Coding Interview Problems by Pattern

Contains Duplicate II Index Tracking for Duplicate Detection

Degree of an Array Frequency Counting with Index Tracking

Design HashMap Separate Chaining

Design HashSet Separate Chaining with Buckets

Find Common Characters in All Strings Frequency Intersection

Find the Difference Frequency Counting

First Unique Character in a String Frequency Counting

Happy Number Cycle Detection via Visited Set

Intersection of Two Arrays Set Membership for Intersection

Intersection of Two Arrays II Frequency Counting

Isomorphic Strings One-to-One Mapping Validation

Jewels and Stones Set Membership for Counting

Logger Rate Limiter Stateful Timestamp Tracking

Longest Harmonious Subsequence Frequency Counting and Complement Lookup

Number of Good Pairs Frequency Counting

Ransom Note Frequency Counting

Set Mismatch Boolean Array for Presence Tracking

Two Sum Complement Lookup

Valid Anagram Frequency Counting

4Sum II Complement Counting

Brick Wall Frequency Counting of Prefix Sums

Bulls and Cows Frequency Counting with Complement Matching

Contiguous Array Prefix Sum with First Occurrence Tracking

Continuous Subarray Sum Prefix Sum Remainder Tracking

Copy List with Random Pointer Node Mapping for Deep Copy

Count Number of Nice Subarrays Prefix Sum Frequency Counting

Design Underground System Stateful Aggregation

Detect Squares Frequency Counting with Coordinate Grouping

Equal Row and Column Pairs Frequency Counting with Tuple Keys

Group Anagrams Grouping by Canonical Key

Insert Delete GetRandom O(1) Index Mapping with Array

K-diff Pairs in an Array Frequency Counting and Complement Lookup

Longest Consecutive Sequence Set Membership for Sequence Detection

Minimum Consecutive Card Pickup Index Tracking for Duplicate Detection

Pairs of Songs With Total Durations Divisible by 60 Frequency Counting with Modular Arithmetic

Sort Characters By Frequency Frequency Counting and Sorting

Subarray Sum Equals K Prefix Sum Frequency Counting

Subdomain Visit Count Frequency Counting with String Manipulation

Valid Sudoku Set Tracking for Uniqueness

Backspace String Compare Greedy Backward Traversal

Two Pointers

31 problems·14 Easy·16 Medium·1 Hard

Is Subsequence Linear Scan for Subsequence

Linked List Cycle Detection Cycle Detection in Linked List

Long Pressed Name Greedy Validation

Merge Strings Alternately Two-Pointer Alternating Merge

Middle of the Linked List Tortoise and Hare

Move Zeroes Stable Partition

Palindrome Linked List Midpoint Detection and In-Place Reversal

Remove Duplicates from Sorted Array In-Place Array Modification

Remove Element In-Place Filtering

Squares of a Sorted Array Greedy Contraction

Two Sum II - Input Array Is Sorted Greedy Contraction

Valid Palindrome Symmetric Character Comparison with Filtering

Valid Palindrome II Greedy with Conditional Branching

3Sum Sorted Array Pair Search

3Sum Closest Sorted Array Pair Search

3Sum Smaller Sorted Array Pair Counting

4Sum Nested Loops with Two-Pointer Pair Search

Boats to Save People Greedy Pairing

Container With Most Water Greedy Contraction

Find the Duplicate Number Cycle Detection in Array as Linked List

Interval List Intersections Linear Merge of Sorted Intervals

Linked List Cycle II Cycle Detection and Entry Point Identification

Longest Palindromic Substring Expand Around Center

Palindromic Substrings Expand Around Center

Remove Duplicates from Sorted Array II In-Place Overwrite

Remove Nth Node From End of List Fixed Gap

Sort Colors Three-Way Partitioning / Dutch National Flag

String Compression In-Place Compression

Valid Triangles Count Sorted Array Pair Search

Trapping Rain Water Greedy Contraction

Maximum Average Subarray I Fixed-Size Window

Sliding Window

24 problems·1 Easy·18 Medium·5 Hard

Find All Anagrams in a String Fixed-Size Frequency Matching

Frequency of the Most Frequent Element Variable Size with Sum Tracking

Fruit Into Baskets Variable Size with Frequency Map

Get Equal Substrings Within Budget Variable Size Window with Sum Constraint

Grumpy Bookstore Owner Fixed-Size Window Sum Optimization

Longest Continuous Subarray With Absolute Diff Less Than or Equal to Limit Monotonic Queue Optimization

