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OJ Practices
  • Introduction
  • Leetcode
    • Tree
      • 94 Binary Tree Inorder Traversal
      • 98 Validate Binary Search Tree
      • 99 Recover Binary Search Tree
      • 100 Same Tree
      • 101 Symmetric Tree
      • 102 Binary Tree Level Order Traversal
      • 103 Binary Tree Zigzag Level Order Traversal
      • 104 Maximum Depth of Binary Tree
      • 105 Construct Binary Tree from Preorder and Inorder Traversal
      • 106 Construct Binary Tree from Inorder and Postorder Traversal
      • 107 Binary Tree Level Order Traversal II
      • 108 Convert Sorted Array to Binary Search Tree
      • 110 Balanced Binary Tree
      • 111 Minimum Depth of Binary Tree
      • 112 Path Sum
      • 113 Path Sum II
      • 114 Flatten Binary Tree to Linked List
      • 116 Populating Next Right Pointers in Each Node
      • 117 Populating Next Right Pointers in Each Node II
      • 124 Binary Tree Maximum Path Sum
      • 129 Sum Root to Leaf Numbers
      • 144 Binary Tree Preorder Traversal
      • 145 Binary Tree Postorder Traversal
      • 156 Binary Tree Upside Down
      • 173 Binary Search Tree Iterator
      • 199 Binary Tree Right Side View
      • 222 Count Complete Tree Nodes
      • 226 Invert Binary Tree
      • 230 Kth Smallest Element in a BST
      • 235 Lowest Common Ancestor of a Binary Search Tree
      • 236 Lowest Common Ancestor of a Binary Tree
      • 250 Count Univalue Subtrees
      • 255 Verify Preorder Sequence in Binary Search Tree
      • 257 Binary Tree Paths
      • 270 Closest Binary Search Tree Value
      • 272 Closest Binary Search Tree Value II
      • 285 Inorder Successor in BST
      • 297 Serialize and Deserialize Binary Tree
      • 314 Binary Tree Vertical Order Traversal
      • 333 Largest BST Subtree
      • 366 Find Leaves of Binary Tree
      • 404 Sum of Left Leaves
      • 437 Path Sum III
      • 449 Serialize and Deserialize BST
      • 450 Delete Node in a BST
      • 501 Find Mode in Binary Search Tree
      • 508 Most Frequent Subtree Sum
      • 513 Find Bottom Left Tree Value
      • 515 Find Largest Value in Each Tree Row
      • 536 Construct Binary Tree from String
      • 538 Convert BST to Greater Tree
      • 543 Diameter of Binary Tree
      • 545 Boundary of Binary Tree
      • 549 Binary Tree Longest Consecutive Sequence II
      • 563 Binary Tree Tilt
      • 572 Subtree of Another Tree
      • 582 Kill Process
      • 606 Construct String from Binary Tree
      • 617 Merge Two Binary Trees
      • 623 Add One Row to Tree
      • 637 Average of Levels in Binary Tree
      • 652 Find Duplicate Subtrees
      • 653 Two Sum IV - Input is a BST
      • 654 Maximum Binary Tree
      • 655 Print Binary Tree
      • 662 Maximum Width of Binary Tree
      • 663 Equal Tree Partition
      • 666 Path Sum IV
      • 669 Trim a Binary Search Tree
      • 671 Second Minimum Node In a Binary Tree
      • 687 Longest Univalue Path
      • 742 Closest Leaf in a Binary Tree
      • 814 Binary Tree Pruning
      • 834 Sum of Distances in Tree
    • Dynamic Programming
      • LIS
        • 300 Longest Increasing Subsequence
        • 334 Increasing Triplet Subsequence
        • 354 Russian Doll Envelopes
        • 368 Largest Divisible Subset
        • 646 Maximum Length of Pair Chain
        • 673 Number of Longest Increasing Subsequence
      • LCS
        • 5 Longest Palindromic Substring
        • 72 Edit Distance
        • 115 Distinct Subsequences
        • 392 Is Subsequence
        • 516 Longest Palindromic Subsequence
        • 583 Delete Operation for Two Strings
        • 647 Palindromic Substrings
        • 712 Minimum ASCII Delete Sum for Two Strings
      • Knapsack Problem
        • 322 Coin Change
        • 377 Combination Sum IV
        • 416 Partition Equal Subset Sum
        • 474 Ones and Zeroes
        • 494 Target Sum
        • 518 Coin Change 2
      • Interval DP
        • 132 Palindrome Partitioning II
        • 312 Burst Balloons
        • 375 Guess Number Higher or Lower II
        • 471 Encode String with Shortest Length
        • 486 Predict the Winner
        • 516 Longest Palindromic Subsequence
