Valid Palindrome — CodeBaobab

Learning Path

Big-O NotationLearn⌄

Arrays and Matrix0 / 2121⌄

ArraysLearnPractice problems1Array Length and EndsFree 2Sum Array ElementsFree 3Count Target OccurrencesFree 4Find Largest ValueFree 5Second Largest Distinct ValueFree 6Running Sum 7Greatest to the Right 8Product Except Self 9In Place Reversal 10Remove Element 11Rotate Array Right 12Matrix Dimensions 13Matrix Row Sums 14Transpose Matrix 15Toeplitz Matrix 16Main Diagonal Sum 17Spiral Matrix Traversal 18Rotate Square Matrix 19Set Matrix Zeroes 20Game of Life In Place 21Recommend Movies

String and Array0 / 1818⌄

StringsLearnPractice problems25String Length and EndsFree 26Count Character OccurrencesFree 27Build Reversed StringFree 28Reverse Character ArrayFree 29Reverse Each WordFree 30Reverse Word Order 31Valid Palindrome 32Palindrome After One Deletion 33Longest Common Prefix 34Run-Length Encode 35In-Place String Compression 36First Substring Match 37Count Overlapping Matches 38Find the Difference 39Is Subsequence 40Trim Matching Ends 41String to Integer (atoi)42Valid Number★

Hash Maps and Sets0 / 3333⌄

Hash Tables and SetsLearnPractice problems45Contains Duplicate 46Is Unique 47Find the Duplicate Number 48Nearby Duplicate 49Maximum Gap★50Nearby Almost Duplicate★51First Unique Character 52Valid Anagram 53Ransom Note 54Palindrome Permutation 55Longest Buildable Palindrome 56Find Common Characters 57Bulls and Cows 58Shared Sorted Values 59Intersection Unique 60Intersection with Multiplicity 61Two Sum Indices 62Earliest Complement Pairs 63Digit Pair Spacing Check 64K-diff Pairs in an Array 65Count Good Meals 664Sum II 67Brick Wall 68Longest Consecutive Sequence 69Group Anagrams 70Equal Row and Column Pairs 71Valid Sudoku 72Top K Frequent Elements 73Sort Characters by Frequency 74Isomorphic Strings 75Word Pattern 76Find and Replace Pattern 77Roman to Integer

Simple Sorting0 / 1818⌄

Simple SortingLearnPractice problems651Already In Order?652Count the Drops 653Smallest to the Front 654Selection Sort From Scratch 655Bubble Sort From Scratch 656How Many Sweeps?657Total Swaps to Sort 658Insertion Sort From Scratch 659Slot One Value In 660Sort Largest First 661Second Smallest Value 662K-th Smallest 663Merge Two Sorted Arrays 664Evens Before Odds 665Sort Small Non-Negative Ints 666Sort Three Colors 667Squares of a Sorted Array 668Sort by How Often

Sorting0 / 1515⌄

Divide-and-Conquer SortingLearnPractice problems78Sort an Array 79Count Inversions★80Partition Around a Pivot 81Implement Quickselect 82Multi-Field Sort 83Relative Sort Array 84Sort by Frequency 85Largest Concatenated Number 86Meeting Rooms Feasibility 87Merge Overlapping Intervals 88Insert Interval 89Remove Covered Intervals 90Minimum Meeting Rooms 91Fewest Arrows for Balloons 92Wiggle Sort

Binary Search0 / 1414⌄

Binary SearchLearnPractice problems93Linear Search in a Sorted Array 94Binary Search 95Search Insert Position 96First Bad Version 97Find First and Last Position in Sorted Array 98Search in Rotated Sorted Array 99Find Minimum in Rotated Sorted Array 100Sqrt(x)101Koko Eating Bananas 102Capacity to Ship Packages Within D Days 103Kth Merged Element★104Median of Two Sorted Arrays★105Magic Index 106Search a 2D Matrix

