652 Find Duplicate Subtrees Solution O(n) O(n) Medium Tree 651 4 Keys Keyboard Solution O(n^2) O(n) Medium DP 650 2 Keys Keyboard Solution O(n^2) O(n) Medium DP 649 Dota2 Senate Solution O(n) O(n) Medium Greedy 648 Replace Words Solution O(n) O(n) Medium Trie 647 Palindromic Substrings Solution O(n^2) O(1) Medium DP 646 Maximum Length of Pair Chain Solution O(nlogn) O(1) Medium DP 645 Set Mismatch Solution O(nlogn) O(1) Easy 644 Maximum Average Subarray II Solution O(1) Hard Binary Search 643 Maximum Average Subarray I Solution O(n) O(1) Easy 642 Design Search Autocomplete System Solution O(n) O(n) Hard Design 640 Solve the Equation Solution O(n) O(n) Medium 639 Decode Ways II Solution O(n) O(n) Hard DP 638 Shopping Offers Solution O(2^n) O(n) Medium DP, DFS 637 Average of Levels in Binary Tree Solution O(n) O(1) Easy 636 Exclusive Time of Functions Solution O(n) O(n/2) Medium Stack 635 Design Log Storage System Solution O(n) O(1) Medium Design 634 Find the Derangement of An Array Solution O(n) O(1) Medium Math 633 Sum of Square Numbers Solution O(logn) O(1) Easy Binary Search 632 Smallest Range Solution O(n*logk) O(k) Hard Heap 631 Design Excel Sum Formula Solution Hard Design, Topological Sort 630 Course Schedule III Solution O(n*logn) O(n) Hard Heap, Greedy 629 K Inverse Pairs Array Solution O(n*k) O(n*k) Hard DP 628 Maximum Product of Three Numbers Solution O(nlogn) O(1) Easy 625 Minimum Factorization Solution O(?) O(?) Medium 624 Maximum Distance in Arrays Solution O(nlogn) O(1) Easy Sort, Array 623 Add One Row to Tree Solution O(n) O(h) Medium Tree 621 Task Scheduler Solution O(n) O(26) Medium Greedy, Queue 617 Merge Two Binary Trees Solution O(n) O(h) Easy Tree, Recursion 616 Add Bold Tag in String Solution O(n*k) (n is length of string, k is size of dict) O(n) Medium String 611 Valid Triangle Number Solution O(n^2logn) O(logn) Medium Binary Search 609 Find Duplicate File in System Solution O(n*x) (x is the average length of each string) O(n*x) Medium HashMap 606 Construct String from Binary Tree Solution O(n) O(n) Easy Tree, Recursion 605 Can Place Flowers Solution O(n) O(1) Easy Array 604 Design Compressed String Iterator Solution O(n) O(n) Easy Design, String 600 Non-negative Integers without Consecutive Ones Solution O(log2(max_int) = 32) O(log2(max_int) = 32) Hard Bit Manipulation, DP 599 Minimum Index Sum of Two Lists Solution O(max(m,n)) O(max(m,n)) Easy HashMap 598 Range Addition II Solution O(n) (n is the number of operations) O(1) Easy 594 Longest Harmonious Subsequence Solution O(n) O(n) Easy Array, HashMap 593 Valid Square Solution O(1) O(1) Medium Math 592 Fraction Addition and Subtraction Solution O(nlogx) O(n) Medium Math 591 Tag Validator Solution O(n) O(n) Hard Stack, String 588 Design In-Memory File System Solution O(n) O(h) Hard Trie, Design 587 Erect the Fence Solution O(?) O(?) Hard Geometry 583 Delete Operation for Two Strings Solution O(m*n) O(m*n) could be optimized to O(n) Medium DP 582 Kill Process Solution O(n) O(h) Medium Stack 581 Shortest Unsorted Continuous Subarray Solution O(n) O(1) Easy Array, Sort 576 Out of Boundary Paths Solution O(Nmn) O(m*n) Hard DP, DFS 575 Distribute Candies Solution O(nlogn) O(1) Easy Array 573 Squirrel Simulation Solution O(n) O(1) Medium Math 572 Subtree of Another Tree Solution O(m*n) O(1) Easy Tree 568 Maximum Vacation Days Solution O(n^2*k) O(n*k) Hard DP 567 Permutation in String Solution O(max(m,n)) O(1) Medium Sliding Windows, Two Pointers 566 Reshape the Matrix Solution O(m*n) O(1) Easy 565 Array Nesting Solution O(n) O(n) Medium 563 Binary Tree Tilt Solution O(n) O(n) Easy Tree Recursion 562 Longest Line of Consecutive One in Matrix Solution O(m*n) O(m*n) Medium Matrix DP 561 Array Partition I Solution O(nlogn) O(1) Easy Array 560 Subarray Sum Equals K Solution O(n) O(n) Medium Array, HashMap 557 Reverse Words in a String III Solution O(n) O(n) Easy String 556 Next Greater Element III Solution O(n) O(1) Medium String 555 Split Concatenated Strings Solution O(n^2) O(n) Medium String 554 Brick Wall Solution O(n) (n is total number of bricks in the wall) O(m) (m is width of the wall) Medium HashMap 553 Optimal Division Solution O(n) O(n) Medium String 552 Student Attendance Record II Solution O(n) O(1) Hard DP 551 Student Attendance