        • 546 Remove Boxes
        • 664 Strange Printer
      • 10 Regular Expression Matching
      • 32 Longest Valid Parentheses
      • 44 Wildcard Matching
      • 53 Maximum Subarray
      • 62 Unique Paths
      • 63 Unique Paths II
      • 64 Minimum Path Sum
      • 70 Climbing Stairs
      • 85 Maximal Rectangle
      • 87 Scramble String
      • 91 Decode Ways
      • 95 Unique Binary Search Trees II
      • 96 Unique Binary Search Trees
      • 97 Interleaving String
      • 120 Triangle
      • 121 Best Time to Buy and Sell Stock
      • 122 Best Time to Buy and Sell Stock II
      • 123 Best Time to Buy and Sell Stock III
      • 139 Word Break
      • 140 Word Break II
      • 152 Maximum Product Subarray
      • 174 Dungeon Game
      • 188 Best Time to Buy and Sell Stock IV
      • 198 House Robber
      • 213 House Robber II
      • 221 Maximal Square
      • 238 Product of Array Except Self
      • 256 Paint House
      • 264 Ugly Number II
      • 265 Paint House II
      • 276 Paint Fence
      • 279 Perfect Squares
      • 303 Range Sum Query - Immutable
      • 304 Range Sum Query 2D - Immutable
      • 309 Best Time to Buy and Sell Stock with Cooldown
      • 321 Create Maximum Number
      • 338 Counting Bits
      • 343 Integer Break
      • 361 Bomb Enemy
      • 363 Max Sum of Rectangle No Larger Than K
      • 403 Frog Jump
      • 410 Split Array Largest Sum
      • 413 Arithmetic Slices
      • 418 Sentence Screen Fitting
      • 446 Arithmetic Slices II - Subsequence
      • 464 Can I Win
      • 467 Unique Substrings in Wraparound String
      • 514 Freedom Trail
      • 517 Super Washing Machines
      • 523 Continuous Subarray Sum
      • 552 Student Attendance Record II
      • 562 Longest Line of Consecutive One in Matrix
      • 568 Maximum Vacation Days
      • 576 Out of Boundary Paths
      • 600 Non-negative Integers without Consecutive Ones
      • 629 K Inverse Pairs Array
      • 638 Shopping Offers
      • 639 Decode Ways II
      • 650 2 Keys Keyboard
      • 651 4 Keys Keyboard
      • 656 Coin Path
      • 688 Knight Probability in Chessboard
      • 689 Maximum Sum of 3 Non-Overlapping Subarrays
      • 691 Stickers to Spell Word
      • 698 Partition to K Equal Sum Subsets
      • 714 Best Time to Buy and Sell Stock with Transaction Fee
      • 718 Maximum Length of Repeated Subarray
      • 727 Minimum Window Subsequence
      • 730 Count Different Palindromic Subsequences
      • 740 Delete and Earn
      • 741 Cherry Pickup
      • 746 Min Cost Climbing Stairs
      • 750 Number Of Corner Rectangles
      • 764 Largest Plus Sign
      • 787 Cheapest Flights Within K Stops
      • 790 Domino and Tromino Tiling
      • 801 Minimum Swaps To Make Sequences Increasing
      • 808 Soup Servings
      • 813 Largest Sum of Averages
    • Depth-First-Search
      • Memoization
        • 139 Word Break
        • 140 Word Break II
        • 276 Paint Fence
        • 294 Flip Game II
        • 329 Longest Increasing Path in a Matrix
        • 337 House Robber III
        • 375 Guess Number Higher or Lower II
        • 514 Freedom Trail
        • 568 Maximum Vacation Days
        • 638 Shopping Offers
        • 808 Soup Servings
    • Breadth-First Search
      • 126 Word Ladder II
      • 127 Word Ladder
      • 130 Surrounded Regions
      • 133 Clone Graph
      • 200 Number of Islands
      • 261 Graph Valid Tree
      • 279 Perfect Squares
      • 286 Walls and Gates
      • 301 Remove Invalid Parentheses
      • 310 Minimum Height Trees
      • 317 Shortest Distance from All Buildings
      • 323 Number of Connected Components in an Undirected Graph
      • 365 Water and Jug Problem
      • 397 Integer Replacement
      • 407 Trapping Rain Water II
      • 417 Pacific Atlantic Water Flow
      • 429 N-ary Tree Level Order Traversal
      • 433 Minimum Genetic Mutation
      • 490 The Maze
      • 499 The Maze III
      • 505 The Maze II
      • 529 Minesweeper
      • 542 01 Matrix
      • 675 Cut Off Trees for Golf Event
      • 690 Employee Importance
      • 743 Network Delay Time
      • 752 Open the Lock
      • 773 Sliding Puzzle
      • 785 Is Graph Bipartite?