Two Pointers0 / 2727⌄

Two PointersLearnPractice problems107Two Sum in Sorted Array 108Unique Two Sum Pairs 109Closest Pair Sum 110Count Pairs Under Target 111Count Pairs Over Target 112Pair Sums in Range 113Smallest Difference Across Arrays 114Merge Sorted Arrays 115Cross-Array Two Sum 116Backward In-Place Merge 117Remove Duplicates Sorted Array 118At Most Two Duplicates 119Move Zeroes 120Sort Array By Parity 121Sort Colors 122Squares of Sorted Array 123Container With Most Water 124Trapping Rain Water★125Three Sum Unique Triplets 126Three Sum Closest 127Count Triplets Below Target 128Four Sum Unique Quadruplets 129Backspace String Compare 130Linked List Cycle 131Circular Array Loop 132Two Sum Data Structure 133Sub Sort

Stacks and Queues0 / 2525⌄

Stacks and QueuesLearnPractice problems134Bracket Matcher 135Validate Parentheses 136Minimum Add for Valid Parentheses 137Minimum Remove for Valid Parentheses 138Evaluate Reverse Polish Notation 139Basic Calculator★140Simplify Unix Path 141Nested Ternary Evaluator 142Decode Repetition String 143Prices After Discounts 144Asteroid Collision 145Remove K Adjacent Duplicates 146Smallest Subsequence of Distinct Characters 147Exclusive Time of Functions 148Baseball Game 149Score of Parentheses 150Folder Navigation Log 151Ticket Queue Timing 152Students Left Without Lunch 153Dota2 Senate 154Moving Average from Data Stream 155Recent Request Counter 156First Nonrepeating Character Stream 157Iterative Flood Fill 158Level Order with Queue

Linked Lists0 / 1919⌄

Linked ListsLearnPractice problems669Length of a Linked List 670Sum of a Linked List 671Value at Position N 672Largest Value in a List 673Append a Node 674Insert Before a Position 675Delete the First Match 676Drop Sorted Duplicates 677Reverse a Linked List 678Middle of the List 679N-th Node From the End 680Remove N-th From the End 681Is the List a Palindrome?682Partition Around a Value 683Group Odd and Even Positions 684Swap Every Two Nodes 685Rotate the List Right 686Merge Two Sorted Lists 687Add Two Numbers★

Trees and Binary Search Trees0 / 1818⌄

Trees and Binary Search TreesLearnPractice problems688In-Order Values 689How Many Nodes 690Height of the Tree 691Sum of All Nodes 692Count the Leaves 693Values by Level 694Right-Side View 695Is It Height-Balanced?696Symmetric Tree?697Invert a Binary Tree 698Path Sum Exists?699Valid Binary Search Tree?700Search a BST 701Range Sum of a BST 702K-th Smallest in a BST 703Lowest Common Ancestor in a BST 704Insert Into a BST 705Sorted Array to Balanced BST★

Recursion and Backtracking0 / 99⌄

RecursionLearn BacktrackingLearnPractice problems159Case Permutations 160Subsets 161Permutations 162Combination Sum 163Pattern Without Boundaries★164Generate Parentheses 165Integer Replacement 166Pow(x, n)167Letter Combinations of a Phone Number

Monotonic Stacks0 / 77⌄

Monotonic StacksLearnPractice problems173Largest Rectangle in Histogram★174Maximal Rectangle via Histogram★176Sum of Subarray Ranges 177Remove K Digits 178Create Maximum Number★179132 Pattern 180Car Fleet

Heaps and Priority Queues0 / 1717⌄

HeapsLearnPractice problems182Kth Largest Element 183Kth Largest in a Stream 184K Closest Points 185Top K Frequent Words 186Merge K Sorted Arrays 187Kth Smallest in a Sorted Matrix 188Find K Pairs with Smallest Sums 189Smallest Range Covering K Lists★191Sliding Window Median★192Smallest Number in Infinite Set 193Minimum Cost to Connect Sticks 194Task Scheduler 195Reorganize String 196Per-Task Cooldown Scheduling★197IPO Capital Maximization★198Seat Reservation Manager 199Twitter Feed Design★

