Count Character Occurrences

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Learning Path

Big-O NotationLearn⌄

Arrays and Matrix20⌄

ArraysLearnPractice problems1Array Length and EndsFree 2Sum Array ElementsFree 3Count Target OccurrencesFree 4Find Largest ValueFree 5Second Largest Distinct ValueFree Running Sum Greatest to the Right Product Except Self In Place Reversal Remove Element Rotate Array Right Matrix Dimensions Matrix Row Sums Transpose Matrix Toeplitz Matrix Main Diagonal Sum Spiral Matrix Traversal Rotate Square Matrix Set Matrix Zeroes Game of Life In Place

String and Array18⌄

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

Hash Maps and Sets33⌄

Hash Tables and SetsLearnPractice problemsContains Duplicate Is Unique Find the Duplicate Number Nearby Duplicate Maximum Gap★Nearby Almost Duplicate★First Unique Character Valid Anagram Ransom Note Palindrome Permutation Longest Buildable Palindrome Find Common Characters Bulls and Cows Shared Sorted Values Intersection Unique Intersection with Multiplicity Two Sum Indices Earliest Complement Pairs Digit Pair Spacing Check K-diff Pairs in an Array Count Good Meals 4Sum II Brick Wall Longest Consecutive Sequence Group Anagrams Equal Row and Column Pairs Valid Sudoku Top K Frequent Elements Sort Characters by Frequency Isomorphic Strings Word Pattern Find and Replace Pattern Roman to Integer

Sorting15⌄

Divide-and-Conquer SortingLearnPractice problemsSort an Array Count Inversions★Partition Around a Pivot Implement Quickselect Multi-Field Sort Relative Sort Array Sort by Frequency Largest Concatenated Number Meeting Rooms Feasibility Merge Overlapping Intervals Insert Interval Remove Covered Intervals Minimum Meeting Rooms Fewest Arrows for Balloons Wiggle Sort

Binary Search14⌄

Binary SearchLearnPractice problemsLinear Search in a Sorted Array Binary Search Search Insert Position First Bad Version Find First and Last Position in Sorted Array Search in Rotated Sorted Array Find Minimum in Rotated Sorted Array Sqrt(x)Koko Eating Bananas Capacity to Ship Packages Within D Days Kth Merged Element★Median of Two Sorted Arrays★Magic Index Search a 2D Matrix

Two Pointers27⌄

Two PointersLearnPractice problemsTwo Sum in Sorted Array Unique Two Sum Pairs Closest Pair Sum Count Pairs Under Target Count Pairs Over Target Pair Sums in Range Smallest Difference Across Arrays Merge Sorted Arrays Cross-Array Two Sum Backward In-Place Merge Remove Duplicates Sorted Array At Most Two Duplicates Move Zeroes Sort Array By Parity Sort Colors Squares of Sorted Array Container With Most Water Trapping Rain Water★Three Sum Unique Triplets Three Sum Closest Count Triplets Below Target Four Sum Unique Quadruplets Backspace String Compare Linked List Cycle Circular Array Loop Two Sum Data Structure Sub Sort

Stacks and Queues25⌄

Stacks and QueuesLearnPractice problemsBracket Matcher Validate Parentheses Minimum Add for Valid Parentheses Minimum Remove for Valid Parentheses Evaluate Reverse Polish Notation Basic Calculator★Simplify Unix Path Nested Ternary Evaluator Decode Repetition String Prices After Discounts Asteroid Collision Remove K Adjacent Duplicates Smallest Subsequence of Distinct Characters Exclusive Time of Functions Baseball Game Score of Parentheses Folder Navigation Log Ticket Queue Timing Students Left Without Lunch Dota2 Senate Moving Average from Data Stream Recent Request Counter First Nonrepeating Character Stream Iterative Flood Fill Level Order with Queue

Recursion and Backtracking9⌄

RecursionLearn BacktrackingLearnPractice problemsCase Permutations Subsets Permutations Combination Sum Pattern Without Boundaries★Generate Parentheses Integer Replacement Pow(x, n)Letter Combinations of a Phone Number

Monotonic Stacks7⌄

Monotonic StacksLearnPractice problemsLargest Rectangle in Histogram★Maximal Rectangle via Histogram★Sum of Subarray Ranges Remove K Digits Create Maximum Number★132 Pattern Car Fleet

