In-Place String Compression

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Big-O NotationLearn⌄

Big-O NotationLearn

Arrays and Matrix0 / 3838⌄

ArraysLearnPractice problems

ProblemDifficulty

1Array Length and EndsFreeEasy 2Sum Array ElementsFreeEasy 3Count Target OccurrencesFreeEasy 4Find Largest ValueFreeEasy 734Two Largest Values, Repeats IncludedFreeMedium 413Largest FourEasy 5Second Largest Distinct ValueEasy 6Running SumEasy 7Greatest to the RightEasy 171Buildings With an Ocean ViewMedium 8Product Except SelfMedium 9In Place ReversalEasy 10Remove ElementEasy 11Rotate Array RightMedium 12Matrix DimensionsEasy 13Matrix Row SumsEasy 14Transpose MatrixEasy 409Reshape MatrixEasy 15Toeplitz MatrixEasy 16Main Diagonal SumEasy 735Group Cells by DiagonalEasy 411Diagonal TraverseMedium 17Spiral Matrix TraversalMedium 410Generate Spiral MatrixMedium 18Rotate Square MatrixMedium 19Set Matrix ZeroesMedium 20Game of Life In PlaceMedium 737Cancel Paired Bit PatternsMedium 414Single NumberEasy 415Missing NumberEasy 736Mark Multiples OnceEasy 412Count PrimesMedium 731Crush Matching CandiesMedium 732Drop CandiesEasy 733Candy CrushHard 753Compare Partially Rated ProfilesMedium 754Predict One Rating from NeighborsEasy 21Recommend MoviesMedium

String and Array0 / 2121⌄

StringsLearnPractice problems

ProblemDifficulty

25String Length and EndsFreeEasy 26Count Character OccurrencesFreeEasy 27Build Reversed StringFreeEasy 28Reverse Character ArrayFreeEasy 29Reverse Each WordFreeEasy 30Reverse Word OrderMedium 33Longest Common PrefixEasy 31Valid PalindromeEasy 32Palindrome After One DeletionMedium 40Trim Matching EndsMedium 730Palindrome from a Common String SplitMedium 34Run-Length EncodeEasy 420Number StreamEasy 35In-Place String CompressionMedium 36First Substring MatchEasy 37Count Overlapping MatchesEasy 38Find the DifferenceEasy 39Is SubsequenceEasy 728Longest Dictionary Word by Deleting CharactersMedium 41String to Integer (atoi)Medium 42Valid NumberHard

Hash Maps and Sets0 / 4444⌄

Hash Tables and SetsPreviewPractice problems

ProblemDifficulty

45Contains DuplicateFreeEasy 738Presence Slots for Bounded ValuesFreeEasy 418Find All Duplicates in an ArrayFreeMedium 419Find All Missing ValuesFreeMedium 46Is UniqueFreeEasy 47Find the Duplicate NumberMedium 48Nearby DuplicateEasy 51First Unique CharacterEasy 52Valid AnagramEasy 416Majority ElementEasy 739Verify Frequency CandidatesEasy 417Majority Elements over One ThirdMedium 53Ransom NoteEasy 54Palindrome PermutationEasy 55Longest Buildable PalindromeEasy 56Find Common CharactersEasy 57Bulls and CowsMedium 59Intersection UniqueEasy 60Intersection with MultiplicityEasy 61Two Sum IndicesEasy 62Earliest Complement PairsMedium 132Two Sum Data StructureMedium 64K-diff Pairs in an ArrayMedium 65Count Good MealsMedium 664Sum IIMedium 63Digit Pair Spacing CheckEasy 67Brick WallMedium 68Longest Consecutive SequenceMedium 69Group AnagramsMedium 740Find Reversed Word PartnersEasy 741Complete a Palindrome on the LeftEasy 422Palindrome PairsHard 766Count K-Word PhrasesEasy 70Equal Row and Column PairsMedium 71Valid SudokuMedium 72Top K Frequent ElementsMedium 73Sort Characters by FrequencyMedium 74Isomorphic StringsEasy 75Word PatternEasy 76Find and Replace PatternMedium 77Roman to IntegerEasy 756Find the Bucket AddressEasy 49Maximum GapHard 50Nearby Almost DuplicateHard