      • 787 Cheapest Flights Within K Stops
      • 815 Bus Routes
      • 818 Race Car
      • 847 Shortest Path Visiting All Nodes
      • 854 K-Similar Strings
      • 864 Shortest Path to Get All Keys
      • 886 Possible Bipartition
      • 909 Snakes and Ladders
      • 913 Cat and Mouse
      • 934 Shortest Bridge
    • Two Pointers
      • 3 Longest Substring Without Repeating Characters
      • 11 Container With Most Water
      • 15 3Sum
      • 16 3Sum Closest
      • 18 4Sum
      • 26 Remove Duplicates from Sorted Array
      • 27 Remove Element
      • 30 Substring with Concatenation of All Words
      • 42 Trapping Rain Water
      • 76 Minimum Window Substring
      • 80 Remove Duplicates from Sorted Array II
      • 88 Merge Sorted Array
      • 125 Valid Palindrome
      • 159 Longest Substring with At Most Two Distinct Characters
      • 167 Two Sum II - Input array is sorted
      • 202 Happy Number
      • 209 Minimum Size Subarray Sum
      • 259 3Sum Smaller
      • 283 Move Zeroes
      • 287 Find the Duplicate Number
      • 340 Longest Substring with At Most K Distinct Characters
      • 344 Reverse String
      • 345 Reverse Vowels of a String
      • 349 Intersection of Two Arrays
      • 350 Intersection of Two Arrays II
      • 360 Sort Transformed Array
      • 395 Longest Substring with At Least K Repeating Characters
      • 424 Longest Repeating Character Replacement
      • 438 Find All Anagrams in a String
      • 487 Max Consecutive Ones II
      • 524 Longest Word in Dictionary through Deleting
      • 532 K-diff Pairs in an Array
      • 567 Permutation in String
      • 611 Valid Triangle Number
      • 632 Smallest Range
      • 713 Subarray Product Less Than K
      • 723 Candy Crush
      • 763 Partition Labels
      • 826 Most Profit Assigning Work
      • 828 Unique Letter String
      • 838 Push Dominoes
      • 844 Backspace String Compare
      • 845 Longest Mountain in Array
      • 881 Boats to Save People
      • 904 Fruit Into Baskets
      • 457 Circular Array Loop
    • Linked List
      • 2 Add Two Numbers
      • 19 Remove Nth Node From End of List
      • 21 Merge Two Sorted Lists
      • 23 Merge k Sorted Lists
      • 24 Swap Nodes in Pairs
      • 25 Reverse Nodes in k-Group
      • 61 Rotate List
      • 82 Remove Duplicates from Sorted List II
      • 83 Remove Duplicates from Sorted List
      • 86 Partition List
      • 92 Reverse Linked List II
      • 109 Convert Sorted List to Binary Search Tree
      • 138 Copy List with Random Pointer
      • 141 Linked List Cycle
      • 142 Linked List Cycle II
      • 143 Reorder List
      • 147 Insertion Sort List
      • 148 Sort List
      • 160 Intersection of Two Linked Lists
      • 203 Remove Linked List Elements
      • 206 Reverse Linked List
      • 234 Palindrome Linked List
      • 237 Delete Node in a Linked List
      • 328 Odd Even Linked List
      • 369 Plus One Linked List
      • 379 Design Phone Directory
      • 426 Convert Binary Search Tree to Sorted Doubly Linked List
      • 430 Flatten a Multilevel Doubly Linked List
      • 445 Add Two Numbers II
      • 707 Design Linked List
      • 708 Insert into a Cyclic Sorted List
      • 725 Split Linked List in Parts
      • 817 Linked List Components
      • 876 Middle of the Linked List
    • Backtracking
      • 17 Letter Combinations of a Phone Number
      • 22 Generate Parentheses
      • 37 Sudoku Solver
      • 39 Combination Sum
      • 40 Combination Sum II