Heaps and Priority Queues17⌄

HeapsLearnPractice problemsKth Largest Element Kth Largest in a Stream K Closest Points Top K Frequent Words Merge K Sorted Arrays Kth Smallest in a Sorted Matrix Find K Pairs with Smallest Sums Smallest Range Covering K Lists★Sliding Window Median★Smallest Number in Infinite Set Minimum Cost to Connect Sticks Task Scheduler Reorganize String Per-Task Cooldown Scheduling★IPO Capital Maximization★Seat Reservation Manager Twitter Feed Design★

Sliding Window20⌄

Sliding WindowLearnPractice problemsDefuse the Bomb Distinct Window Max Sum Maximum Average Subarray I Vowels per Window Count Fixed-Length Target Windows Distinct Count per Window Permutation in String Longest Substring Without Repeats Longest Repeating Character Replacement Frequency at Most K Longest Ones After K Flips Subarrays with K Distinct★Binary Subarrays to Goal Shortest Sum-at-Least Window Count Product Below K Minimum Window Substring★Minimum Window Subsequence★Find All Anagram Starts Longest Window within Budget★Shortest K-Sum Subarray★

Subarrays and Prefix Sums16⌄

Prefix SumsLearnPractice problemsLeft and Right Sum Differences Range Addition Array Longest Subarray Sum K Subarray Sum Divisible by K Continuous Multiple of K Balanced Zeros and Ones Subarrays with K Odds Maximum Subarray Sum Maximum Circular Subarray Maximum Product Subarray Count Range Sum★2D Prefix Sum Queries Count Submatrices Sum Target★Maximum Sum Rectangle No Larger Than K★Corporate Flight Bookings Median-K Subarray Count★

Greedy Algorithms14⌄

Greedy AlgorithmsLearnPractice problemsAssign Cookies Boats to Save People Bag of Tokens Lemonade Change Can Place Flowers Gas Station Circuit Jump Game Jump Game II Non-overlapping Intervals Partition Labels Candy Distribution★Queue Reconstruction by Height Stack-Robot Smallest String Maximum Average Pass Ratio

Graphs I — Fundamentals12⌄

GraphsLearnPractice problemsFind Center of Star Graph Reachability Between Vertices Uniform-Edge Shortest Reach All Paths from Source to Target Count Connected Components Count Unreachable Vertex Pairs Keys and Rooms Graph Valid Tree Clone Graph Is Graph Bipartite Possible Bipartition Evaluate Division

Graphs II — Grid Graphs13⌄

GraphsLearnPractice problemsNumber of Islands Island Perimeter Maximum Island Area Number of Distinct Islands Word Search Surrounded Regions Count Closed Islands Number of Enclaves Pacific Atlantic Water Flow Shortest Bridge Making a Large Island★Jump Game III The Maze

Graphs III — Search and Applications20⌄

GraphsLearnPractice problemsToroidal Nearest Enemy Shortest Path in Binary Matrix Nearest Exit from Maze Shortest Path with Obstacle Eliminations★Rotting Oranges 01 Matrix Distances Walls and Gates Farthest Water from Land Shortest Distance from All Buildings★Minimum Genetic Mutation Open the Lock Snakes and Ladders Word Ladder Length★Word Ladder Paths★Bus Routes★Detonate Maximum Bombs Shortest Alternating Paths Minimum Edge Reversals to Reach All Nodes Valid Arrangement of Pairs★Reconstruct Itinerary★

Topological Sort13⌄

Topological SortLearnPractice problemsBuild Order Course Schedule Feasibility Course Schedule Order Alien Dictionary★Sequence Reconstruction Minimum Semesters Course Duration Critical Path★Recipes from Supplies Largest Color Value in Directed Graph★Eventual Safe Nodes Find All Ancestors in DAG Minimum Height Tree Roots Sort Items by Groups★

Dynamic Programming I — Foundations19⌄

Dynamic ProgrammingLearnPractice problemsFibonacci Number Tribonacci Number Climbing Stairs Custom-Step Stair Climb Min Cost Climbing Stairs Robot in a Grid Domino and Tromino Tiling House Robber House Robber Circular Delete and Earn House Robber Tree Decode Ways Decode Ways with Wildcards★Word Break Word Break II★Coin Change Minimum Coin Change Combinations Combination Sum IV Perfect Squares

