rectangle In Histogram - Medium 🟡 🔥 Array
  21. Maximal Rectangle - Medium 🟡 Array
  22. Partition List - Medium 🟡 Linked List
  23. Scramble String - Medium 🟡 Array
  24. Merge Sorted Array - Medium 🟡 Array
  25. Gray Code - Medium 🟡 Array
  26. Subsets II - Medium 🟡 Array
  27. Decode Ways - Medium 🟡 Array
  28. Validate Binary Search Tree - Medium 🟡 🔥 Tree
  29. Recover Binary Search Tree - Medium 🟡 Tree
  30. Same Tree - Medium 🟡 Tree
  31. Symmetric Tree - Medium 🟡 Tree
  32. Binary Tree Level Order Traversal - Medium 🟡 🔥 Tree
  33. Binary Tree Zigzag Level Order Traversal - Medium 🟡 Tree
  34. Maximum Depth Of Binary Tree - Medium 🟡 🔥 Tree
  35. Construct Binary Tree From Preorder And Inorder Traversal - Medium 🟡 Tree
  36. Construct Binary Tree From Inorder And Postorder Traversal - Medium 🟡 Tree
  37. Flatten Binary Tree To Linked List - Medium 🟡 Tree Linked List
  38. Path Sum - Medium 🟡 DP
  39. Path Sum II - Medium 🟡 DP
  40. Populating Next Right Pointers In Each Node - Medium 🟡 Two Pointers
  41. Sum Root To Leaf Numbers - Medium 🟡 Array
  42. Longest Consecutive Sequence - Medium 🟡 🔥 Array
  43. Surrounded Regions - Medium 🟡 Array
  44. Palindrome Partitioning - Medium 🟡 Array
  45. Copy List With Random Pointer - Medium 🟡 Linked List Two Pointers
  46. Word Break - Medium 🟡 🔥 Array
  47. Linked List Cycle - Medium 🟡 🔥 Linked List
  48. Linked List Cycle II - Medium 🟡 Linked List
  49. Reorder List - Medium 🟡 Linked List
  50. Binary Tree Maximum Path Sum - Medium 🟡 🔥 Tree DP

101–150

  1. Symmetric Tree - Easy 🟢 Tree
  2. Binary Tree Level Order Traversal - Medium 🟡 🔥 Tree
  3. Binary Tree Zigzag Level Order Traversal - Medium 🟡 Tree
  4. Maximum Depth of Binary Tree - Easy 🟢 🔥 Tree
  5. Construct Binary Tree from Preorder and Inorder Traversal - Medium 🟡 🔥 Tree
  6. Construct Binary Tree from Inorder and Postorder Traversal - Medium 🟡 Tree
  7. Binary Tree Level Order Traversal II - Medium 🟡 Tree
  8. Convert Sorted Array to Binary Search Tree - Easy 🟢 Tree
  9. Convert Sorted List to Binary Search Tree - Medium 🟡 Tree Linked List
  10. Balanced Binary Tree - Easy 🟢 Tree
  11. Minimum Depth of Binary Tree - Easy 🟢 Tree
  12. Path Sum - Easy 🟢 Tree
  13. Path Sum II - Medium 🟡 Tree
  14. Flatten Binary Tree to Linked List - Medium 🟡 🔥 Tree
  15. Distinct Subsequences - Hard 🔴 DP String
  16. Populating Next Right Pointers in Each Node - Medium 🟡 🔥 Tree
  17. Populating Next Right Pointers in Each Node II - Medium 🟡 Tree
  18. Pascal’s Triangle - Easy 🟢 Array
  19. Pascal’s Triangle II - Easy 🟢 Array
  20. Triangle - Medium 🟡 DP
  21. Best Time to Buy and Sell Stock - Easy 🟢 🔥 Array DP
  22. Best Time to Buy and Sell Stock II - Medium 🟡 Array
  23. Best Time to Buy and Sell Stock III - Hard 🔴 DP
  24. Binary Tree Maximum Path Sum - Hard 🔴 🔥 Tree DP
  25. Valid Palindrome - Easy 🟢 Two Pointers
  26. Word Ladder II - Hard 🔴 Graph BFS
  27. Word Ladder - Hard 🔴 🔥 Graph BFS
  28. Longest Consecutive Sequence - Medium 🟡 🔥 Hash Table
  29. Sum Root to Leaf Numbers - Medium 🟡 Tree
  30. Surrounded Regions - Medium 🟡 🔥 Graph DFS
  31. Palindrome Partitioning - Medium 🟡 Backtracking
  32. Palindrome Partitioning II - Hard 🔴 DP
  33. Clone Graph - Medium 🟡 🔥 Graph
  34. Gas Station - Medium 🟡 Greedy
  35. Candy - Hard 🔴 Greedy
  36. Single Number - Easy 🟢 🔥 Bit Manipulation
  37. Single Number II - Medium 🟡 Bit Manipulation
  38. Copy List with Random Pointer - Medium 🟡 🔥 Linked List
  39. Word Break - Medium 🟡 🔥 DP String
  40. Word Break II - Hard 🔴 DP Backtracking
  41. Linked List Cycle - Easy 🟢 🔥 Linked List
  42. Linked List Cycle II - Medium 🟡 🔥 Linked List
  43. Reorder List - Medium 🟡 🔥 Linked List
  44. LRU Cache - Medium 🟡 🔥 Design Hash Table
  45. Binary Tree Postorder Traversal - Easy 🟢 Tree
  46. Binary Tree Preorder Traversal - Easy 🟢 Tree
  47. Insertion Sort List - Medium 🟡 Linked List
  48. Sort List - Medium 🟡 🔥 Linked List
  49. Max Points on a Line - Hard 🔴 Math
  50. Evaluate Reverse Polish Notation - Medium 🟡 🔥 Stack