      • 46 Permutations
      • 47 Permutations II
      • 51 N-Queens
      • 52 N-Queens II
      • 60 Permutation Sequence
      • 77 Combinations
      • 78 Subsets
      • 79 Word Search
      • 89 Gray Code
      • 90 Subsets II
      • 93 Restore IP Addresses
      • 131 Palindrome Partitioning
      • 212 Word Search II
      • 216 Combination Sum III
      • 254 Factor Combinations
      • 267 Palindrome Permutation II
      • 282 Expression Add Operators
      • 291 Word Pattern II
      • 301 Remove Invalid Parentheses
      • 306 Additive Number
      • 320 Generalized Abbreviation
      • 357 Count Numbers with Unique Digits
      • 401 Binary Watch
      • 411 Minimum Unique Word Abbreviation
      • 425 Word Squares
      • 473 Matchsticks to Square
      • 491 Increasing Subsequences
      • 526 Beautiful Arrangement
      • 679 24 Game
      • 698 Partition to K Equal Sum Subsets
      • 784 Letter Case Permutation
      • 797 All Paths From Source to Target
      • 842 Split Array into Fibonacci Sequence
    • Binary Search
      • 4 Median of Two Sorted Arrays
      • 29 Divide Two Integers
      • 33 Search in Rotated Sorted Array
      • 34 Search for a Range
      • 35 Search Insert Position
      • 50 Pow(x, n)
      • 69 Sqrt(x)
      • 74 Search a 2D Matrix
      • 81 Search in Rotated Sorted Array II
      • 153 Find Minimum in Rotated Sorted Array
      • 154 Find Minimum in Rotated Sorted Array II
      • 162 Find Peak Element
      • 209 Minimum Size Subarray Sum
      • 240 Search a 2D Matrix II
      • 275 H-Index II
      • 278 First Bad Version
      • 287 Find the Duplicate Number
      • 300 Longest Increasing Subsequence
      • 302 Smallest Rectangle Enclosing Black Pixels
      • 349 Intersection of Two Arrays
      • 350 Intersection of Two Arrays II
      • 354 Russian Doll Envelopes
      • 367 Valid Perfect Square
      • 374 Guess Number Higher or Lower
      • 378 Kth Smallest Element in a Sorted Matrix
      • 410 Split Array Largest Sum
      • 436 Find Right Interval
      • 441 Arranging Coins
      • 475 Heaters
      • 483 Smallest Good Base
      • 497 Random Point in Non-overlapping Rectangles
      • 528 Random Pick with Weight
      • 540 Single Element in a Sorted Array
      • 644 Maximum Average Subarray II
      • 658 Find K Closest Elements
      • 668 Kth Smallest Number in Multiplication Table
      • 702 Search in a Sorted Array of Unknown Size
      • 704 Binary Search
      • 710 Random Pick with Blacklist
      • 718 Maximum Length of Repeated Subarray
      • 719 Find K-th Smallest Pair Distance
      • 744 Find Smallest Letter Greater Than Target
      • 774 Minimize Max Distance to Gas Station
      • 778 Swim in Rising Water
      • 786 K-th Smallest Prime Fraction
      • 793 Preimage Size of Factorial Zeroes Function
      • 852 Peak Index in a Mountain Array
      • 862 Shortest Subarray with Sum at Least K
      • 875 Koko Eating Bananas
      • 878 Nth Magical Number
      • 887 Super Egg Drop
      • 911 Online Election
    • Union Find
      • 128 Longest Consecutive Sequence
      • 130 Surrounded Regions
      • 200 Number of Islands
      • 261 Graph Valid Tree
      • 305 Number of Islands II
      • 323 Number of Connected Components in an Undirected Graph
      • 399 Evaluate Division
      • 547 Friend Circles
      • 684 Redundant Connection
      • 685 Redundant Connection II
      • 695 Max Area of Island
      • 721 Accounts Merge