Record I Solution O(n) O(1) Easy String 549 Binary Tree Longest Consecutive Sequence II Solution O(n) O(n) Medium Tree 548 Split Array with Equal Sum Solution O(n^2) O(1) Medium Array 547 Friend Circles Solution O(n^2) O(n) Medium Union Find 546 Remove Boxes Solution O(n^3) O(n^3) Hard DFS, DP 545 Boundary of Binary Tree Solution O(n) O(n) Medium Recursion 544 Output Contest Matches Solution O(n) O(n) Medium Recursion 543 Diameter of Binary Tree Solution O(n) O(h) Easy Tree/DFS/Recursion 542 01 Matrix Solution O(m*n) O(n) Medium BFS 541 Reverse String II Solution O(n) O(1) Easy String 540 Single Element in a Sorted Array Solution O(n) O(1) Medium 539 Minimum Time Difference Solution O(n) O(1) Medium String 538 Convert BST to Greater Tree Solution O(nlogn) O(n) Medium Tree 537 Complex Number Multiplication Solution O(1) O(1) Medium Math, String 536 Construct Binary Tree from String Solution O(n) O(h) Medium Recursion 535 Encode and Decode TinyURL Solution O(1) O(n) Medium Design 533 Lonely Pixel II Solution O(m*n) O(m) (m is number of rows) Medium HashMap 532 K-diff Pairs in an Array Solution O(n) O(n) Easy HashMap 531 Lonely Pixel I Solution O(m*n) O(1) Medium 530 Minimum Absolute Difference in BST Solution O(n) O(n) Easy DFS 529 Minesweeper Solution O(m*n) O(k) Medium BFS 527 Word Abbreviation Solution O(n^2) O(n) Hard 526 Beautiful Arrangement Solution O(n) O(h) Medium Backtracking 525 Contiguous Array Solution O(n) O(n) Medium HashMap 524 Longest Word in Dictionary through Deleting Solution O(n) O(n) Medium Sort 523 Continuous Subarray Sum Solution O(n) O(1) Medium DP 522 Longest Uncommon Subsequence II Solution O(x*n^2) (x is average length of strings) O(1) Medium 521 Longest Uncommon Subsequence I Solution O(max(x,y)) (x and y are length of strings) O(1) Easy 520 Detect Capital Solution O(n) O(1) Easy 517 Super Washing Machines Solution Hard DP 516 Longest Palindromic Subsequence Solution O(n^2) O(n^2) Medium DP 515 Find Largest Value in Each Tree Row Solution O(n) O(k) Medium BFS 514 Freedom Trail Solution O(?) O(?) Hard DP 513 Find Bottom Left Tree Value Solution O(n) O(k) Medium BFS 508 Most Frequent Subtree Sum Solution O(n) O(n) Medium DFS, Tree 507 Perfect Number Solution O(sqrt(n)) O(1) Easy Math 506 Relative Ranks Solution O(nlogn) O(n) Easy 505 The Maze II Solution O(m*n) O(m*n) Medium BFS 504 Base 7 Solution O(1) O(1) Easy 503 Next Greater Element II Solution O(n) O(n) Medium Stack 502 IPO Solution O(nlogn) O(n) Hard Heap, Greedy 501 Find Mode in Binary Tree Solution O(n) O(k) Easy Binary Tree 500 Keyboard Row Solution O(n) O(1) Easy 499 The Maze III Solution O(m*n) O(m*n) Hard BFS 496 Next Greater Element I Solution O(n*m) O(1) Easy 498 Diagonal Traverse Solution O(m*n) O(1) Medium 495 Teemo Attacking Solution O(n) O(1) Medium Array 494 Target Sum Solution O(2^n) O(1) Medium 493 Reverse Pairs Solution O(?) O(?) Hard 492 Construct the Rectangle Solution O(n) O(1) Easy Array 491 Increasing Subsequences Solution O(n!) O(n) Medium Backtracking, DFS 490 The Maze Solution O(m*n) O(m*n) Medium BFS 488 Zuma Game Solution O(?) O(?) Hard DFS, Backtracking 487 Max Consecutive Ones II Solution O(n) O(n) Medium Array 486 Predict the Winner Solution O(2^n) O(n^2) Medium DP 485 Max Consecutive Ones Solution O(n) O(1) Easy Array 484 Find Permutation Solution O(n) O(1) Medium Array, String, Greedy 483 Smallest Good Base Solution O(logn) O(1) Hard Binary Search, Math 482 License Key Formatting Solution O(n) O(n) Medium 481 Magical String Solution O(?) O(?) Medium 480 Sliding Window Median Solution O(nlogk) O(k) Hard Heap 479 Largest Palindrome Product Solution O(n) O(1) Easy 477 Total Hamming Distance Solution O(n) O(1) Medium Bit Manipulation 476 Number Complement Solution O(n) O(1) Easy Bit Manipulation 475 Heaters Solution max(O(nlogn), O(mlogn)) - m is the length of houses, n is the length of heaters O(1) Easy Array Binary Search 474 Ones and Zeroes Solution O(n) O(m*n) Medium DP 473 Matchsticks to Square Solution O(n!) O(n) Medium Backtracking, DFS 472 Concatenated Words Solution O(n^2) O(n) Hard Trie, DP, DFS 471 Encode String with Shortest Length Solution O(n^3) O(n^2) Hard DP 469 Convex Polygon Solution O(n) O(1) Medium Math 468 Validate IP Address Solution O(n) O(1) Medium String 467 Unique Substrings in