Simple Sorting0 / 1818⌄

Simple SortingPreviewPractice problems

ProblemDifficulty

651Already In Order?FreeEasy 652Count the DropsFreeEasy 653Smallest to the FrontFreeEasy 654Selection Sort From ScratchFreeEasy 655Bubble Sort From ScratchFreeEasy 656How Many Sweeps?Medium 657Total Swaps to SortMedium 659Slot One Value InEasy 658Insertion Sort From ScratchEasy 660Sort Largest FirstEasy 661Second Smallest ValueEasy 662K-th SmallestMedium 663Merge Two Sorted ArraysMedium 664Evens Before OddsMedium 665Sort Small Non-Negative IntsMedium 666Sort Three ColorsMedium 667Squares of a Sorted ArrayMedium 668Sort by How OftenMedium

Sorting0 / 1616⌄

Divide-and-Conquer SortingPreviewPractice problems

ProblemDifficulty

755Sum by Splitting and ReturningFreeMedium 78Sort an ArrayFreeMedium 79Count InversionsFreeHard 80Partition Around a PivotFreeMedium 81Implement QuickselectFreeMedium 82Multi-Field SortEasy 83Relative Sort ArrayMedium 84Sort by FrequencyEasy 85Largest Concatenated NumberMedium 86Meeting Rooms FeasibilityEasy 87Merge Overlapping IntervalsMedium 88Insert IntervalMedium 89Remove Covered IntervalsMedium 90Minimum Meeting RoomsMedium 91Fewest Arrows for BalloonsMedium 92Wiggle SortMedium

Binary Search0 / 1515⌄

Binary SearchPreviewPractice problems

ProblemDifficulty

93Linear Search in a Sorted ArrayFreeEasy 94Binary SearchFreeEasy 95Search Insert PositionFreeEasy 96First Bad VersionFreeEasy 97Find First and Last Position in Sorted ArrayFreeMedium 105Magic IndexEasy 106Search a 2D MatrixMedium 98Search in Rotated Sorted ArrayMedium 99Find Minimum in Rotated Sorted ArrayMedium 100Sqrt(x)Easy 101Koko Eating BananasMedium 102Capacity to Ship Packages Within D DaysMedium 757Certify a Cut Across Two Sorted ArraysMedium 103Kth Merged ElementHard 104Median of Two Sorted ArraysHard

Two Pointers0 / 2525⌄

Two PointersPreviewPractice problems

ProblemDifficulty

107Two Sum in Sorted ArrayEasy 108Unique Two Sum PairsMedium 109Closest Pair SumMedium 110Count Pairs Under TargetMedium 111Count Pairs Over TargetMedium 112Pair Sums in RangeMedium 113Smallest Difference Across ArraysMedium 114Merge Sorted ArraysEasy 58Shared Sorted ValuesEasy 115Cross-Array Two SumMedium 116Backward In-Place MergeEasy 117Remove Duplicates Sorted ArrayEasy 118At Most Two DuplicatesMedium 119Move ZeroesEasy 120Sort Array By ParityEasy 121Sort ColorsMedium 122Squares of Sorted ArrayEasy 123Container With Most WaterMedium 124Trapping Rain WaterHard 125Three Sum Unique TripletsMedium 126Three Sum ClosestMedium 127Count Triplets Below TargetMedium 128Four Sum Unique QuadrupletsMedium 129Backspace String CompareMedium 133Sub SortMedium