      • 737 Sentence Similarity II
      • 765 Couples Holding Hands
      • 778 Swim in Rising Water
      • 803 Bricks Falling When Hit
      • 839 Similar String Groups
      • 924 Minimize Malware Spread
      • 928 Minimize Malware Spread II
      • 947 Most Stones Removed with Same Row or Column
      • 952 Largest Component Size by Common Factor
      • 959 Regions Cut By Slashes
    • Trie
      • 208 Implement Trie (Prefix Tree)
      • 211 Add and Search Word - Data structure design
      • 212 Word Search II
      • 336 Palindrome Pairs
      • 421 Maximum XOR of Two Numbers in an Array
      • 425 Word Squares
      • 472 Concatenated Words
      • 527 Word Abbreviation
      • 588 Design In-Memory File System
      • 642 Design Search Autocomplete System
      • 648 Replace Words
      • 676 Implement Magic Dictionary
      • 677 Map Sum Pairs
      • 692 Top K Frequent Words
      • 720 Longest Word in Dictionary
      • 745 Prefix and Suffix Search
    • Sort
      • 49 Group Anagrams
      • 75 Sort Colors
      • 164 Maximum Gap
      • 179 Largest Number
      • 274 H-Index
      • 280 Wiggle Sort
      • 296 Best Meeting Point
      • 315 Count of Smaller Numbers After Self
      • 324 Wiggle Sort II
      • 327 Count of Range Sum
      • 406 Queue Reconstruction by Height
      • 462 Minimum Moves to Equal Array Elements II
      • 493 Reverse Pairs
      • 846 Hand of Straights
    • Heap
      • 215 Kth Largest Element in an Array
      • 218 The Skyline Problem
      • 239 Sliding Window Maximum
      • 253 Meeting Rooms II
      • 264 Ugly Number II
      • 295 Find Median from Data Stream
      • 313 Super Ugly Number
      • 347 Top K Frequent Elements
      • 355 Design Twitter
      • 358 Rearrange String k Distance Apart
      • 373 Find K Pairs with Smallest Sums
      • 378 Kth Smallest Element in a Sorted Matrix
      • 407 Trapping Rain Water II
      • 451 Sort Characters By Frequency
      • 480 Sliding Window Median
      • 502 IPO
      • 632 Smallest Range
      • 659 Split Array into Consecutive Subsequences
      • 668 Kth Smallest Number in Multiplication Table
      • 692 Top K Frequent Words
      • 703 Kth Largest Element in a Stream
      • 719 Find K-th Smallest Pair Distance
      • 743 Network Delay Time
      • 759 Employee Free Time
      • 767 Reorganize String
      • 778 Swim in Rising Water
      • 786 K-th Smallest Prime Fraction
      • 787 Cheapest Flights Within K Stops
      • 818 Race Car
      • 855 Exam Room
      • 857 Minimum Cost to Hire K Workers
      • 864 Shortest Path to Get All Keys
      • 871 Minimum Number of Refueling Stops
      • 882 Reachable Nodes In Subdivided Graph
    • Randomness
      • 384 Shuffle an Array
      • 470 Implement Rand10() Using Rand7()
      • 478 Generate Random Point in a Circle
      • 497 Random Point in Non-overlapping Rectangles
      • 519 Random Flip Matrix
      • 528 Random Pick with Weight
      • 710 Random Pick with Blacklist
    • Topological Sort
      • 207 Course Schedule
      • 210 Course Schedule II
      • 269 Alien Dictionary
      • 444 Sequence Reconstruction
      • 802 Find Eventual Safe States
    • Stack
      • 20 Valid Parentheses
      • 42 Trapping Rain Water
      • 71 Simplify Path
      • 84 Largest Rectangle in Histogram
      • 85 Maximal Rectangle
      • 150 Evaluate Reverse Polish Notation
      • 155 Min Stack
      • 224 Basic Calculator
      • 227 Basic Calculator II
      • 225 Implement Stack using Queues