151–200

  1. Find Minimum In Rotated Sorted Array - Medium 🟡 Array
  2. Find Minimum In Rotated Sorted Array II - Medium 🟡 Array
  3. Minimum Size Subarray Sum - Medium 🟡 Array
  4. Compare Version Numbers - Medium 🟡 Array
  5. Excel Sheet Column Title - Medium 🟡 Array
  6. Excel Sheet Column Number - Medium 🟡 Array
  7. Factorial Trailing Zeroes - Medium 🟡 Array
  8. Binary Search Tree Iterator - Medium 🟡 Tree
  9. Majority Element - Medium 🟡 Array
  10. Majority Element II - Medium 🟡 Array
  11. Reverse Bits - Medium 🟡 Array
  12. Number Of 1 Bits - Medium 🟡 Array
  13. Happy Number - Medium 🟡 Array
  14. Remove Linked List Elements - Medium 🟡 Linked List
  15. Isomorphic Strings - Medium 🟡 Array
  16. Count Primes - Medium 🟡 Array
  17. House Robber - Medium 🟡 Array
  18. House Robber II - Medium 🟡 Array
  19. Rotate Array - Medium 🟡 Array
  20. Reverse Linked List - Medium 🟡 🔥 Linked List
  21. Course Schedule - Medium 🟡 🔥 Graph
  22. Course Schedule II - Medium 🟡 Graph
  23. Implement Trie Prefix Tree - Medium 🟡 Tree Trie
  24. Add And Search Word Data Structure Design - Medium 🟡 Binary Search
  25. Word Search II - Medium 🟡 Binary Search
  26. Summary Ranges - Medium 🟡 Array
  27. Number Of Islands - Medium 🟡 🔥 Graph
  28. Bitwise And Of Numbers Range - Medium 🟡 Array
  29. Power Of Two - Medium 🟡 Two Pointers
  30. Implement Queue Using Stacks - Medium 🟡 Stack
  31. Implement Stack Using Queues - Medium 🟡 Stack
  32. Invert Binary Tree - Medium 🟡 🔥 Tree
  33. Kth Smallest Element In A BST - Medium 🟡 Array
  34. Lowest Common Ancestor Of A Binary Search Tree - Medium 🟡 Tree
  35. Lowest Common Ancestor Of A Binary Tree - Medium 🟡 Tree
  36. Product Of Array Except Self - Medium 🟡 🔥 Array
  37. Sliding Window Maximum - Medium 🟡 🔥 Sliding Window
  38. Search A 2D Matrix II - Medium 🟡 Binary Search
  39. Different Ways To Add Parentheses - Medium 🟡 Array
  40. Valid Anagram - Medium 🟡 Array
  41. Shortest Word Distance - Medium 🟡 Array
  42. Meeting Rooms - Medium 🟡 Array
  43. Meeting Rooms II - Medium 🟡 Array
  44. Factor Combinations - Medium 🟡 Backtracking
  45. Flatten 2D Vector - Medium 🟡 Array
  46. Move Zeroes - Medium 🟡 Array
  47. Peeking Iterator - Medium 🟡 Array
  48. Maximum Product Subarray - Medium 🟡 Array
  49. Search A 2D Matrix - Medium 🟡 Binary Search
  50. Count Complete Tree Nodes - Medium 🟡 Tree