Wraparound String Solution O(n) O(1) Medium DP 466 Count The Repetitions Solution O(max(m,n)) O(1) Hard DP 465 Optimal Account Balancing Solution Hard DP 464 Can I Win Solution O(2^n) O(n) Medium DP 463 Island Perimeter Solution O(m*n) O(1) Easy 462 Minimum Moves to Equal Array Elements II Solution O(nlogn) O(1) Medium 461 Hamming Distance Solution O(n) O(1) Easy 460 LFU Cache Solution O(1) O(n) Hard Design, LinkedHashMap, HashMap 459 Repeated Substring Pattern Solution O(n) O(n) Easy KMP 458 Poor Pigs Solution O(1) O(1) Easy Math 456 132 Pattern Solution O(n) O(n) Medium Stack 455 Assign Cookies Solution O(n) O(1) Easy 454 4Sum II Solution O(n) O(n) Medium HashMap 453 Minimum Moves to Equal Array Elements Solution O(n) O(1) Easy 452 Minimum Number of Arrows to Burst Balloons Solution O(nlogn) O(1) Medium Array, Greedy 451 Sort Characters By Frequency Solution O(nlogn) O(n) Medium HashMap 450 Delete Node in a BST Solution O(?) O(?) Medium Tree, Recursion 449 Serialize and Deserialize BST Solution O(n) O(h) Medium BFS 448 Find All Numbers Disappeared in an Array Solution O(n) O(1) Easy Array, HashMap 447 Number of Boomerangs Solution O(n^2) O(n) Easy HashMap 446 Arithmetic Slices II - Subsequence Solution O(n^2) O(n^2) Hard DP 445 Add Two Numbers II Solution O(max(m,n) O(max(m,n)) Medium Stack, LinkedList 444 Sequence Reconstruction Solution O(n) O(n) Medium Topological Sort, Graph 442 Find All Duplicates in an Array Solution O(n) O(1) Medium Array 441 Arranging Coins Solution O(n) O(1) Easy 440 K-th Smallest in Lexicographical Order Solution O(n^2) O(1) Hard 439 Ternary Expression Parser Solution O(n) O(n) Medium Stack 438 Find All Anagrams in a String Solution O(n) O(1) Easy Sliding Window 437 Path Sum III Solution O(n^2) O(n) Easy DFS, recursion 436 Find Right Interval Solution O(nlogn) O(n) Medium Binary Search 435 Non-overlapping Intervals Solution O(nlogn) O(1) Medium Greedy 434 Number of Segments in a String Solution O(n) O(1) Easy 432 All O`one Data Structure Solution O(1) O(n) Hard Design 425 Word Squares Solution O(n!) O(n) Hard Trie, Backtracking, Recursion 424 Longest Repeating Character Replacement Solution O(n) O(1) Medium Sliding Window 423 Reconstruct Original Digits from English Solution O(n) O(1) Medium Math 422 Valid Word Square Solution O(n) O(1) Easy 421 Maximum XOR of Two Numbers in an Array Solution O(n) O(1) Medium Bit Manipulation, Trie 420 Strong Password Checker Solution ? ? Hard 419 Battleships in a Board Solution O(m*n) O(1) Medium DFS 418 Sentence Screen Fitting Solution O(n) O(1) Medium 417 Pacific Atlantic Water Flow Solution O(mnMax(m,n)) O(m*n) Medium DFS 416 Partition Equal Subset Sum Solution O(m*n) O(m*n) Medium DP 415 Add Strings Solution O(n) O(1) Easy 414 Third Maximum Number Solution O(n) O(1) Easy 413 Arithmetic Slices Solution O(n) O(1) Medium DP 412 Fizz Buzz Solution O(n) O(1) Easy 411 Minimum Unique Word Abbreviation Solution O(?) O(?) Hard NP-Hard, Backtracking, Trie, Recursion 410 Split Array Largest Sum Solution O(nlogn) O(1) Hard Binary Search, DP 408 Valid Word Abbreviation Solution O(n) O(1) Easy 407 Trapping Rain Water II Solution Hard Heap 406 Queue Reconstruction by Height Solution O(nlogn) O(1) Medium LinkedList, PriorityQueue 405 Convert a Number to Hexadecimal Solution O(n) O(1) Easy 404 Sum of Left Leaves Solution O(n) O(h) Easy 403 Frog Jump Solution O(n^2) O(n^2) Hard DP 402 Remove K Digits Solution O(n) O(n) Medium Greedy, Stack 401 Binary Watch Solution O(1) O(1) Easy 400 Nth Digit Solution O(n) O(1) Easy 399 Evaluate Division Solution O(n*n!) O(n) Medium Graph, DFS, Backtracking 398 Random Pick Index Solution Medium Reservoir Sampling 397 Integer Replacement Solution ? ? Easy BFS 396 Rotate Function Solution O(n^2) could be optimized to O(n) O(1) Easy 393 UTF-8 Validation Solution O(?) O(?) Medium Bit Manipulation 392 Is Subsequence Solution O(m*n) O(1) Medium Array, String 391 Perfect Rectangle Solution O(n) O(1) Hard 390 Elimination Game Solution O(logn) O(1) Medium 389 Find the Difference Solution O(n) O(1) Easy 388 Longest Absolute File Path Solution O(n) O(d) Medium Stack 387 First Unique Character in a String Solution O(n) O(n) Easy HashMap 386 Lexicographical Numbers Solution O(n) O(1) Medium 385 Mini Parser Solution O(n) O(h) Medium Stack 