LeetCode Book

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Bitwise | Number & Math | String | Linked List | Array | Tree | BST | Binary Search | Priority Queue | Backtracking | Dynamic Programming | Greedy | Graph | Trie | Design

[Review Policy] By repeatedly reviewing I can get: 🍉(x1), 🍭(x2), 🐼(x3), 🚁(x4), 🚀(x5), …, TBD.

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Notes (Summary)

• Recursion Runtime Patterns 🍉
• Divide And Conquer Notes Recursion TreeMaster Theorem 🍉
• Dynamic Programming Notes 👀 Top-DownBottom-Up
• Backtracking Notes 🥊 Backtracking vs. DFS
• Binary Search Template Notes Lower/Upper Bound [lo, hi) & [lo, hi] 🍉
• TreeSet/TreeMap Usage & binarySearch() Tree MapArrays & Collections

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Solve a Problem, Write a Post

Reference: LeetCode & Elements of Programming Interviews in Java

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Bitwise

• 136. Single Number I, II, III MediumEPI 11.10
• 191. Number of 1 Bits Easyx & (x - 1)x & ~(x - 1)EPI 4.1
• 338. Counting Bits MediumDP 🍉
• 477. Total Hamming Distance MediumEPI 4.4
• 50. Pow(x, n) Medium
• 371. Sum of Two Integers EasyAdd Two NumbersNot Easy

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Number & Math

• Duplicate & Missing
  • 268. Missing Number Easy 🍉
  • 287. Find the Duplicate Number MediumCycle Detection 🍉
  • 645. Set Mismatch EasyBit ManipulationEPI 11.10 🍉
  • 41. First Missing Positive Hard
• Primality
  • 204. Count Primes EasySieve of Eratosthenes
• Majority
  • 169. Majority Element EasyBoyer-Moore Majority Vote Algorithm
  • 229. Majority Element II Medium
• Uncategorized
  • 9. Palindrome Number EasyEPI 4.9
  • 66. Plus One / 369. Plus One Linked List EasyIt is easy compared to 2. Add Two Numbers
  • 231. Power of Two / 326. Power of Three / 342. Power of Four Easy
  • 13. Roman to Integer / 12. Integer to Roman EasyEPI 6.8
  • 202. Happy Number EasyCycle Detection
  • 263. Ugly Number I & II MediumPrimePriority QueueDP
  • 780. Reaching Points Hard
  • 273. Integer to English Words Hard

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String

• Basic
  • EPI 6.1 | Interconvert Strings and Integers String <-> IntegerEPI 6.1
  • 8. String to Integer (atoi) EasyPhD in Corner Cases
  • 541. Reverse String II EasyGroup Index Practice
  • 151. Reverse Words in a String I Medium
  • 186. Reverse Words in a String II MediumIn-PlaceEPI 6.6
  • 557. Reverse Words in a String III EasySimplified Version of II
  • 819. Most Common Word EasyRegex
  • 412. Fizz Buzz Easy
• Palindrome
  • 125. Valid Palindrome I & II EasyTwo PointersEPI 6.5
  • 266. Palindrome Permutation I EasyHash Table
  • 267. Palindrome Permutation II MediumBacktracking
  • EPI 15.8 | Generate Palindromic Decompositions BacktrackingTime ComplexityEPI 15.8
  • 647. Palindromic Substrings MediumExpansionDP 🍉
  • 5. Longest Palindromic Substring MediumExpansionDP 🍉
  • 516. Longest Palindromic Subsequence MediumDFSDP 🍉
• Uncategorized
  • 1002. Find Common Characters EasyA bit strange
  • 451. Sort Characters By Frequency Medium
  • 271. Encode and Decode Strings Medium
  • 937. Reorder Data in Log Files EasyComparatorSucks 🍉🍭
  • 6. ZigZag Conversion MediumIndex Practice
  • 38. Count and Say EasyIndex Practice
  • 468. Validate IP Address Medium
  • OA | Substring of Size K with K Distinct Chars MediumSliding WindowHash Set
  • 49. Group Anagrams MediumHash Mapsorting
  • 953. Verifying an Alien Dictionary Easy