201–250

  1. Bitwise And Of Numbers Range - Medium 🟡 Array
  2. Happy Number - Medium 🟡 Array
  3. Remove Linked List Elements - Medium 🟡 Linked List
  4. Count Primes - Medium 🟡 Array
  5. Isomorphic Strings - Medium 🟡 Array
  6. Reverse Linked List - Medium 🟡 🔥 Linked List
  7. Course Schedule - Medium 🟡 🔥 Graph
  8. Implement Trie Prefix Tree - Medium 🟡 Tree Trie
  9. Minimum Size Subarray Sum - Medium 🟡 Array
  10. Course Schedule II - Medium 🟡 Graph
  11. Add And Search Word Data Structure Design - Medium 🟡 Binary Search
  12. Word Search II - Medium 🟡 Binary Search
  13. House Robber II - Medium 🟡 Array
  14. Shortest Palindrome - Medium 🟡 Array
  15. Kth Largest Element In An Array - Medium 🟡 Array
  16. Combination Sum III - Medium 🟡 Backtracking
  17. Contains Duplicate - Medium 🟡 Array
  18. The Skyline Problem - Medium 🟡 Array
  19. Contains Duplicate II - Medium 🟡 Array
  20. Contains Duplicate III - Medium 🟡 Array
  21. Maximal Square - Medium 🟡 Array
  22. Count Complete Tree Nodes - Medium 🟡 Tree
  23. Rectangle Area - Medium 🟡 Array
  24. Basic Calculator - Medium 🟡 Array
  25. Implement Stack Using Queues - Medium 🟡 Stack
  26. Invert Binary Tree - Medium 🟡 🔥 Tree
  27. Basic Calculator II - Medium 🟡 Array
  28. Summary Ranges - Medium 🟡 Array
  29. Majority Element II - Medium 🟡 Array
  30. Kth Smallest Element In A BST - Medium 🟡 Array
  31. Power Of Two - Medium 🟡 Two Pointers
  32. Implement Queue Using Stacks - Medium 🟡 Stack
  33. Number Of Digit One - Medium 🟡 Array
  34. Palindrome Linked List - Medium 🟡 Linked List
  35. Lowest Common Ancestor Of A Binary Search Tree - Medium 🟡 Tree
  36. Lowest Common Ancestor Of A Binary Tree - Medium 🟡 Tree
  37. Delete Node In A Linked List - Medium 🟡 Linked List
  38. Product Of Array Except Self - Medium 🟡 🔥 Array
  39. Sliding Window Maximum - Medium 🟡 🔥 Sliding Window
  40. Search A 2D Matrix II - Medium 🟡 Binary Search
  41. Different Ways To Add Parentheses - Medium 🟡 Array
  42. Valid Anagram - Medium 🟡 Array
  43. Shortest Word Distance - Medium 🟡 Array
  44. Shortest Word Distance II - Medium 🟡 Array
  45. Shortest Word Distance III - Medium 🟡 Array
  46. Strobogrammatic Number - Medium 🟡 Array
  47. Strobogrammatic Number II - Medium 🟡 Array
  48. Strobogrammatic Number III - Medium 🟡 Array
  49. Group Shifted Strings - Medium 🟡 Array
  50. Count Univalue Subtrees - Medium 🟡 Tree