384 Shuffle an Array Solution O(n) O(n) Medium 383 Ransom Note Solution O(n) O(n) Medium 382 Linked List Random Node Solution O(1) O(n) Medium Reservoir Sampling 381 Insert Delete GetRandom O(1) - Duplicates allowed Solution Hard 379 Design Phone Directory Solution O(1) O(n) Medium 377 Combination Sum IV Solution O(?) O(?) Medium 376 Wiggle Subsequence Solution O(n) O(1) Medium DP, Greedy 375 Guess Number Higher or Lower II Solution O(n^2) O(n^2) Medium DP 374 Guess Number Higher or Lower Solution O(logn) O(1) Easy Binary Search 373 Find K Pairs with Smallest Sums Solution O(?) O(?) Medium Heap 372 Super Pow Solution O(n) O(1) Medium Math 371 Sum of Two Integers Solution O(n) O(1) Easy 370 Range Addition Solution O(n+k) O(1) Medium 369 Plus One Linked List Solution O(n) O(1) Medium Linked List 368 Largest Divisible Subset Solution O(n^2) O(n) Medium DP 367 Valid Perfect Square Solution O(n) O(1) Medium 366 Find Leaves of Binary Tree Solution O(n) O(h) Medium DFS 365 Water and Jug Problem Solution O(n) O(1) Medium Math 364 Nested List Weight Sum II Solution O(n) O(h) Medium DFS 363 Max Sum of Rectangle No Larger Than K Solution Hard DP 362 Design Hit Counter Solution O(1) amortized O(k) Medium Design 361 Bomb Enemy Solution O(?) O(?) Medium 360 Sort Transformed Array Solution O(n) O(1) Medium Two Pointers, Math 359 Logger Rate Limiter Solution amortized O(1) O(k) Easy HashMap 358 Rearrange String k Distance Apart Solution O(n) O(n) Hard HashMap, Heap, Greedy 357 Count Numbers with Unique Digits Solution O(n) O(1) Medium DP, Math 356 Line Reflection Solution O(n) O(n) Medium HashSet 355 Design Twitter Solution O(n) O(n) Medium Design, HashMap, Heap 354 Russian Doll Envelopes Solution O(nlogn) O(1) Hard DP, Binary Search 353 Design Snake Game Solution O(?) O(?) Medium 352 Data Stream as Disjoint Intervals Solution O(logn) O(n) Hard TreeMap 351 Android Unlock Patterns Solution O(?) O(?) Medium 350 Intersection of Two Arrays II Solution O(m+n) O((m+n)) could be optimized Easy HashMap, Binary Search 349 Intersection of Two Arrays Solution O(m+n) O(min(m,n)) Easy Two Pointers, Binary Search 348 Design Tic-Tac-Toe Solution O(1) O(n) Medium Design 347 Top K Frequent Elements Solution O(n) O(1) Medium HashTable, Heap 346 Moving Average from Data Stream Solution O(1) O(w)) Easy Queue 345 Reverse Vowels of a String Solution O(n) O(1) Easy String 344 Reverse String Solution O(n) O(1) Easy String 343 Integer Break Solution O(1) O(1) Medium Math 342 Power of Four Solution O(n) O(1) Easy Math 341 Flatten Nested List Iterator Solution O(n) O(n) Medium Stack 340 Longest Substring with At Most K Distinct Characters Solution O(n) O(1) Hard Sliding Window 339 Nested List Weight Sum Solution O(n) O(h)) Easy DFS 338 Counting Bits Solution O(nlogn) O(h) Medium 337 House Robber III Solution O(n) O(n) Medium DP 336 Palindrome Pairs Solution O(n^2) O(n) Hard 335 Self Crossing Solution O(n) O(1) Hard Math 334 Increasing Triplet Subsequence Solution O(n^2) O(1) Medium 333 Largest BST Subtree Solution O(n) O(n) Medium Tree 332 Reconstruct Itinerary Solution O(n) O(n) Medium Graph, DFS 331 Verify Preorder Serialization of a Binary Tree Solution O(n) O(n) Medium Stack 330 Patching Array Solution O(m+logn) O(1) Hard Greedy 329 Longest Increasing Path in a Matrix Solution O(?) O(?) Hard 328 Odd Even Linked List Solution O(n) O(1) Medium Linked List 327 Count of Range Sum Solution O(?) O(?) Hard 326 Power of Three Solution O(1) O(1) Easy Math 325 Maximum Size Subarray Sum Equals k Solution O(n) O(n) Medium Sort 324 Wiggle Sort II Solution O(n) O(n) Medium Sort 323 Number of Connected Components in an Undirected Graph Solution O(?) O(?) Medium 322 Coin Change Solution O(?) O(?) Medium 321 Create Maximum Number Solution O(?) O(?) Hard 320 Generalized Abbreviation Solution O(n*2^n) O(n) Medium Backtracking, Bit Manipulation 319 Bulb Switcher Solution O(1) O(1) Medium Brainteaser 318 Maximum Product of Word Lengths Solution O(n^2) O(n) Medium 317 Shortest Distance from All Buildings Solution O(?) O(?) Hard 316 Remove Duplicate Letters Solution O(n) O(1) Hard Stack, Recursion, Greedy 315 Count of Smaller Numbers After Self Solution O(?) O(?) Hard Tree 314 Binary Tree Vertical Order Traversal Solution O(n) O(n) Medium HashMap, BFS 313 Super Ugly