Longest Repeating Character Replacement Variable Size with Frequency Tracking

Longest Subarray of 1's After Deleting One Element Variable Size with Constraint

Longest Substring with At Most K Distinct Characters Variable Size with Frequency Map

Longest Substring Without Repeating Characters Dynamic Window Adjustment

Max Consecutive Ones III Variable Size with Constraint

Maximize the Confusion of an Exam Variable Size with Constraint

Maximum Erasure Value Dynamic Window with Set for Uniqueness

Maximum Number of Vowels in a Substring of Given Length Fixed-Size Window

Maximum Sum of Distinct Subarrays With Length K Fixed-Size with Distinctness Constraint

Minimum Size Subarray Sum Variable Size Window

Permutation in String Fixed-Size Frequency Matching

Subarray Product Less Than K Variable Size with Multiplicative Constraint

Minimum Window Substring Variable Size with Frequency Counting

Shortest Subarray with Sum at Least K Prefix Sums + Monotonic Deque

Sliding Window Maximum Monotonic Queue

Subarrays with K Different Integers Counting Subarrays with Constraints

Substring with Concatenation of All Words Fixed-Size Chunk Partitioning

Prefix Sum

6 problems·1 Easy·5 Medium

Find Pivot Index

Binary Subarrays With Sum Frequency Counting

Car Pooling Difference Array

Corporate Flight Bookings Difference Array for Range Updates

Product of Array Except Self Prefix and Postfix Product Arrays

Ways to Split Array Range Sum Queries

Binary Search Modified Binary Search

Binary Search

16 problems·5 Easy·9 Medium·2 Hard

First Bad Version Boundary Search

Lowest Common Ancestor of a Binary Search Tree Directed Tree Traversal

Search Insert Position Lower Bound Search

Sqrt(x) Search for Integer Square Root

Capacity To Ship Packages Within D Days Feasibility Check

Count Complete Tree Nodes Height-Based Pruning in Complete Binary Trees

Find Minimum in Rotated Sorted Array Rotated Sorted Array

Find Peak Element Slope-Based Search for Peak

Koko Eating Bananas Search Space Reduction on Monotonic Function

Random Pick with Weight Binary Search on Prefix Sums for Weighted Random Selection