251–300

  1. Flatten 2D Vector - Medium 🟡 Array
  2. Meeting Rooms - Medium 🟡 Array
  3. Meeting Rooms II - Medium 🟡 Array
  4. Factor Combinations - Medium 🟡 Backtracking
  5. Verify Preorder Sequence In Binary Search Tree - Medium 🟡 Tree
  6. Paint House - Medium 🟡 Array
  7. Binary Tree Paths - Medium 🟡 Tree DP
  8. Add Digits - Medium 🟡 Array
  9. 3sum Smaller - Medium 🟡 Array
  10. Single Number III - Medium 🟡 Array
  11. Graph Valid Tree - Medium 🟡 Tree Graph
  12. Trips And Users - Medium 🟡 Array
  13. Ugly Number - Medium 🟡 Array
  14. Ugly Number II - Medium 🟡 Array
  15. Paint House II - Medium 🟡 Array
  16. Palindrome Permutation - Medium 🟡 Backtracking
  17. Palindrome Permutation II - Medium 🟡 Backtracking
  18. Missing Number - Medium 🟡 Array
  19. Alien Dictionary - Medium 🟡 Array
  20. Closest Binary Search Tree Value - Medium 🟡 Tree
  21. Encode And Decode Strings - Medium 🟡 Array
  22. Closest Binary Search Tree Value II - Medium 🟡 Tree
  23. Integer To English Words - Medium 🟡 Array
  24. H Index - Medium 🟡 Array
  25. H Index II - Medium 🟡 Array
  26. Paint Fence - Medium 🟡 Array
  27. Find The Celebrity - Medium 🟡 Array
  28. First Bad Version - Medium 🟡 Array
  29. Perfect Squares - Medium 🟡 Array
  30. Wiggle Sort - Medium 🟡 Array
  31. Zigzag Iterator - Medium 🟡 Array
  32. Expression Add Operators - Medium 🟡 Array
  33. Move Zeroes - Medium 🟡 Array
  34. Peeking Iterator - Medium 🟡 Array
  35. Inorder Successor In BST - Medium 🟡 Array
  36. Walls And Gates - Medium 🟡 Array
  37. Find The Duplicate Number - Medium 🟡 Array
  38. Unique Word Abbreviation - Medium 🟡 Array
  39. Game Of Life - Medium 🟡 Array
  40. Word Pattern - Medium 🟡 Array
  41. Word Pattern II - Medium 🟡 Array
  42. Nim Game - Medium 🟡 Array
  43. Flip Game - Medium 🟡 Array
  44. Flip Game II - Medium 🟡 Array
  45. Find Median From Data Stream - Medium 🟡 Array
  46. Best Meeting Point - Medium 🟡 Array
  47. Serialize And Deserialize Binary Tree - Medium 🟡 🔥 Tree
  48. Binary Tree Longest Consecutive Sequence - Medium 🟡 Tree
  49. Bulls And Cows - Medium 🟡 Array
  50. Longest Increasing Subsequence - Medium 🟡 Array

301–350

  1. Remove Invalid Parentheses - Medium 🟡 Array
  2. Smallest Rectangle Enclosing Black Pixels - Medium 🟡 Array
  3. Range Sum Query Immutable - Medium 🟡 Array
  4. Range Sum Query 2D Immutable - Medium 🟡 Array
  5. Number Of Islands II - Medium 🟡 Graph
  6. Additive Number - Medium 🟡 Array
  7. Range Sum Query Mutable - Medium 🟡 Array
  8. Range Sum Query 2D Mutable - Medium 🟡 Array
  9. Best Time To Buy And Sell Stock With Cooldown - Medium 🟡 Array
  10. Minimum Height Trees - Medium 🟡 Tree
  11. Sparse Matrix Multiplication - Medium 🟡 Array
  12. Burst Balloons - Medium 🟡 Array
  13. Super Ugly Number - Medium 🟡 Array
  14. Binary Tree Vertical Order Traversal - Medium 🟡 Tree
  15. Count Of Smaller Numbers After Self - Medium 🟡 Array
  16. Remove Duplicate Letters - Medium 🟡 Array
  17. Shortest Distance From All Buildings - Medium 🟡 Array
  18. Maximum Product Of Word Lengths - Medium 🟡 Array
  19. Bulb Switcher - Medium 🟡 Array
  20. Generalized Abbreviation - Medium 🟡 Array
  21. Create Maximum Number - Medium 🟡 Array
  22. Coin Change - Medium 🟡 Array
  23. Number Of Connected Components In An Undirected Graph - Medium 🟡 Graph
  24. Wiggle Sort II - Medium 🟡 Array
  25. Maximum Size Subarray Sum Equals K - Medium 🟡 Array
  26. Power Of Three - Medium 🟡 Array
  27. Count Of Range Sum - Medium 🟡 Array
  28. Odd Even Linked List - Medium 🟡 Linked List
  29. Longest Increasing Path In A Matrix - Medium 🟡 DP
  30. Patching Array - Medium 🟡 Array
  31. Verify Preorder Serialization Of A Binary Tree - Medium 🟡 Tree
  32. Reconstruct Itinerary - Medium 🟡 Array
  33. Largest BST Subtree - Medium 🟡 Tree
  34. Increasing Triplet Subsequence - Medium 🟡 Array
  35. Self Crossing - Medium 🟡 Array
  36. Palindrome Pairs - Medium 🟡 Array
  37. House Robber III - Medium 🟡 Array
  38. Counting Bits - Medium 🟡 Array
  39. Nested List Weight Sum - Medium 🟡 Linked List
  40. Longest Substring With At Most K Distinct Characters - Medium 🟡 Array
  41. Flatten Nested List Iterator - Medium 🟡 Linked List
  42. Power Of Four - Medium 🟡 Array
  43. Integer Break - Medium 🟡 Array
  44. Reverse String - Medium 🟡 Array
  45. Reverse Vowels Of A String - Medium 🟡 Array
  46. Moving Average From Data Stream - Medium 🟡 Array
  47. Top K Frequent Elements - Medium 🟡 Array
  48. Design Tic Tac Toe - Medium 🟡 Array
  49. Intersection Of Two Arrays - Medium 🟡 Two Pointers
  50. Intersection Of Two Arrays II - Medium 🟡 Two Pointers