Number Solution O(?) O(?) Medium 312 Burst Balloons Solution O(?) O(?) Hard DP 311 Sparse Matrix Multiplication Solution O(mnl) O(m*l) Medium 310 Minimum Height Trees Solution ? ? Medium 309 Best Time to Buy and Sell Stock with Cooldown Solution O(n) O(1) Medium DP 308 Range Sum Query 2D - Mutable Solution ? ? Hard Tree 307 Range Sum Query - Mutable Solution ? ? Medium Tree 306 Additive Number Solution O(n^2) O(n) Medium 305 Number of Islands II Solution ? ? Hard Union Find 304 Range Sum Query 2D - Immutable Solution ? ? Medium 303 Range Sum Query - Immutable Solution O(n) O(1) Easy 302 Smallest Rectangle Enclosing Black Pixels Solution ? O(m*n) Hard DFS, BFS 301 Remove Invalid Parentheses Solution ? ? Hard BFS 300 Longest Increasing Subsequence Solution O(logn) O(n) Medium DP 299 Bulls and Cows Solution O(n) O(1) Easy 298 Binary Tree Longest Consecutive Sequence Solution O(n) O(n) Medium Tree 297 Serialize and Deserialize Binary Tree Solution O(n) O(h) Hard BFS 296 Best Meeting Point Solution ? ? Hard 295 Find Median from Data Stream Solution O(nlogn) O(n) Hard Heap 294 Flip Game II Solution O(?) O(?) Medium Backtracking 293 Flip Game Solution O(n) O(1) Easy 292 Nim Game Solution O(1) O(1) Easy 291 Word Pattern II Solution O(n) O(n) Hard 290 Word Pattern Solution O(n) O(n) Easy 289 Game of Life Solution O(m*n) O(m*n), could be optimized to O(1) Medium 288 Unique Word Abbreviation Solution O(n) O(1) Easy 287 Find the Duplicate Number Solution O(n) O(1) Medium 286 Walls and Gates Solution O(m*n) O(g) Medium BFS 285 Inorder Successor In BST Solution O(h) O(1) Medium Tree 284 Peeking Iterator Solution O(n) O(n) Medium Design 283 Move Zeroes Solution O(n) O(1) Easy 282 Expression Add Operators Solution O(?) O(?) Hard 281 Zigzag Iterator Solution O(n) O(n) Medium 280 Wiggle Sort Solution O(n) O(1) Medium 279 Perfect Squares Solution O(n) O(1) Medium 278 First Bad Version Solution O(logn) O(1) Easy Binary Search 277 Find the Celebrity Solution O(n) O(1) Medium 276 Paint Fence Solution O(n) O(1) Easy DP 275 H-Index II Solution O(logn) O(1) Medium Binary Search 274 H-Index Solution O(nlogn) O(1) Medium 273 Integer to English Words Solution O(n) O(1) Hard Math, String 272 Closest Binary Search Tree Value II Solution O(h+k) O(h) Hard Stack 271 Encode and Decode Strings Solution O(n) O(1) Medium 270 Closest Binary Search Tree Value Solution O(h) O(1) Easy DFS 269 Alien Dictionary Solution O(?) O(?) Hard Topological Sort 268 Missing Number Solution O(n) O(1) Easy Bit Manipulation 267 Palindrome Permutation II Solution O(n*n!) O(n) Medium 266 Palindrome Permutation Solution O(n) O(1) Easy 265 Paint House II Solution ? ? Hard 264 Ugly Number II Solution O(n) O(n) Medium DP 263 Ugly Number Solution O(n) O(1) Easy 261 Graph Valid Tree Solution O(V+E) O(V+E) Medium 260 Single Number III Solution O(n) O(n) Medium 259 3Sum Smaller Solution O(n^2) O(1) Medium 258 Add Digits Solution O(1) O(1) Easy 257 Binary Tree Paths Solution O(n*h) O(h) DFS/Recursion 256 Paint House Solution O(n) O(1) Medium DP 255 Verify Preorder Sequence in Binary Search Tree Solution O(n) O(h) Medium Tree 254 Factor Combinations Solution O(nlogn) O(nlogn) Medium Backtracking 253 Meeting Rooms II Solution O(nlogn) O(h) Medium Heap 252 Meeting Rooms Solution O(nlogn) O(1) Easy 251 Flatten 2D Vector Solution O(1) O(m*n) Medium 250 Count Univalue Subtrees Solution O(n) O(h) Medium DFS 249 Group Shifted Strings Solution O(nlogn) O(n) 248 Strobogrammatic Number III Solution O(?) O(?) Hard Recursion, DFS 247 Strobogrammatic Number II Solution O(n^2) O(n) Medium Recursion 246 Strobogrammatic Number Solution O(n) O(1) Easy 245 Shortest Word Distance III Solution O(n) O(1) Medium 244 Shortest Word Distance II Solution O(n) O(n) Medium HashMap 243 Shortest Word Distance Solution O(n) O(1) Easy 242 Valid Anagram Solution O(n) O(1) Easy 241 Different Ways to Add Parentheses Solution O(O(n * 4^n / n^(3/2))) O(n * 4^n / n^(3/2)) Medium Divide and Conquer 240 Search a 2D Matrix II Solution O(log(m*n)) O(1) Medium Binary Search 239 Sliding Window Maximum Solution O(nlogn) O(k) Hard Heap 238 Product of Array Except Self Solution O(n) O(1) Medium Array 237 Delete Node in a Linked List Solution O(1) O(1) Easy LinkedList 236 Lowest Common Ancestor of a Binary Tree Solution