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Linked List

• Add / Merge Two Lists
  • 2. Add Two Numbers Medium 🍉🍭
  • 445. Add Two Numbers II Medium 🍉🍭
  • 21. Merge Two Sorted Lists EasyEPI 7.1 🍉🍭
  • 23. Merge k Sorted Lists HardEPI 10.1 🍉🍭
• Reverse / Swap List
  • 206. Reverse Linked List Easy (gist) 🍉🍭
  • 92. Reverse Linked List II MediumEPI 7.2 🍉🍭
  • 25. Reverse Nodes in k-Group HardEPI 7.2 🍉🍭
  • 24. Swap Nodes in Pairs Medium 🍉🍭
• Cycle Detection
  • 141. Linked List Cycle I EasyTwo Pointers 🍉🍭
  • 142. Linked List Cycle II MediumTwo PointersEPI 7.3 🍉🍭
• Sort List
  • 147. Insertion Sort List Medium 🍉🍭
  • 148. Sort List MediumSlow/Fast Pointers 🍉🍭
• Uncategorized
  • 234. Palindrome Linked List EasyTwo Pointers 🍉
  • 160. Intersection of Two Linked Lists EasyTwo PointersEPI 7.4 🍉🍭
  • 61. Rotate List Medium 🍉
  • 817. Linked List Components Medium 🍉
  • 328. Odd Even Linked List Medium
  • 725. Split Linked List in Parts Medium
  • 138. Copy List with Random Pointer MediumDeep Copy 🍉
  • 203. Remove Linked List Elements EasyDummy

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Array

• N-Sum
  • 1. Two Sum EasyHash TableUsed OnceElements are not necessarily unique 🍉
  • 167. Two Sum II (sorted) EasyHash TableBinary SearchTwo PointersUsed Once 🍉
  • 170. Two Sum III (data structure design) EasyTwo SetsCount Map 🍉
  • 653. Two Sum IV (BST) EasyBSTA Tree Problem 🍉
  • 1214. Two Sum BSTs MediumTwo Pointers 🍉
  • 1099. Two Sum Less Than K EasySorting & Two Pointers 🍉
  • EPI 17.4 | The 3-Sum Problem Hash TableBinary SearchTwo PointersUsed >1EPI 17.4 🍉
  • 15. 3Sum MediumTwo PointersUsed OnceDifficult to handle duplicate triplets 🍉
  • 259. 3Sum Smaller MediumTwo PointersAllow Duplicate Pairs 🍉
  • 18. 4Sum MediumTwo Pointers 🍉
• Subarray Sum
  • 560. Subarray Sum Equals K MediumHash Map
  • 325. Maximum Size Subarray Sum Equals K MediumHash Map
    • 525. Contiguous Array MediumTo Be Done
  • 209. Minimum Size Subarray Sum MediumTree MapBinary SearchTwo Pointers
    • 862. Shortest Subarray with Sum at Least K HardTo Be Done (Negative Allowed)
    • 76. Minimum Window Substring HardTo Be Done (Same Idea on Two Pointers)
• Sliding Window
  • 219. Contains Duplicate II EasyHash Mapk-size Set 🍉
  • 220. Contains Duplicate III MediumTree Set 🍉
  • 239. Sliding Window Maximum HardPriority QueueDequeDP 🍉
  • 992. Subarrays with K Distinct Integers HardSliding Window
• Intersection
  • 349. Intersection of Two Arrays EasyHash Table 🍉
  • 350. Intersection of Two Arrays II EasyHash TableBinary SearchTwo Pointers 🍉
• Duplicates
  • 26. Remove Duplicates from Sorted Array EasyEPI 5.5 🍉
  • 217. Contains Duplicate I EasyHash SetSorting 🍉
• In-Place Removal / Swapping
  • EPI 5.1 | The Dutch National Flag Problem 3-Way PartitionSortingEPI 5.1
  • 561. Array Partition I EasyPermutation
  • 27. Remove Element EasyTwo PointersRare vs. Many
  • 283. Move Zeroes EasyTwo Pointers
• Buy and Sell Stocks & Similar Problems
  • EPI 5.6 | Longest Subarray with Equal elements One-PassEPI 5.6 Variant
  • 53. Maximum Subarray Easydivide-and-conquerDP
  • 121. Best Time to Buy and Sell Stock Easydivide-and-conquerOne-PassEPI 5.6
• Uncategorized
  • 88. Merge Sorted Array EasyTwo PointersEPI 13.2
  • EPI 5.4 | Advancing Through an Array GreedyDFSEPI 5.4
  • 189. Rotate Array EasyIn-Place
  • 11. Container With Most Water MediumTwo Pointers
  • 56. Merge Intervals Medium
  • 945. Minimum Increment to Make Array Unique MediumSorting
  • 532. K-diff Pairs in an Array EasyHash Map
  • 739. Daily Temperatures MediumStack
  • 695. Max Area of Island MediumDFS
  • 200. Number of Islands MediumDFSBFS
  • 694. Number of Distinct Islands MediumDFSHash Table
  • 289. Game of Life Medium
  • 73. Set Matrix Zeroes Medium
  • 31. Next Permutation Medium