351–400

  1. Android Unlock Patterns - Medium 🟡 Array
  2. Data Stream As Disjoint Intervals - Medium 🟡 Array
  3. Design Snake Game - Medium 🟡 Array
  4. Russian Doll Envelopes - Medium 🟡 Array
  5. Design Twitter - Medium 🟡 Array
  6. Line Reflection - Medium 🟡 Array
  7. Count Numbers With Unique Digits - Medium 🟡 Array
  8. Rearrange String K Distance Apart - Medium 🟡 Array
  9. Logger Rate Limiter - Medium 🟡 Array
  10. Sort Transformed Array - Medium 🟡 Array
  11. Bomb Enemy - Medium 🟡 Array
  12. Design Hit Counter - Medium 🟡 Array
  13. Max Sum Of Rectangle No Larger Than K - Medium 🟡 Array
  14. Nested List Weight Sum II - Medium 🟡 Linked List
  15. Water And Jug Problem - Medium 🟡 Array
  16. Find Leaves Of Binary Tree - Medium 🟡 Tree
  17. Valid Perfect Square - Medium 🟡 Array
  18. Largest Divisible Subset - Medium 🟡 Array
  19. Plus One Linked List - Medium 🟡 Linked List
  20. Range Addition - Medium 🟡 Array
  21. Sum Of Two Integers - Medium 🟡 Two Pointers
  22. Super Pow - Medium 🟡 Array
  23. Find K Pairs With Smallest Sums - Medium 🟡 Array
  24. Guess Number Higher Or Lower - Medium 🟡 Array
  25. Guess Number Higher Or Lower II - Medium 🟡 Array
  26. Wiggle Subsequence - Medium 🟡 Array
  27. Combination Sum IV - Medium 🟡 Backtracking
  28. Kth Smallest Element In A Sorted Matrix - Medium 🟡 Array
  29. Design Phone Directory - Medium 🟡 Array
  30. Insert Delete Getrandom O(1) - Medium 🟡 Array
  31. Insert Delete Getrandom O(1) Duplicates Allowed - Medium 🟡 Array
  32. Linked List Random Node - Medium 🟡 Linked List
  33. Ransom Note - Medium 🟡 Array
  34. Shuffle An Array - Medium 🟡 Array
  35. Mini Parser - Medium 🟡 Array
  36. Lexicographical Numbers - Medium 🟡 Graph
  37. First Unique Character In A String - Medium 🟡 Array
  38. Longest Absolute File Path - Medium 🟡 DP
  39. Find The Difference - Medium 🟡 Array
  40. Elimination Game - Medium 🟡 Array
  41. Perfect Rectangle - Medium 🟡 Array
  42. Is Subsequence - Medium 🟡 Array
  43. Utf 8 Validation - Medium 🟡 Array
  44. Decode String - Medium 🟡 Array
  45. Longest Substring With At Least K Repeating Characters - Medium 🟡 Array
  46. Rotate Function - Medium 🟡 Array
  47. Integer Replacement - Medium 🟡 Array
  48. Random Pick Index - Medium 🟡 Array
  49. Evaluate Division - Medium 🟡 Array
  50. Nth Digit - Medium 🟡 Array