Search a 2D Matrix Two-Level Search in Sorted Matrix

Search in Rotated Sorted Array Rotated Sorted Array

Time Based Key-Value Store Rightmost Element Search in Sorted Array

Median of Two Sorted Arrays Partitioning Technique

Split Array Largest Sum Feasibility Check

Find All Numbers Disappeared in an Array In-place Marking via Index Negation

Sorting

3 problems·1 Easy·2 Medium

Find All Duplicates in an Array In-Place Index Marking

Wiggle Sort II Sorting with Controlled Interleaving

Min Stack Auxiliary Stack for State Tracking

Stacks

17 problems·4 Easy·10 Medium·3 Hard

Next Greater Element I Monotonic Stack for Next Greater Element

Remove All Adjacent Duplicates In String Simulated State Reduction

Valid Parentheses Bracket Matching

Asteroid Collision Collision Simulation

Basic Calculator II Operator Precedence Handling

Daily Temperatures Next Greater Element

Decode String State Preservation for Nested Structures

Evaluate Reverse Polish Notation Expression Evaluation

Minimum Remove to Make Valid Parentheses Parentheses Matching

Online Stock Span Compress Consecutive Smaller Values

Remove All Adjacent Duplicates in String II Count Tracking for Adjacent Removal

Simplify Path Path Simplification

Sum of Subarray Minimums Counting Subarray Contributions

Largest Rectangle in Histogram Nearest Smaller Elements for Range Queries

Longest Valid Parentheses Index Tracking for Valid Parentheses

Maximum Frequency Stack Frequency Grouping with Max Frequency Tracking

Merge Two Sorted Lists Two-Pointer Merge

Linked Lists

7 problems·2 Easy·4 Medium·1 Hard

Reverse Linked List In-place Reversal of a Linked List

Add Two Numbers Digit-by-Digit Simulation

Reorder List Fast & Slow Pointers + In-place Reversal + Merge

Reverse Linked List II Partial Sublist

Swap Nodes in Pairs Pointer Manipulation for Node Swapping

Reverse Nodes in k-Group Interval Reversal

Balanced Binary Tree Post-Order Traversal with State Aggregation

DFS

41 problems·11 Easy·29 Medium·1 Hard

Diameter of Binary Tree Post-Order Traversal with State Propagation

Find if Path Exists in Graph Graph Connectivity

Flood Fill Grid Connected Component Traversal

Invert Binary Tree Recursive Tree Traversal

Maximum Depth of Binary Tree Post-Order Traversal for Tree Depth

Minimum Absolute Difference in BST Inorder Traversal on BST

Path Sum in Binary Tree Recursive State Exploration on Trees

Range Sum of BST Pruned Search on BST

Same Tree Recursive Tree Traversal

Subtree of Another Tree Recursive Tree Traversal and Comparison

Accounts Merge Graph Connected Components

All Paths From Source to Target Stateful Exploration with Backtracking

Binary Search Tree Iterator Controlled Inorder Traversal with Stack

Binary Tree Longest Consecutive Sequence II Post-Order Traversal with State Propagation

Clone Graph Graph Cloning with Memoization

Construct Binary Tree from Preorder and Inorder Traversal Recursive Tree Construction

Count Good Nodes in Binary Tree Stateful Exploration

Course Schedule Cycle Detection in Directed Graph

Course Schedule II Cycle Detection and Topological Sort

Evaluate Division Graph Traversal with Accumulated State

Find Leaves of Binary Tree Post-Order Traversal with State Aggregation

Flatten Binary Tree to Linked List Preorder Traversal with Postprocessing

Graph Valid Tree Cycle Detection and Connectivity Check

House Robber III Stateful Exploration with Dual State Tracking

Keys and Rooms Graph Reachability

Kth Smallest Element in a BST Inorder Traversal with Early Termination

Lowest Common Ancestor of a Binary Tree Postorder Traversal for State Propagation

Max Area of Island Connected Components in Grid

Minimum Number of Edge Reversals to Reach City Zero Graph Traversal with Edge Direction Tracking

Minimum Score of a Path Between Two Cities Graph Connectivity and State Propagation

Number of Islands Connected Components in Grid

Number of Provinces Connected Components in Graph

Pacific Atlantic Water Flow Reverse Reachability in Grid

Path Sum III Prefix Sum with Backtracking

Serialize and Deserialize Binary Tree Preorder Traversal with State Preservation

Sum Root to Leaf Numbers Stateful Exploration

Surrounded Regions Boundary-Connected Region Marking

Validate Binary Search Tree Boundary-Constrained Validation

Word Search Stateful Exploration with Backtracking

Binary Tree Maximum Path Sum Post-Order Traversal with State Propagation

01 Matrix Multi-Source Traversal

BFS

18 problems·14 Medium·4 Hard

All Nodes Distance K in Binary Tree Shortest Path on Unweighted Graph

As Far from Land as Possible Multi-Source Traversal

Binary Tree Level Order Traversal Level Order Traversal on Tree

Binary Tree Right Side View Level-Order Traversal for Layered State Exploration

Binary Tree Vertical Order Traversal Layered Graph Traversal with State Tracking

Binary Tree Zigzag Level Order Traversal Level Order Traversal with Direction Toggle

Even Odd Tree Level-Order Traversal with Conditional Validation

Find Largest Value in Each Tree Row Level-Order Traversal with Aggregation

Nearest Exit from Entrance in Maze Shortest Path on Unweighted Grid

Open the Lock Shortest Path on Unweighted Graph

Rotting Oranges Shortest Path on Unweighted Grid

Shortest Path in Binary Matrix Shortest Path on Unweighted Graph

Walls and Gates Multi-Source Traversal

Bus Routes Shortest Path on Unweighted Graph

Shortest Path in a Grid with Obstacles Elimination Shortest Path with State Augmentation