Stacks and Queues0 / 2626⌄

Stacks and QueuesPreviewPractice problems

ProblemDifficulty

134Bracket MatcherMedium 135Validate ParenthesesEasy 43Valid ParenthesesEasy 742Report Matched Parenthesis IndicesEasy 44Longest Valid ParenthesesHard 136Minimum Add for Valid ParenthesesMedium 137Minimum Remove for Valid ParenthesesMedium 148Baseball GameEasy 138Evaluate Reverse Polish NotationMedium 149Score of ParenthesesMedium 139Basic CalculatorHard 141Nested Ternary EvaluatorMedium 142Decode Repetition StringMedium 140Simplify Unix PathMedium 150Folder Navigation LogEasy 145Remove K Adjacent DuplicatesMedium 144Asteroid CollisionMedium 143Prices After DiscountsEasy 146Smallest Subsequence of Distinct CharactersMedium 147Exclusive Time of FunctionsMedium 154Moving Average from Data StreamEasy 155Recent Request CounterEasy 156First Nonrepeating Character StreamMedium 151Ticket Queue TimingEasy 152Students Left Without LunchEasy 153Dota2 SenateMedium

Linked Lists0 / 2020⌄

Linked ListsPreviewPractice problems

ProblemDifficulty

669Length of a Linked ListEasy 670Sum of a Linked ListEasy 671Value at Position NEasy 672Largest Value in a ListEasy 673Append a NodeEasy 674Insert Before a PositionMedium 675Delete the First MatchMedium 676Drop Sorted DuplicatesMedium 677Reverse a Linked ListMedium 678Middle of the ListMedium 130Linked List CycleEasy 679N-th Node From the EndMedium 680Remove N-th From the EndMedium 686Merge Two Sorted ListsMedium 681Is the List a Palindrome?Medium 682Partition Around a ValueMedium 683Group Odd and Even PositionsMedium 684Swap Every Two NodesMedium 685Rotate the List RightMedium 687Add Two NumbersHard

Trees and Binary Search Trees0 / 1919⌄

Trees and Binary Search TreesPreviewPractice problems

ProblemDifficulty

689How Many NodesEasy 691Sum of All NodesEasy 690Height of the TreeEasy 692Count the LeavesEasy 688In-Order ValuesEasy 158Level Order with QueueMedium 693Values by LevelMedium 694Right-Side ViewMedium 695Is It Height-Balanced?Medium 697Invert a Binary TreeMedium 696Symmetric Tree?Medium 698Path Sum Exists?Medium 700Search a BSTEasy 704Insert Into a BSTMedium 699Valid Binary Search Tree?Medium 701Range Sum of a BSTMedium 702K-th Smallest in a BSTMedium 703Lowest Common Ancestor in a BSTMedium 705Sorted Array to Balanced BSTHard

Recursion and Backtracking0 / 99⌄

RecursionPreview BacktrackingPreviewPractice problems

ProblemDifficulty

159Case PermutationsMedium 167Letter Combinations of a Phone NumberMedium 160SubsetsMedium 161PermutationsMedium 164Generate ParenthesesMedium 162Combination SumMedium 166Pow(x, n)Medium 165Integer ReplacementMedium 163Pattern Without BoundariesHard

Monotonic Stacks0 / 1515⌄

Monotonic StacksPreviewPractice problems

ProblemDifficulty

743First Greater Position to the RightMedium 168Next Greater Element IEasy 170Daily TemperaturesMedium 169Next Greater Element CircularMedium 172Stock SpanMedium 744Nearest Smaller BarsEasy 173Largest Rectangle in HistogramHard 174Maximal Rectangle via HistogramHard 745Count Intervals Owned by Each MinimumEasy 175Sum of Subarray MinimumsMedium 176Sum of Subarray RangesMedium 177Remove K DigitsMedium 178Create Maximum NumberHard 179132 PatternMedium 180Car FleetMedium