O(n) O(h) Medium DFS 235 Lowest Common Ancestor of a Binary Search Tree Solution O(h) O(1) Easy DFS 234 Palindrome Linked List Solution O(n) O(1) Easy 233 Number of Digit One Solution O(n) O(1) Hard Math 232 Implement Queue using Stacks Solution O(n) O(n) Medium Stack, Design 231 Power of Two Solution O(1) O(1) Easy 230 Kth Smallest Element in a BST Solution O(n) O(k) Medium Tree 229 Majority Element II Solution O(n) O(n) Medium 228 Summary Ranges Solution O(n) O(1) Medium Array 227 Basic Calculator II Solution O(n) O(n) Medium String 226 Invert Binary Tree Solution O(n) O(h) Easy DFS, recursion 225 Implement Stack using Queues Solution O(n) O(n) Easy Stack, Queue 224 Basic Calculator Solution ? ? Hard 223 Rectangle Area Solution O(1) O(1) Easy 222 Count Complete Tree Nodes Solution O(?) O(h) Medium Recursion 221 Maximal Square Solution O(?) O(h) Medium Recursion 220 Contains Duplicate III Solution O(nlogn) O(n) Medium TreeSet 219 Contains Duplicate II Solution O(n) O(n) Easy HashMap 218 The Skyline Problem Solution O(n) O(n) Hard TreeMap, Design 217 Contains Duplicate Solution O(n) O(n) Easy HashSet 216 Combination Sum III Solution O(k * C(n, k)) O(k) Medium Backtracking 215 Kth Largest Element in an Array Solution O(nlogn) O(n) Medium Heap 214 Shortest Palindrome Solution O(?) O(?) Hard KMP 213 House Robber II Solution O(n) O(n) Medium DP 212 Word Search II Solution O(mnl) O(l) Hard Trie 211 Add and Search Word - Data structure design Solution O(n) O(h) Medium Trie 210 Course Schedule II Solution O(?) O(?) Medium 209 Minimum Size Subarray Sum Solution O(n) O(1) Medium 208 Implement Trie Solution O(n) O(1) Medium Trie 207 Course Schedule Solution O(?) O(?) Medium 206 Reverse Linked List Solution O(n) O(1) Easy Linked List 205 Isomorphic Strings Solution O(n) O(1) Easy 204 Count Primes Solution O(sqrt(n)) O(n) Easy 203 Remove Linked List Elements Solution O(n) O(1) Easy 202 Happy Number Solution O(k) O(k) Easy 201 Bitwise AND of Numbers Range Solution O(min(m,n)) O(1) Medium Bit Manipulation 200 Number of Islands Solution O(m*n) O(m*n) Medium Union Find, DFS 199 Binary Tree Right Side View Solution O(n) O(n) Medium BFS 198 House Robber Solution O(n) O(n) Easy DP 191 Number of 1 Bits Solution O(n) O(1) Easy Bit Manipulation 190 Reverse Bits Solution O(n) O(1) Easy Bit Manipulation 189 Rotate Array Solution O(n) O(n), could be optimized to O(1) Easy 188 Best Time to Buy and Sell Stock IV Solution O(n*k) O(n*k) Hard DP 187 Repeated DNA Sequences Solution O(n) O(n) Medium 186 Reverse Words in a String II Solution O(n) O(1) Medium 179 Largest Number Solution O(?) O(?) Medium 174 Dungeon Game Queue Stack O(m*n) O(m*n) Hard DP 173 Binary Search Tree Iterator Queue Stack O(1) O(h) Medium Stack, Design 172 Factorial Trailing Zeroes Solution O(logn) O(1) Easy 171 Excel Sheet Column Number Solution O(n) O(1) Easy 170 Two Sum III - Data structure design Solution O(n) O(n) Easy 169 Majority Element Solution O(n) O(1) Easy 168 Excel Sheet Column Title Solution O(n) O(1) Easy 167 Two Sum II - Input array is sorted Solution O(logn) O(1) Easy 166 Fraction to Recurring Decimal Solution O(1) O(1) Medium HashMap 165 Compare Version Numbers Solution O(n) O(1) Easy 164 Maximum Gap Solution O(n) O(n) Hard 163 Missing Ranges Solution O(n) O(1) 162 Find Peak Element Solution O(1) O(logn)/O(n) Binary Search 161 One Edit Distance Solution O(n) O(1) 160 Intersection of Two Linked Lists Solution O(m+n) O(1) Easy Linked List 159 Longest Substring with At Most Two Distinct Characters Solution O(n) O(1) Hard String, Sliding Window 158 Read N Characters Given Read4 II - Call multiple times Solution O(n) O(1) Hard 157 Read N Characters Given Read4 Solution O(n) O(1) Easy 156 Binary Tree Upside Down Solution O(n) O(h) Medium Tree, Recursion 155 Min Stack Solution O(1) O(n) Easy Stack 154 Find Minimum in Rotated Sorted Array II Solution O(logn) O(1) Hard Array, Binary Search 153 Find Minimum in Rotated Sorted Array Solution O(logn) O(1) Medium Array, Binary Search 152 Maximum Product Subarray Solution O(n) O(1) Medium Array 151 Reverse Words in a String Solution O(n) O(n) Medium 150 Evaluate Reverse Polish Notation Solution O(?) O(?) Medium 149 Max Points on a Line Solution O(?) O(?) Hard 148 Sort List Solution O(nlogn) O(h) Medium Linked List, Sort 