401–450

  1. Binary Watch - Medium 🟡 Tree Binary Search
  2. Remove K Digits - Medium 🟡 Array
  3. Frog Jump - Medium 🟡 Array
  4. Sum Of Left Leaves - Medium 🟡 Array
  5. Convert A Number To Hexadecimal - Medium 🟡 Array
  6. Queue Reconstruction By Height - Medium 🟡 Stack
  7. Trapping Rain Water II - Medium 🟡 Array
  8. Valid Word Abbreviation - Medium 🟡 Array
  9. Longest Palindrome - Medium 🟡 Array
  10. Split Array Largest Sum - Medium 🟡 Array
  11. Minimum Unique Word Abbreviation - Medium 🟡 Array
  12. Fizz Buzz - Medium 🟡 Array
  13. Arithmetic Slices - Medium 🟡 Array
  14. Third Maximum Number - Medium 🟡 Array
  15. Add Strings - Medium 🟡 Array
  16. Partition Equal Subset Sum - Medium 🟡 Array
  17. Pacific Atlantic Water Flow - Medium 🟡 Array
  18. Sentence Screen Fitting - Medium 🟡 Array
  19. Battleships In A Board - Medium 🟡 Array
  20. Strong Password Checker - Medium 🟡 Array
  21. Maximum Xor Of Two Numbers In An Array - Medium 🟡 Two Pointers
  22. Valid Word Square - Medium 🟡 Array
  23. Reconstruct Original Digits From English - Medium 🟡 Array
  24. Longest Repeating Character Replacement - Medium 🟡 Array
  25. Word Squares - Medium 🟡 Array
  26. Convert Binary Search Tree To Sorted Doubly Linked List - Medium 🟡 Tree Linked List
  27. Construct Quad Tree - Medium 🟡 Tree
  28. Serialize And Deserialize N Ary Tree - Medium 🟡 Tree
  29. N Ary Tree Level Order Traversal - Medium 🟡 Tree
  30. Flatten A Multilevel Doubly Linked List - Medium 🟡 Linked List
  31. Encode N Ary Tree To Binary Tree - Medium 🟡 Tree
  32. All Oone Data Structure - Medium 🟡 Array
  33. Minimum Genetic Mutation - Medium 🟡 Array
  34. Number Of Segments In A String - Medium 🟡 Array
  35. Non Overlapping Intervals - Medium 🟡 Array
  36. Find Right Interval - Medium 🟡 Array
  37. Path Sum III - Medium 🟡 DP
  38. Find All Anagrams In A String - Medium 🟡 Array
  39. Ternary Expression Parser - Medium 🟡 Array
  40. K Th Smallest In Lexicographical Order - Medium 🟡 Graph
  41. Arranging Coins - Medium 🟡 Array
  42. Find All Duplicates In An Array - Medium 🟡 Array
  43. String Compression - Medium 🟡 Array
  44. Sequence Reconstruction - Medium 🟡 Array
  45. Add Two Numbers II - Medium 🟡 Two Pointers
  46. Arithmetic Slices II Subsequence - Medium 🟡 Array
  47. Number Of Boomerangs - Medium 🟡 Array
  48. Find All Numbers Disappeared In An Array - Medium 🟡 Array
  49. Serialize And Deserialize BST - Medium 🟡 Array
  50. Delete Node In A BST - Medium 🟡 Array