Heaps and Priority Queues0 / 2020⌄

HeapsPreviewPractice problems

ProblemDifficulty

182Kth Largest ElementMedium 181Last Stone WeightEasy 183Kth Largest in a StreamEasy 184K Closest PointsMedium 185Top K Frequent WordsMedium 186Merge K Sorted ArraysMedium 187Kth Smallest in a Sorted MatrixMedium 188Find K Pairs with Smallest SumsMedium 189Smallest Range Covering K ListsHard 746Keep Two Sorted HalvesMedium 190Find Median from Data StreamHard 191Sliding Window MedianHard 192Smallest Number in Infinite SetMedium 198Seat Reservation ManagerMedium 193Minimum Cost to Connect SticksMedium 194Task SchedulerMedium 195Reorganize StringMedium 196Per-Task Cooldown Policy SimulationMedium 197IPO Capital MaximizationHard 199Twitter Feed DesignHard

Sliding Window0 / 2525⌄

Sliding WindowPreviewPractice problems

ProblemDifficulty

202Maximum Average SubarrayEasy 203Vowels per WindowMedium 204Count Fixed-Length Target WindowsEasy 200Defuse the BombEasy 205Distinct Count per WindowMedium 201Distinct Window Max SumMedium 206Permutation in StringMedium 218Find All Anagram StartsMedium 207Longest Substring Without RepeatsMedium 208Fruit Into BasketsMedium 210Frequency at Most KMedium 211Longest Ones After K FlipsMedium 747Keep Only Future Maximum CandidatesEasy 240Sliding Window MaximumHard 748Spread of Each Fixed WindowEasy 242Longest Subarray Max-Min Within LimitHard 209Longest Repeating Character ReplacementMedium 212Subarrays with K DistinctHard 213Binary Subarrays to GoalMedium 214Shortest Sum-at-Least WindowMedium 215Count Product Below KMedium 216Minimum Window SubstringHard 219Longest Window within BudgetHard 217Minimum Window SubsequenceHard 222Shortest K-Sum SubarrayHard

Subarrays and Prefix Sums0 / 2020⌄

Prefix SumsPreviewPractice problems

ProblemDifficulty

220Left and Right Sum DifferencesEasy 221Immutable Range Sum QueriesEasy 223Pivot IndexEasy 749Count Target-Sum Intervals Ending HereEasy 225Count Subarrays Sum KMedium 226Longest Subarray Sum KMedium 227Subarray Sum Divisible by KMedium 228Continuous Multiple of KMedium 229Balanced Zeros and OnesMedium 230Subarrays with K OddsMedium 224Range Addition ArrayMedium 238Corporate Flight BookingsMedium 231Maximum Subarray SumMedium 232Maximum Circular SubarrayMedium 233Maximum Product SubarrayMedium 2352D Prefix Sum QueriesMedium 236Count Submatrices Sum TargetHard 237Maximum Sum Rectangle No Larger Than KHard 234Count Range SumHard 239Median-K Subarray CountHard

Greedy Algorithms0 / 1818⌄

Greedy AlgorithmsPreviewPractice problems

ProblemDifficulty

243Assign CookiesEasy 244Boats to Save PeopleMedium 247Can Place FlowersMedium 246Lemonade ChangeEasy 245Bag of TokensMedium 249Jump GameMedium 250Jump Game IIMedium 251Non-overlapping IntervalsMedium 252Partition LabelsMedium 750Pack Words into LinesEasy 751Justify One Complete LineEasy 421Text JustificationHard 253Candy DistributionHard 729Minimum Number of Increments on Subarrays to Form a Target ArrayHard 248Gas Station CircuitMedium 254Queue Reconstruction by HeightMedium 255Stack-Robot Smallest StringMedium 256Maximum Average Pass RatioMedium