147 Insertion Sort List Solution O(n^2) O(1) Medium Linked List 146 LRU Cache Solution amortized O(1) O(n) Hard Linked List 145 Binary Tree Postorder Traversal Solution O(n) O(h) Hard Binary Tree 144 Binary Tree Preorder Traversal Solution O(n) O(h) Medium Binary Tree 143 Reorder List Solution O(n) O(1) Medium 142 Linked List Cycle II Solution O(n) O(1) Medium Linked List 141 Linked List Cycle Solution O(n) O(1) Easy Linked List 140 Word Break II Solution ? O(n^2) Hard Backtracking/DFS 139 Word Break Solution O(n^2) O(n) Medium DP 138 Copy List with Random Pointer Solution O(n) O(n) Medium LinkedList, HashMap 137 Single Number II Solution O(n) O(n) Medium 136 Single Number Solution O(n) O(n) Medium 135 Candy Solution O(n) O(1) Hard Greedy 134 Gas Station Solution O(n) O(1) Medium Greedy 133 Clone Graph Solution O(n) O(n) Medium HashMap, BFS, Graph 132 Palindrome Partitioning II Solution O(n^2) O(n^2) Hard 131 Palindrome Partitioning Solution O(n^2) O(n^2) Medium 130 Surrounded Regions Solution O(?) O(?) Medium 129 Sum Root to Leaf Numbers Solution O(n) O(h) Medium DFS 128 Longest Consecutive Sequence Solution O(?) O(?) Hard Union Find 127 Word Ladder Solution O(?) O(?) Medium BFS 126 Word Ladder II Solution O(?) O(?) Hard BFS 125 Valid Palindrome Solution O(n) O(1) Easy Two Pointers 124 Binary Tree Maximum Path Sum Solution O(n) O(h) Hard Tree, DFS 123 Best Time to Buy and Sell Stock III Solution O(?) O(?) Hard 122 Best Time to Buy and Sell Stock II Solution O(n) O(1) Medium Greedy 121 Best Time to Buy and Sell Stock Solution O(n) O(1) Easy DP 120 Triangle Solution O(m*n) O(n) Medium DP 119 Pascal's Triangle II Solution O(n^2) O(1) Easy 118 Pascal's Triangle Solution O(n^2) O(1) Easy 117 Populating Next Right Pointers in Each Node II Solution O(n) O(1) Medium BFS 116 Populating Next Right Pointers in Each Node Solution O(n) O(1) Medium BFS 115 Distinct Subsequences Solution O(m*n) O(m*n) Hard DP 114 Flatten Binary Tree to Linked List Solution O(n) O(h) Medium Tree 113 Path Sum II Solution O(n) O(h) Medium DFS 112 Path Sum Solution O(n) O(1) Easy DFS 111 Minimum Depth of Binary Tree Solution O(n) O(1)~O(h) Easy BFS, DFS 110 Balanced Binary Tree Solution O(n) O(1)~O(h) Easy DFS 109 Convert Sorted List to Binary Search Tree Solution O(n) O(h) Medium DFS, Recursion 108 Convert Sorted Array to Binary Search Tree Solution O(n) O(h) Medium Tree 107 Binary Tree Level Order Traversal II Solution O(nlogn) O(h) Easy BFS 106 Construct Binary Tree from Inorder and Postorder Traversal Solution O(n) O(n) Medium Recursion, Tree 105 Construct Binary Tree from Preorder and Inorder Traversal Solution O(n) O(n) Medium Recursion, Tree 104 Maximum Depth of Binary Tree Solution O(n) O(h) Easy DFS 103 Binary Tree Zigzag Level Order Traversal Solution O(n) O(h) Medium BFS,DFS 102 Binary Tree Level Order Traversal Solution O(n) O(h) Medium BFS 101 Symmetric Tree Solution O(n) O(h) Easy DFS 100 Same Tree Solution O(n) O(h) Easy DFS 99 Recover Binary Search Tree Solution O(?) O(?) Hard 98 Validate Binary Search Tree Solution O(n) O(h) Medium DFS/Recursion 97 Interleaving String Solution O(?) O(?) Hard DP 96 Unique Binary Search Trees Solution O(n^2) O(n) Medium Recursion, DP 95 Unique Binary Search Trees II Solution O(?) O(?) Medium Recursion 94 Binary Tree Inorder Traversal Solution O(n) O(h) Medium Binary Tree 93 Restore IP Addresses Solution O(1) O(1) Medium Backtracking 92 Reverse Linked List II Solution O(n) O(1) Medium 91 Decode Ways Solution O(n) O(n) Medium DP 90 Subsets II Solution O(n^2) O(1) Medium Backtracking 89 Gray Code Solution O(n) O(1) Medium Bit Manipulation 88 Merge Sorted Array Solution O(max(m,n)) O(1) Easy 87 Scramble String Solution O(?) O(?) Hard Recursion 86 Partition List Solution O(n) O(1) Medium 85 Maximal Rectangle Solution O(m*n) O(n) Hard DP 84 Largest Rectangle in Histogram Solution O(n) O(n) Hard Array, Stack 83 Remove Duplicates from Sorted List Solution O(n) O(1) Medium Linked List 82 Remove Duplicates from Sorted List II Solution O(n) O(1) Medium Linked List 81 Search in Rotated Sorted Array II Solution O(logn) O(1) Medium Binary Search 80 Remove Duplicates from Sorted Array II Solution O(n) O(n) Medium 79 Word Search Solution O(mnl) ? O(m*n) Medium Backtracking, DFS 78 Subsets Solution