Dynamic Programming II — Grid DP12⌄

Dynamic ProgrammingLearnPractice problemsUnique Paths Unique Paths with Obstacles Unique Paths III★Minimum Path Sum Minimum Falling Path Sum Triangle Minimum Total Minimum Survival Health★Maximal Square Count Square Submatrices Count Submatrices With All Ones Longest Increasing Path Matrix★Two-Robot Cherry Harvest★

Dynamic Programming III — Strings and Palindromes15⌄

Dynamic ProgrammingLearnPractice problemsLongest Common Subsequence Reconstruct the Longest Common Subsequence Longest Common Substring Extract the Longest Common Substring Distinct Subsequences★Shortest Common Supersequence★Edit Distance★Interleaving String Longest Palindromic Subsequence Minimum Insertions Palindrome Count Palindromic Substrings Longest Palindromic Substring Palindrome Partition Minimum Cuts★Partition String into Palindromes Palindrome Partition III★

Dynamic Programming IV — Knapsack, Intervals and Games14⌄

Union-Find (Disjoint Set Union)LearnDynamic ProgrammingLearnPractice problemsPartition Equal Subset Sum Last Stone Weight II Target Sum Ways Ones and Zeroes Partition to K Equal Subsets★Profitable Schemes★Matrix Chain Multiplication★Minimum Cost Cut Stick★Burst Balloons★Boolean Evaluation★Stone Game Predict the Winner Stone Game II★Pizza Slices★

Dynamic Programming V — Sequences, Scheduling and Stocks20⌄

Dynamic ProgrammingLearnPractice problemsLongest Increasing Subsequence Count Longest Increasing Subsequences Russian Doll Envelopes★Largest Divisible Subset Longest Arithmetic Subsequence Arithmetic Slices Longest Fibonacci-like Subsequence Maximum Profit Job Scheduling★Maximum Length Pair Chain Partition Array for Maximum Sum Minimum Difficulty Job Schedule★Split Array Largest Sum★Rod Cutting Egg Drop★Super Egg Drop★Best Time to Buy and Sell Stock Best Time to Buy and Sell Stock II Best Time to Buy and Sell Stock with Cooldown Best Time to Buy and Sell Stock with Transaction Fee Best Time to Buy and Sell Stock IV★

Dynamic Programming VI — State Machines, Trees and Patterns13⌄

Dynamic ProgrammingLearnPractice problemsPaint House Paint House K Colors★Binary Tree Cameras★Tree Diameter Maximum Path Sum Tree★Paths with Sum Sum of Distances in Tree★Count Vowel Permutations Knight Dialer Attendance Record II★Regex Matching★Wildcard Matching★Stack of Boxes★

Simple SortingSoon⌄

Simple SortingLearnPractice problems

ProblemsComing soon

Linked ListsSoon⌄

Linked ListsLearnPractice problems

ProblemsComing soon

Trees and Binary Search TreesSoon⌄

Trees and Binary Search TreesLearnPractice problems

ProblemsComing soon

TriesSoon⌄

TriesLearnPractice problems

ProblemsComing soon

IntervalsSoon⌄

IntervalsLearnPractice problems

ProblemsComing soon

Bit ManipulationSoon⌄

Bit ManipulationLearnPractice problems

ProblemsComing soon

Miscellaneous32⌄

Practice problemsValid Parentheses Longest Valid Parentheses★Next Greater Element I Next Greater Element Circular Daily Temperatures Buildings With an Ocean View Stock Span Sum of Subarray Minimums Last Stone Weight Find Median from Data Stream★Fruit Into Baskets Immutable Range Sum Queries Pivot Index Count Subarrays Sum K Sliding Window Maximum★Constrained Subsequence Sum★Longest Subarray Max-Min Within Limit★Reshape Matrix Generate Spiral Matrix Diagonal Traverse Count Primes Largest Four Single Number Missing Number Majority Element Majority Elements over One Third Find All Duplicates in an Array Find All Missing Values Number Stream Text Justification★Palindrome Pairs★Course Schedule

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Count Character Occurrences — CodeBaobab