Graphs I — Fundamentals0 / 1313⌄

GraphsPreviewPractice problems

ProblemDifficulty

258Find Center of Star GraphEasy 259Reachability Between VerticesEasy 263Keys and RoomsMedium 257Uniform-Edge Shortest ReachMedium 261Count Connected ComponentsMedium 262Count Unreachable Vertex PairsMedium 264Graph Valid TreeMedium 314Minimum Height Tree RootsMedium 265Clone GraphMedium 260All Paths from Source to TargetMedium 293Is Graph BipartiteMedium 294Possible BipartitionMedium 295Evaluate DivisionMedium

Graphs II — Grid Graphs0 / 1414⌄

GraphsPreviewPractice problems

ProblemDifficulty

266Number of IslandsMedium 157Iterative Flood FillEasy 268Maximum Island AreaMedium 269Island PerimeterEasy 270Number of Distinct IslandsMedium 271Surrounded RegionsMedium 272Count Closed IslandsMedium 273Number of EnclavesMedium 274Pacific Atlantic Water FlowMedium 275Shortest BridgeMedium 276Making a Large IslandHard 267Word SearchMedium 301Jump Game IIIMedium 302The MazeMedium

Graphs III — Search and Applications0 / 2323⌄

GraphsPreviewPractice problems

ProblemDifficulty

278Shortest Path in Binary MatrixMedium 279Nearest Exit from MazeMedium 277Toroidal Nearest EnemyEasy 281Rotting OrangesMedium 28201 Matrix DistancesMedium 283Walls and GatesMedium 284Farthest Water from LandMedium 285Shortest Distance from All BuildingsHard 288Minimum Genetic MutationMedium 289Open the LockMedium 290Snakes and LaddersMedium 286Word Ladder LengthHard 287Word Ladder PathsHard 291Bus RoutesHard 298Detonate Maximum BombsMedium 296Shortest Alternating PathsMedium 764Reach a Grid Goal with an Obstacle BudgetMedium 280Shortest Path with Obstacle EliminationsHard 131Circular Array LoopMedium 297Minimum Edge Reversals to Reach All NodesMedium 765Follow a Forced Edge TrailEasy 299Valid Arrangement of PairsHard 300Reconstruct ItineraryHard

Topological Sort0 / 1313⌄

Topological SortPreviewPractice problems

ProblemDifficulty

303Build OrderMedium 304Course Schedule FeasibilityMedium 292Course ScheduleMedium 305Course Schedule OrderMedium 306Alien DictionaryHard 307Sequence ReconstructionMedium 310Recipes from SuppliesMedium 308Minimum SemestersMedium 309Course Duration Critical PathHard 313Find All Ancestors in DAGMedium 311Largest Color Value in Directed GraphHard 312Eventual Safe NodesMedium 315Sort Items by GroupsHard

Dynamic Programming I — Foundations0 / 2121⌄

Dynamic ProgrammingPreviewPractice problems

ProblemDifficulty

316Fibonacci NumberEasy 317Tribonacci NumberEasy 318Climbing StairsEasy 319Custom-Step Stair ClimbMedium 320Min Cost Climbing StairsEasy 321Robot in a GridMedium 323House RobberMedium 324House Robber CircularMedium 325Delete and EarnMedium 326House Robber TreeMedium 327Decode WaysMedium 759Count Valid Wildcard TokensEasy 328Decode Ways with WildcardsHard 329Word BreakMedium 330Word Break IIHard 331Coin Change MinimumMedium 334Perfect SquaresMedium 332Coin Change CombinationsMedium 333Combination Sum IVMedium 758Tile a Two-Row Board with DominoesMedium 322Domino and Tromino TilingMedium

Dynamic Programming II — Grid DP0 / 1111⌄

Dynamic ProgrammingPreviewPractice problems

ProblemDifficulty

341Unique PathsMedium 342Unique Paths with ObstaclesMedium 344Minimum Path SumMedium 345Minimum Falling Path SumMedium 346Triangle Minimum TotalMedium 347Minimum Survival HealthHard 348Maximal SquareMedium 349Count Square SubmatricesMedium 350Count Submatrices With All OnesMedium 351Longest Increasing Path MatrixHard 352Two-Robot Cherry HarvestHard