O(n^2) ? O(1) Medium Backtracking 77 Combinations Solution O(n^2) ? O(1) Medium Backtracking 76 Minimum Window Substring Solution O(n) O(k) Hard Two Pointers 75 Sort Colors Solution O(n) O(1) Medium Two Pointers 74 Search a 2D Matrix Solution O(logn) O(1) Medium Binary Search 73 Set Matrix Zeroes Solution O(mn) O(mn) Medium 72 Edit Distance Solution O(m*n) O(m+n) Hard 71 Simplify Path Solution O(n) O(n) Medium Stack 70 Climbing Stairs Solution O(n) O(n) Easy DP 69 Sqrt(x) Solution O(logn) O(1) Easy 68 Text Justification Solution O(n) O(1) Hard 67 Add Binary Solution O(n) O(1) Easy 66 Plus One Solution O(n) O(1) Easy 65 Valid Number Solution O(n) O(1) Hard 64 Minimum Path Sum Solution O(m*n) O(m*n) Medium DP 63 Unique Paths II Solution O(m*n) O(m*n) Medium DP 62 Unique Paths Solution O(m*n) O(m*n) Medium DP 61 Rotate List Solution O(n) O(1) Medium Linked List 60 Permutation Sequence Solution ? ? Medium 59 Spiral Matrix II Solution O(n) O(n) Medium 58 Length of Last Word Solution O(n) O(1) Easy 57 Insert Intervals Solution O(n) O(1) Hard Array, Sort 56 Merge Intervals Solution O(n*logn) O(1) Medium Array, Sort 55 Jump Game Solution O(n) O(1) Medium Greedy 54 Spiral Matrix Solution O(m*n) O(m*n) Medium Array 53 Maximum Subarray Solution O(n) O(1) Easy 52 N-Queens II Solution O(?) O(?) Hard 51 N-Queens Solution O(?) O(?) Hard 50 Pow(x, n) Solution O(logn) O(logn) Medium 49 Group Anagrams Solution O(m*logn) O(m*n) Medium HashMap 48 Rotate Image Solution O(n^2) O(1) Medium Array 47 Permutations II Solution O(n*n!) O(n) Medium Backtracking 46 Permutations Solution O(n*n!) O(n) Medium Backtracking 45 Jump Game II Solution O(?) O(?) Hard 44 Wildcard Matching Solution O(m*n) O(m*n) Hard Backtracking, DP, Greedy, String 43 Multiply Strings Solution O(n) O(1) Medium Array, String 42 Trapping Rain Water Solution O(n) O(1) Hard 41 First Missing Positive Solution O(n) O(1) Hard 40 Combination Sum II Solution O(k*n^k) O(k) Medium Backtracking 39 Combination Sum Solution O(k*n^k) O(k) Medium Backtracking 38 Count and Say Solution O(n*2^n) O(2^n) Easy Recursion, LinkedList 37 Sudoku Solver Solution O((9!)^9) O(1) Hard 36 Valid Sudoku Solution O(1) O(1) Medium 35 Search Insert Position Solution O(n) O(1) Medium Array 34 Search for a Range Solution O(logn) O(1) Medium Array, Binary Search 33 Search in Rotated Sorted Array Solution O(logn) O(1) Medium Binary Search 32 Longest Valid Parentheses Solution O(n) O(n) Hard Stack, DP 31 Next Permutation Solution O(n) O(1) Medium Array 30 Substring with Concatenation of All Words Solution O(n^2) O(n) Hard HashMap 29 Divide Two Integers Solution O(?) O(?) Medium 28 Implement strStr() Solution O(n) O(1) Easy String 27 Remove Element Solution O(n) O(1) Easy 26 Remove Duplicates from Sorted Array Solution O(n) O(1) Easy Array 25 Reverse Nodes in k-Group Solution O(n) O(1) Hard Recursion, LinkedList 24 Swap Nodes in Pairs Solution O(n) O(h) Medium Recursion, LinkedList 23 Merge k Sorted Lists Solution O(n*logk) O(logk) Hard Heap 22 Generate Parentheses Solution TBD O(n) Medium Backtracking 21 Merge Two Sorted Lists Solution O(n) O(h) Easy Recursion 20 Valid Parentheses Solution O(n) O(n) Easy Stack 19 Remove Nth Node From End of List Solution O(n) O(1) Medium Linked List 18 4 Sum Solution O(n^2) O(1) Medium Two Pointers 17 Letter Combinations of a Phone Number Solution O(n*4^n) O(n) Medium Backtracking 16 3Sum Closest Solution O(nlogn) O(1) Medium Two Pointers 15 3Sum Solution O(n^2) O(1) Medium Two Pointers 14 Longest Common Prefix Solution O(n*min(wordLength in this array)) O(1) Easy 13 Roman to Integer Solution O(1) O(1) Easy 12 Integer to Roman Solution O(1) O(1) Medium 11 Container With Most Water Solution O(n) O(1) Medium 10 Regular Expression Matching Solution O(m*n) O(m*n) Hard DP 9 Palindrome Number Solution O(logn)/(n) O(1) Easy 8 String to Integer (atoi) Solution O(n) O(1) Medium 7 Reverse Integer Solution O(1) O(1) Easy 6 ZigZag Conversion Solution O(n) O(n) Easy 5 Longest Palindromic Substring Solution O(n^2) O(1) Medium 4 Median of Two Sorted Arrays Solution ? ? Hard Divide and Conquer 3 Longest Substring Without Repeating Characters Solution O(n) O(1) Medium HashMap, Sliding Window 2 Add Two Numbers Solution O(max(m,n)) O(1) Medium LinkedList 1 Two Sum Solution O(n) O(n) Easy HashMap