451–500

  1. Sort Characters By Frequency - Medium 🟡 Array
  2. Minimum Number Of Arrows To Burst Balloons - Medium 🟡 Array
  3. Minimum Moves To Equal Array Elements - Medium 🟡 Array
  4. 4sum II - Medium 🟡 Array
  5. Assign Cookies - Medium 🟡 Array
  6. 132 Pattern - Medium 🟡 Array
  7. Circular Array Loop - Medium 🟡 Array
  8. Poor Pigs - Medium 🟡 Array
  9. Repeated Substring Pattern - Medium 🟡 Array
  10. LFU Cache - Medium 🟡 Array
  11. Hamming Distance - Medium 🟡 Array
  12. Minimum Moves To Equal Array Elements II - Medium 🟡 Array
  13. Island Perimeter - Medium 🟡 Graph
  14. Can I Win - Medium 🟡 Array
  15. Optimal Account Balancing - Medium 🟡 Array
  16. Count The Repetitions - Medium 🟡 Array
  17. Unique Substrings In Wraparound String - Medium 🟡 Array
  18. Validate Ip Address - Medium 🟡 Array
  19. Convex Polygon - Medium 🟡 Array
  20. Implement Rand10 Using Rand7 - Medium 🟡 Array
  21. Encode String With Shortest Length - Medium 🟡 Array
  22. Concatenated Words - Medium 🟡 Array
  23. Matchsticks To Square - Medium 🟡 Array
  24. Ones And Zeroes - Medium 🟡 Array
  25. Heaters - Medium 🟡 Array
  26. Number Complement - Medium 🟡 Array
  27. Total Hamming Distance - Medium 🟡 Array
  28. Generate Random Point In A Circle - Medium 🟡 Array
  29. Largest Palindrome Product - Medium 🟡 Array
  30. Sliding Window Median - Medium 🟡 Sliding Window
  31. Magical String - Medium 🟡 Array
  32. License Key Formatting - Medium 🟡 Array
  33. Smallest Good Base - Medium 🟡 Array
  34. Find Permutation - Medium 🟡 Backtracking
  35. Max Consecutive Ones - Medium 🟡 Array
  36. Predict The Winner - Medium 🟡 Array
  37. Max Consecutive Ones II - Medium 🟡 Array
  38. Zuma Game - Medium 🟡 Array
  39. Robot Room Cleaner - Medium 🟡 Array
  40. The Maze - Medium 🟡 Array
  41. Non Decreasing Subsequences - Medium 🟡 Array
  42. Construct The Rectangle - Medium 🟡 Array
  43. Reverse Pairs - Medium 🟡 Array
  44. Target Sum - Medium 🟡 Array
  45. Teemo Attacking - Medium 🟡 Array
  46. Next Greater Element I - Medium 🟡 Array
  47. Random Point In Non Overlapping Rectangles - Medium 🟡 Array
  48. Diagonal Traverse - Medium 🟡 Array
  49. The Maze III - Medium 🟡 Array
  50. Keyboard Row - Medium 🟡 Array

Pattern-Based Study Guide

TIER 1: FOUNDATION PATTERNS

Learn these patterns first - they form the foundation for 70% of interview problems.

Array & Hashing

When to use: Problems involving frequency counting, lookups, or duplicate detection. Key techniques: Hash maps, hash sets, prefix sums.

####Two Pointers When to use: Problems involving pairs, triplets, or palindromes in sorted/unsorted arrays. Key techniques: Left-right pointers, fast-slow pointers.

Container With Most Water

3Sum

Trapping Rain Water

Remove Nth Node From End of List

Sort Colors

Move Zeroes Sliding Window (0/7)

Longest Substring Without Repeating Characters

Minimum Window Substring

Longest Repeating Character Replacement

Permutation in String

Sliding Window Maximum

Minimum Size Subarray Sum

Fruit Into Baskets Binary Search (0/6)

Binary Search

Search in Rotated Sorted Array

Find Minimum in Rotated Sorted Array

Koko Eating Bananas

Search a 2D Matrix

Time Based Key-Value Store Linked List (0/6)

Reverse Linked List

Merge Two Sorted Lists

Reorder List

Remove Nth Node From End of List

Copy List with Random Pointer

LRU Cache Trees – Core (0/9)

Invert Binary Tree

Maximum Depth of Binary Tree

Diameter of Binary Tree

Balanced Binary Tree

Lowest Common Ancestor of a BST

Binary Tree Level Order Traversal

Validate Binary Search Tree

Serialize and Deserialize Binary Tree

Binary Tree Maximum Path Sum Heap / Priority Queue (0/4)

Kth Largest Element in Array

K Closest Points to Origin

Find Median from Data Stream

Top K Frequent Elements (Heap approach) Backtracking (0/4)

Combination Sum

Permutations

Subsets

Word Search TIER 2: PATTERN MASTERY (0/70) Advanced Array (0/8)

Maximum Subarray

Jump Game II

Rotate Array

Next Permutation

First Missing Positive

4Sum

Set Matrix Zeroes

Spiral Matrix String Manipulation (0/8)