Dynamic Programming III — Strings and Palindromes0 / 1515⌄

Dynamic ProgrammingPreviewPractice problems

ProblemDifficulty

353Longest Common SubsequenceMedium 354Reconstruct the Longest Common SubsequenceMedium 358Shortest Common SupersequenceHard 355Longest Common SubstringMedium 356Extract the Longest Common SubstringMedium 357Distinct SubsequencesHard 359Edit DistanceHard 360Interleaving StringMedium 363Count Palindromic SubstringsMedium 364Longest Palindromic SubstringMedium 361Longest Palindromic SubsequenceMedium 362Minimum Insertions PalindromeMedium 366Partition String into PalindromesMedium 365Palindrome Partition Minimum CutsHard 367Palindrome Partition IIIHard

Dynamic Programming IV — Knapsack, Intervals and Games0 / 1717⌄

Union-Find (Disjoint Set Union)PreviewDynamic ProgrammingPreviewPractice problems

ProblemDifficulty

335Partition Equal Subset SumMedium 336Last Stone Weight IIMedium 337Target Sum WaysMedium 338Ones and ZeroesMedium 340Profitable SchemesHard 760Record a Growing Used SetEasy 339Partition to K Equal SubsetsHard 343Unique Paths IIIHard 761Compare Two Matrix Multiplication OrdersEasy 368Matrix Chain MultiplicationHard 369Minimum Cost Cut StickHard 370Burst BalloonsHard 371Boolean EvaluationHard 372Stone GameMedium 373Predict the WinnerMedium 374Stone Game IIHard 375Pizza SlicesHard

Dynamic Programming V — Sequences, Scheduling and Stocks0 / 2424⌄

Dynamic ProgrammingPreviewPractice problems

ProblemDifficulty

376Longest Increasing SubsequenceMedium 752Best Sum Ending Here with Bounded JumpsMedium 241Constrained Subsequence SumHard 377Count Longest Increasing SubsequencesMedium 378Russian Doll EnvelopesHard 379Largest Divisible SubsetMedium 381Arithmetic SlicesMedium 380Longest Arithmetic SubsequenceMedium 382Longest Fibonacci-like SubsequenceMedium 384Maximum Length Pair ChainMedium 383Maximum Profit Job SchedulingHard 385Partition Array for Maximum SumMedium 386Minimum Difficulty Job ScheduleHard 387Split Array Largest SumHard 388Rod CuttingMedium 762Choose a Trial by Its Worst OutcomeEasy 389Egg DropHard 390Super Egg DropHard 763Track Legal Single-Share ActionsEasy 391Best Time to Buy and Sell StockEasy 392Best Time to Buy and Sell Stock IIMedium 393Best Time to Buy and Sell Stock with CooldownMedium 394Best Time to Buy and Sell Stock with Transaction FeeMedium 395Best Time to Buy and Sell Stock IVHard

Dynamic Programming VI — State Machines, Trees and Patterns0 / 1313⌄

Dynamic ProgrammingPreviewPractice problems

ProblemDifficulty

396Paint HouseMedium 397Paint House K ColorsHard 399Tree DiameterMedium 400Maximum Path Sum TreeHard 398Binary Tree CamerasHard 401Paths with SumMedium 402Sum of Distances in TreeHard 403Count Vowel PermutationsMedium 404Knight DialerMedium 405Attendance Record IIHard 407Wildcard MatchingHard 406Regex MatchingHard 408Stack of BoxesHard

TriesSoon⌄

TriesPreviewPractice problems

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IntervalsSoon⌄

IntervalsPreviewPractice problems

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Bit ManipulationSoon⌄

Bit ManipulationPreviewPractice problems

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In-Place String Compression — CodeBaobab