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Tree

• Traversal
  • 94. Binary Tree Inorder Traversal MediumMorris Traversal 🍉🍭
  • 144. Binary Tree Preorder Traversal MediumMorris Traversal 🍉🍭
  • 145. Binary Tree Postorder Traversal HardPreorder-likeInorder-like 🍉🍭
  • EPI 9.10 | Binary Tree Traversals with Parent EPI 9.10Not Common 🍉
  • 102. Binary Tree Level Order Traversal I & II MediumPreorderBFS 🍉🍭
  • 429. N-ary Tree Level Order Traversal Easy 🍉🍭
  • 589. N-ary Tree Preorder Traversal Easy 🍉🍭
  • 987. Vertical Order Traversal of a Binary Tree Medium 🍉🍭
  • 114. Flatten Binary Tree to Linked List Medium 🍉🍭
• Equality & Subtree
  • 100. Same Tree Easy 🍉🍭
  • 101. Symmetric Tree EasyDFSBFSEPI 9.2 🍉🍭
  • 572. Subtree of Another Tree EasyPreorder Sequence Comparison 🍉🍭
• Height & Depth
  • 104. Maximum Depth of Binary Tree Easy 🍉
  • 110. Balanced Binary Tree Easy 🍉
  • 111. Minimum Depth of Binary Tree Easy 🍉
  • 236. Lowest Common Ancestor of a Binary Tree MediumEPI 9.3EPI 9.4
  • 235. Lowest Common Ancestor of a Binary Search Tree EasyEPI 14.4
• Sum & Path
  • EPI 9.5 | Sum Root-To-Leaf Paths of Binary Tree (Binary Data) EPI 9.5 🍉
  • 129. Sum Root to Leaf Numbers (Decimal Data) Medium EPI 9.5 🍉
  • 112. Path Sum EasyEPI 9.6 🍉
  • 113. Path Sum II Medium 🍉
  • 437. Path Sum III EasyNot Easy, Prefix Sum is difficult to understand 🍉
  • 257. Binary Tree Paths Easy 🍉
  • Using Max / Min Field
    • 543. Diameter of Binary Tree EasySimple Version of 124
    • 124. Binary Tree Maximum Path Sum Hard 🍉
    • 687. Longest Univalue Path EasyAnother Version of 124. Not Easy
• Pruning & Trimming
  • 814. Binary Tree Pruning MediumGreat Problem 🍉
  • 669. Trim a Binary Search Tree Easy 🍉
• Recovery & Construction
  • 105. Construct Binary Tree from Preorder / Postorder and Inorder Traversal Medium EPI 9.11
  • 108. Convert Sorted Array / List to Binary Search Tree EasyMedium EPI 14.8 🍉
  • 99. Recover Binary Search Tree Hard
  • 1028. Recover a Tree From Preorder Traversal Hard
  • 297. Serialize and Deserialize Binary Tree Hard
  • 428. Serialize and Deserialize N-ary Tree Medium
  • 449. Serialize and Deserialize BST Medium
• Binary Search Tree (BST)
  • 98. Validate Binary Search Tree MediumEPI 14.1 🍉
  • 285. Inorder Successor in BST Medium 🍉
  • 510. Inorder Successor in BST II MediumEPI 9.9 🍉
  • 701. Insert into a Binary Search Tree Medium 🍉
  • 450. Delete Node in a BST Medium 🍉
  • 230. Kth Smallest Element in a BST MediumEPI 14.3 🍉
  • 530. Minimum Absolute Difference in BST Easy 🍉
  • 501. Find Mode in Binary Search Tree Easy
  • 173. Binary Search Tree Iterator Medium
• Uncategorized
  • 617. Merge Two Binary Trees Easy
  • 654. Maximum Binary Tree Medium