Sliding Window0 / 2020⌄

Sliding WindowLearnPractice problems200Defuse the Bomb 201Distinct Window Max Sum 202Maximum Average Subarray I 203Vowels per Window 204Count Fixed-Length Target Windows 205Distinct Count per Window 206Permutation in String 207Longest Substring Without Repeats 209Longest Repeating Character Replacement 210Frequency at Most K 211Longest Ones After K Flips 212Subarrays with K Distinct★213Binary Subarrays to Goal 214Shortest Sum-at-Least Window 215Count Product Below K 216Minimum Window Substring★217Minimum Window Subsequence★218Find All Anagram Starts 219Longest Window within Budget★222Shortest K-Sum Subarray★

Subarrays and Prefix Sums0 / 1616⌄

Prefix SumsLearnPractice problems220Left and Right Sum Differences 224Range Addition Array 226Longest Subarray Sum K 227Subarray Sum Divisible by K 228Continuous Multiple of K 229Balanced Zeros and Ones 230Subarrays with K Odds 231Maximum Subarray Sum 232Maximum Circular Subarray 233Maximum Product Subarray 234Count Range Sum★2352D Prefix Sum Queries 236Count Submatrices Sum Target★237Maximum Sum Rectangle No Larger Than K★238Corporate Flight Bookings 239Median-K Subarray Count★

Greedy Algorithms0 / 1414⌄

Greedy AlgorithmsLearnPractice problems243Assign Cookies 244Boats to Save People 245Bag of Tokens 246Lemonade Change 247Can Place Flowers 248Gas Station Circuit 249Jump Game 250Jump Game II 251Non-overlapping Intervals 252Partition Labels 253Candy Distribution★254Queue Reconstruction by Height 255Stack-Robot Smallest String 256Maximum Average Pass Ratio

Graphs I — Fundamentals0 / 1212⌄

GraphsLearnPractice problems258Find Center of Star Graph 259Reachability Between Vertices 257Uniform-Edge Shortest Reach 260All Paths from Source to Target 261Count Connected Components 262Count Unreachable Vertex Pairs 263Keys and Rooms 264Graph Valid Tree 265Clone Graph 293Is Graph Bipartite 294Possible Bipartition 295Evaluate Division

Graphs II — Grid Graphs0 / 1313⌄

GraphsLearnPractice problems266Number of Islands 269Island Perimeter 268Maximum Island Area 270Number of Distinct Islands 267Word Search 271Surrounded Regions 272Count Closed Islands 273Number of Enclaves 274Pacific Atlantic Water Flow 275Shortest Bridge 276Making a Large Island★301Jump Game III 302The Maze

Graphs III — Search and Applications0 / 2020⌄

GraphsLearnPractice problems277Toroidal Nearest Enemy 278Shortest Path in Binary Matrix 279Nearest Exit from Maze 280Shortest Path with Obstacle Eliminations★281Rotting Oranges 28201 Matrix Distances 283Walls and Gates 284Farthest Water from Land 285Shortest Distance from All Buildings★288Minimum Genetic Mutation 289Open the Lock 290Snakes and Ladders 286Word Ladder Length★287Word Ladder Paths★291Bus Routes★298Detonate Maximum Bombs 296Shortest Alternating Paths 297Minimum Edge Reversals to Reach All Nodes 299Valid Arrangement of Pairs★300Reconstruct Itinerary★

Topological Sort0 / 1313⌄

Topological SortLearnPractice problems303Build Order 304Course Schedule Feasibility 305Course Schedule Order 306Alien Dictionary★307Sequence Reconstruction 308Minimum Semesters 309Course Duration Critical Path★310Recipes from Supplies 311Largest Color Value in Directed Graph★312Eventual Safe Nodes 313Find All Ancestors in DAG 314Minimum Height Tree Roots 315Sort Items by Groups★

Dynamic Programming I — Foundations0 / 1919⌄

Dynamic ProgrammingLearnPractice problems316Fibonacci Number 317Tribonacci Number 318Climbing Stairs 319Custom-Step Stair Climb 320Min Cost Climbing Stairs 321Robot in a Grid 322Domino and Tromino Tiling 323House Robber 324House Robber Circular 325Delete and Earn 326House Robber Tree 327Decode Ways 328Decode Ways with Wildcards★329Word Break 330Word Break II★331Coin Change Minimum 332Coin Change Combinations 333Combination Sum IV 334Perfect Squares