      • 232 Implement Queue using Stacks
      • 316 Remove Duplicate Letters
      • 331 Verify Preorder Serialization of a Binary Tree
      • 341 Flatten Nested List Iterator
      • 385 Mini Parser
      • 394 Decode String
      • 402 Remove K Digits
      • 439 Ternary Expression Parser
      • 456 132 Pattern
      • 484 Find Permutation
      • 496 Next Greater Element I
      • 503 Next Greater Element II
      • 591 Tag Validator
      • 636 Exclusive Time of Functions
      • 682 Baseball Game
      • 726 Number of Atoms
      • 735 Asteroid Collision
      • 739 Daily Temperatures
      • 768 Max Chunks To Make Sorted II
      • 770 Basic Calculator IV
      • 772 Basic Calculator III
      • 844 Backspace String Compare
      • 853 Car Fleet
      • 856 Score of Parentheses
      • 880 Decoded String at Index
      • 895 Maximum Frequency Stack
      • 901 Online Stock Span
      • 907 Sum of Subarray Minimums
      • 946 Validate Stack Sequences
      • 962 Maximum Width Ramp
      • 975 Odd Even Jump
    • Hash Table
      • 1 Two Sum
      • 3 Longest Substring Without Repeating Characters
      • 18 4Sum
      • 30 Substring with Concatenation of All Words
      • 37 Sudoku Solver
      • 49 Group Anagrams
      • 76 Minimum Window Substring
      • 136 Single Number
      • 138 Copy List with Random Pointer
      • 149 Max Points on a Line
      • 159 Longest Substring with At Most Two Distinct Characters
      • 166 Fraction to Recurring Decimal
      • 170 Two Sum III - Data structure design
      • 187 Repeated DNA Sequences
      • 202 Happy Number
      • 204 Count Primes
      • 205 Isomorphic Strings
      • 217 Contains Duplicate
      • 219 Contains Duplicate II
      • 242 Valid Anagram
      • 244 Shortest Word Distance II
      • 246 Strobogrammatic Number
      • 249 Group Shifted Strings
      • 266 Palindrome Permutation
      • 274 H-Index
      • 288 Unique Word Abbreviation
      • 290 Word Pattern
      • 299 Bulls and Cows
      • 311 Sparse Matrix Multiplication
      • 314 Binary Tree Vertical Order Traversal
      • 325 Maximum Size Subarray Sum Equals k
      • 336 Palindrome Pairs
      • 340 Longest Substring with At Most K Distinct Characters
      • 347 Top K Frequent Elements
      • 349 Intersection of Two Arrays
      • 350 Intersection of Two Arrays II
      • 355 Design Twitter
      • 356 Line Reflection
      • 358 Rearrange String k Distance Apart
      • 359 Logger Rate Limiter
      • 380 Insert Delete GetRandom O(1)
      • 381 Insert Delete GetRandom O(1) - Duplicates allowed
      • 387 First Unique Character in a String
      • 389 Find the Difference
      • 409 Longest Palindrome
      • 438 Find All Anagrams in a String
      • 447 Number of Boomerangs
      • 451 Sort Characters By Frequency
      • 454 4Sum II
      • 463 Island Perimeter
      • 500 Keyboard Row
      • 508 Most Frequent Subtree Sum
      • 525 Contiguous Array
      • 554 Brick Wall
      • 560 Subarray Sum Equals K
      • 575 Distribute Candies
      • 594 Longest Harmonious Subsequence
      • 599 Minimum Index Sum of Two Lists
      • 609 Find Duplicate File in System
      • 624 Maximum Distance in Arrays
      • 632 Smallest Range
      • 645 Set Mismatch
      • 648 Replace Words
      • 676 Implement Magic Dictionary
      • 692 Top K Frequent Words
      • 694 Number of Distinct Islands
      • 705 Design HashSet
      • 706 Design HashMap
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