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Binary Search

• Basic Search
  • 35. Search Insert Position Easy
  • 34. Find First and Last Position of Element in Sorted Array Medium
  • 278. First Bad Version Easy
  • 374. Guess Number Higher or Lower Easy
  • EPI 11.2 | Search a Sorted Array for Entry Equal to Its Index Standard BSEPI 11.2
• Rotated Sorted Array
  • 153. Find Minimum in Rotated Sorted Array MediumFor Problem 33divide-and-conquer
  • 154. Find Minimum in Rotated Sorted Array II HardFor Problem 81divide-and-conquer
  • 33 / 81. Search in Rotated Sorted Array I & II Medium
• Matrix
  • 74. Search a 2D Matrix MediumEPI 11.6
  • 240. Search a 2D Matrix II MediumEPI 11.6
  • 378. Kth Smallest Element in a Sorted Matrix MediumEPI 11.6
• Uncategorized
  • 367. Valid Perfect Square EasyEPI 11.4
  • 633. Sum of Square Numbers Easy
  • 29. Divide Two Integers Medium
  • 4. Median of Two Sorted Arrays Hard
  • 162. Find Peak Element MediumVery Strange

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Priority Queue

• 347. Top K Frequent Elements / 692. Top K Frequent Words Medium 🍉🍭
• 215. Kth Largest Element in an Array MediumQuick SelectEPI 11.8
• 973. K Closest Points to Origin Medium Very Similar to 215Quick SelectEPI 10.4 🍉🍭
• 295. Find Median from Data Stream HardBinary Search Templates 🍉🍭
• 659. Split Array into Consecutive Subsequences MediumGreedy
• 407. Trapping Rain Water II Hard
• 1102. Path With Maximum Minimum Value MediumBacktrackingHeapVariant -> DP

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Backtracking

• Basic
  • 46. Permutations I & II MediumEPI 15.4
  • 77. CombinationsMediumEPI 15.6
  • 78. Subsets I & II MediumStartIndexInteresting!EPI 15.5
  • 17. Letter Combinations of a Phone Number MediumStartIndexSimilar to SubsetsEPI 15.2
  • 39. Combination Sum I & II & III Medium
• Parentheses
  • 20. Valid Parentheses EasyStackNot Backtracking
  • 22. Generate Parentheses MediumEPI 15.7
  • 301. Remove Invalid Parentheses Hard
• Uncategorized
  • 89. Gray Code MediumEPI 15.11
  • 93. Restore IP Addresses MediumEPI 6.9
  • 51. N-Queens I & II Hard
  • 36. Valid Sudoku MediumHash Table
  • 37. Sudoku Solver Hard
  • 79. Word Search I & II Medium
  • 139. Word Break I MediumDP
  • 140. Word Break II HardDP

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Dynamic Programming

• 70. Climbing Stairs EasyCounting
• 746. Min Cost Climbing Stairs EasyOptimizationIndex!
• 分界线
• 62. Unique Paths I & II MediumCountingProblem II is very great!
• 分界线
• 96. Unique Binary Search Trees I MediumCountingEPI 15.9
• 95. Unique Binary Search Trees II MediumCountingClone a Tree
• 分界线
• 845. Longest Mountain in Array MediumOne Pointer
• 42. Trapping Rain Water HardTwo Pointersdivide-and-conquerDP
• 1D w/ Multiple States / dp[i][0], dp[i][1], dp[i][2]
  • 256. Paint House EasyOptimization
  • 926. Flip String to Monotone Increasing Mediumdp[i][0] & dp[i][1]Challenging

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Greedy

• 134. Gas Station MediumEPI 17.6

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Graph

• 133. Clone Graph MediumDeep Copy 🍉
• 785. Is Graph Bipartite? MediumDFS
• 547. Friend Circles MediumDFS
• 1192. Critical Connections in a Network HardDFSTarjan’s Algorithm
• 207. Course Schedule I & II MediumCycleDFSBFSTopological Sort
• 269. Alien Dictionary HardBFSTopological Sort953. Verifying an Alien Dictionary

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Trie

• 208. Implement Trie (Prefix Tree) MediumCS 61B Tries 🍉
• 421. Maximum XOR of Two Numbers in an Array MediumTrie Practice

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Design

• 707. Design Linked List Easy 🍉🍭
• 155. Min Stack EasyOne Stack 🍉🍭
• 716. Max Stack EasyTwo Stacks 🍉🍭
• 146. LRU Cache MediumLinkedHashMap
• 460. LFU Cache HardLinkedHashSet
• 341. Flatten Nested List Iterator MediumStackRecursion
• 794. Valid Tic-Tac-Toe State Medium