Longest Palindromic Substring

Palindromic Substrings

Decode Ways

Valid Parentheses

Generate Parentheses

Longest Common Prefix

String to Integer (atoi)

Multiply Strings Stack (0/5)

Valid Parentheses

Min Stack

Evaluate Reverse Polish Notation

Daily Temperatures

Largest Rectangle in Histogram Graph – BFS/DFS (0/12)

Number of Islands

Clone Graph

Rotting Oranges

Pacific Atlantic Water Flow

Course Schedule

Course Schedule II

Word Ladder

Surrounded Regions

01 Matrix

Shortest Path in Binary Matrix

Walls and Gates

Number of Provinces Graph – Advanced (0/7)

Network Delay Time

Cheapest Flights Within K Stops

Swim in Rising Water

Min Cost to Connect All Points

Find the City With the Smallest Number of Neighbors

Alien Dictionary

Reconstruct Itinerary Dynamic Programming – 1D (0/10)

Climbing Stairs

House Robber

House Robber II

Longest Increasing Subsequence

Longest Decreasing Subsequence (custom variant)

Word Break

Coin Change

Maximum Product Subarray

Coin Change 2

Palindrome Partitioning II Dynamic Programming – 2D (0/10)

Unique Paths

Longest Common Subsequence

Best Time to Buy and Sell Stock III

Best Time to Buy and Sell Stock IV

Interleaving String

Edit Distance

Regular Expression Matching

Unique Paths II

Minimum Path Sum

Maximal Square Trees – Advanced (0/10)

Kth Smallest Element in BST

Construct Binary Tree from Preorder and Inorder Traversal

Binary Tree Right Side View

Count Good Nodes in Binary Tree

House Robber III

Lowest Common Ancestor of Binary Tree

Binary Tree Zigzag Level Order Traversal

Flatten Binary Tree to Linked List

Binary Tree Vertical Order Traversal

Populating Next Right Pointers in Each Node TIER 3: COMPANY-SPECIFIC PATTERNS (0/80) Trie (0/5)

Implement Trie

Design Add and Search Words Data Structure

Word Search II

Implement Magic Dictionary

Replace Words Advanced DP (0/10)

Burst Balloons

Minimum Cost to Cut a Stick

Stone Game

Predict the Winner

Partition Equal Subset Sum

Target Sum

Ones and Zeroes

Minimum Difficulty of a Job Schedule

Wildcard Matching

Distinct Subsequences Intervals (0/8)

Merge Intervals

Insert Interval

Non-overlapping Intervals

Meeting Rooms II

Minimum Number of Arrows to Burst Balloons

My Calendar I

Meeting Rooms

Employee Free Time Greedy (0/8)

Jump Game

Gas Station

Hand of Straights

Partition Labels

Valid Parenthesis String

Reorganize String

Task Scheduler

Candy Math & Bit Manipulation (0/8)

Single Number

Number of 1 Bits

Counting Bits

Reverse Integer

Pow(x, n)

Missing Number

Reverse Bits

Sum of Two Integers Design (0/8)

LRU Cache

LFU Cache

Design Twitter

Design HashMap

Flatten Nested List Iterator

Design Hit Counter

Snapshot Array

Design In-Memory File System Union Find (0/7)

Number of Connected Components

Redundant Connection

Accounts Merge

Satisfiability of Equality Equations

Graph Valid Tree

Longest Consecutive Sequence (Union Find approach)

Most Stones Removed with Same Row or Column Advanced Sliding Window (0/5)

Max Consecutive Ones III

Longest Substring with At Most K Distinct Characters

Subarrays with K Different Integers

Count Number of Nice Subarrays

Replace the Substring for Balanced String Monotonic Stack / Queue (0/5)

Next Greater Element I

Next Greater Element II

Online Stock Span

Sum of Subarray Minimums

Shortest Subarray with Sum at Least K Binary Search - Advanced (0/8)

Median of Two Sorted Arrays

Split Array Largest Sum

Capacity To Ship Packages Within D Days

Find Peak Element

Search in Rotated Sorted Array II

Find First and Last Position of Element

Minimum Number of Days to Make m Bouquets

Aggressive Cows Linked List - Advanced (0/8)

Add Two Numbers

Merge K Sorted Lists

Reverse Nodes in k-Group

Rotate List

Partition List

Linked List Cycle

Linked List Cycle II

Intersection of Two Linked Lists