Dynamic Programming II — Grid DP0 / 1212⌄

Dynamic ProgrammingLearnPractice problems341Unique Paths 342Unique Paths with Obstacles 343Unique Paths III★344Minimum Path Sum 345Minimum Falling Path Sum 346Triangle Minimum Total 347Minimum Survival Health★348Maximal Square 349Count Square Submatrices 350Count Submatrices With All Ones 351Longest Increasing Path Matrix★352Two-Robot Cherry Harvest★

Dynamic Programming III — Strings and Palindromes0 / 1515⌄

Dynamic ProgrammingLearnPractice problems353Longest Common Subsequence 354Reconstruct the Longest Common Subsequence 355Longest Common Substring 356Extract the Longest Common Substring 357Distinct Subsequences★358Shortest Common Supersequence★359Edit Distance★360Interleaving String 361Longest Palindromic Subsequence 362Minimum Insertions Palindrome 363Count Palindromic Substrings 364Longest Palindromic Substring 365Palindrome Partition Minimum Cuts★366Partition String into Palindromes 367Palindrome Partition III★

Dynamic Programming IV — Knapsack, Intervals and Games0 / 1414⌄

Union-Find (Disjoint Set Union)LearnDynamic ProgrammingLearnPractice problems335Partition Equal Subset Sum 336Last Stone Weight II 337Target Sum Ways 338Ones and Zeroes 339Partition to K Equal Subsets★340Profitable Schemes★368Matrix Chain Multiplication★369Minimum Cost Cut Stick★370Burst Balloons★371Boolean Evaluation★372Stone Game 373Predict the Winner 374Stone Game II★375Pizza Slices★

Dynamic Programming V — Sequences, Scheduling and Stocks0 / 2020⌄

Dynamic ProgrammingLearnPractice problems376Longest Increasing Subsequence 377Count Longest Increasing Subsequences 378Russian Doll Envelopes★379Largest Divisible Subset 380Longest Arithmetic Subsequence 381Arithmetic Slices 382Longest Fibonacci-like Subsequence 383Maximum Profit Job Scheduling★384Maximum Length Pair Chain 385Partition Array for Maximum Sum 386Minimum Difficulty Job Schedule★387Split Array Largest Sum★388Rod Cutting 389Egg Drop★390Super Egg Drop★391Best Time to Buy and Sell Stock 392Best Time to Buy and Sell Stock II 393Best Time to Buy and Sell Stock with Cooldown 394Best Time to Buy and Sell Stock with Transaction Fee 395Best Time to Buy and Sell Stock IV★

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

Dynamic ProgrammingLearnPractice problems396Paint House 397Paint House K Colors★398Binary Tree Cameras★399Tree Diameter 400Maximum Path Sum Tree★401Paths with Sum 402Sum of Distances in Tree★403Count Vowel Permutations 404Knight Dialer 405Attendance Record II★406Regex Matching★407Wildcard Matching★408Stack of Boxes★

TriesSoon⌄

TriesLearnPractice problems

ProblemsComing soon

IntervalsSoon⌄

IntervalsLearnPractice problems

ProblemsComing soon

Bit ManipulationSoon⌄

Bit ManipulationLearnPractice problems

ProblemsComing soon

Miscellaneous0 / 3232⌄

Practice problems43Valid Parentheses 44Longest Valid Parentheses★168Next Greater Element I 169Next Greater Element Circular 170Daily Temperatures 171Buildings With an Ocean View 172Stock Span 175Sum of Subarray Minimums 181Last Stone Weight 190Find Median from Data Stream★208Fruit Into Baskets 221Immutable Range Sum Queries 223Pivot Index 225Count Subarrays Sum K 240Sliding Window Maximum★241Constrained Subsequence Sum★242Longest Subarray Max-Min Within Limit★409Reshape Matrix 410Generate Spiral Matrix 411Diagonal Traverse 412Count Primes 413Largest Four 414Single Number 415Missing Number 416Majority Element 417Majority Elements over One Third 418Find All Duplicates in an Array 419Find All Missing Values 420Number Stream 421Text Justification★422Palindrome Pairs★292Course Schedule

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Valid Palindrome — CodeBaobab