Sliding Puzzle Shortest Path on Unweighted Graph

Word Ladder Shortest Path on Unweighted Graph

Last Stone Weight Max Heap Simulation

Heaps

19 problems·1 Easy·14 Medium·4 Hard

Design Twitter K-Way Merge of Sorted Lists

Hand of Straights Greedy Sequential Grouping

K Closest Points to Origin Partial Sorting

Kth Largest Element in a Stream Fixed-Size Min-Heap for Top K Elements

Kth Largest Element in an Array Fixed-Size Min-Heap for Top K Elements

Maximum Subsequence Score Sorted Greedy with Min-Heap over Values

Meeting Rooms II Interval Scheduling Resource Allocation

Min Cost to Connect All Points Prim's Algorithm for MST

Minimum Interval to Include Each Query Dynamic Candidate Filtering

Minimum Operations to Halve Array Sum Greedy Maximum Extraction

Reorganize String Greedy Frequency Scheduling

Top K Frequent Elements Top K Elements

Top K Frequent Words Frequency Sorting with Custom Comparator

Total Cost to Hire K Workers Dynamic Candidate Pool Management

Find Median from Data Stream Dual Heap Median Maintenance

Merge k Sorted Lists Priority Queue for Merging Sorted Sequences

Smallest Range Covering Elements from K Lists K-Way Merge with Range Tracking

Swim in Rising Water Best-First Search on Grid

Employee Free Time Flatten and Merge

Intervals

3 problems·3 Medium

Insert Interval Linear Scan with Boundary Updates

Merge Intervals Sorting and Linear Merge

Assign Cookies Resource Allocation

Greedy

25 problems·5 Easy·18 Medium·2 Hard

Best Time to Buy and Sell Stock Two Pointers for Optimal Substructure

Best Time to Buy and Sell Stock II Incremental Profit Accumulation

Majority Element Boyer-Moore Voting

Meeting Rooms Interval Scheduling Overlap Detection

Asteroids Destroyed Incremental Accumulation

Best Sightseeing Pair Prefix Tracking

Car Fleet Monotonic Stack for Grouping

Gas Station Single Pass Feasibility and Reset

Increasing Triplet Subsequence State Compression for Subsequence Detection

Integer to Roman Greedy Decomposition

Jump Game Backward Reachability

Jump Game II Range Expansion

Least Number of Unique Integers after K Removals Frequency-Based Removal

Merge Triplets to Form Target Selective Filtering and Coverage

Minimum Number of Arrows to Burst Balloons Interval Scheduling

Non-overlapping Intervals Interval Scheduling

Pancake Sorting Greedy Placement via Reversal

Partition Labels Interval Partitioning

Remove Covered Intervals Interval Coverage via Sorting

Task Scheduler Priority Scheduling with Cooldown

Valid Parenthesis String Range Tracking

Wiggle Sort Local Correction

Candy Two Pass Local Constraint Enforcement

Set Intersection Size At Least Two Interval Sorting

Combination Sum State Restoration

Backtracking

12 problems·11 Medium·1 Hard

Combination Sum II State Restoration with Duplicate Pruning

Combinations State Restoration

Generate Parentheses State Restoration

Letter Combinations of a Phone Number State Restoration

Palindrome Partitioning State Restoration

Permutations State Restoration

Reconstruct Itinerary State Restoration with Lexicographical Ordering

Restore IP Addresses State Restoration

Subsets State Restoration

Subsets II State Restoration with Duplicate Skipping

N-Queens State Restoration

Climbing Stairs Top-Down Memoization

Dynamic Programming

33 problems·4 Easy·23 Medium·6 Hard

Counting Bits Bit Manipulation

Maximum Subarray Kadane's Algorithm

Min Cost Climbing Stairs Top-Down Memoization

Best Time to Buy and Sell Stock with Cooldown State Machine with Cooldown

Best Time to Buy and Sell Stock with Transaction Fee State Machine

Cheapest Flights Within K Stops Bellman-Ford Relaxation with Edge Constraints

Coin Change Top-Down Memoization

Coin Change 2 Top-Down Memoization for Combinatorial Counting

Decode Ways Top-Down Memoization

Delete and Earn House Robber Variant

Distinct Subsequences Top-Down Memoization for Counting Subsequences

House Robber Top-Down Memoization

House Robber II Linear Subproblem Decomposition

Interleaving String Top-Down Memoization for Sequence Interleaving

Length of Longest Fibonacci Subsequence Pair-State Tracking

Longest Common Subsequence Top-Down Memoization

Longest Increasing Subsequence Top-Down Memoization

Longest Palindromic Subsequence Top-Down Memoized Recursion

Maximum Product Subarray Tracking Dual States

Maximum Sum Circular Subarray Kadane's Algorithm with Circular Array Adaptation

Minimum Cost For Tickets Interval Coverage

Partition Equal Subset Sum Top-Down Memoized Subset Sum

Target Sum Top-Down Memoization

Unique Paths Top-Down Memoization

Unique Paths II Grid Path Counting with Obstacles

Word Break Top-Down Memoization

Best Time to Buy and Sell Stock IV State Machine

Burst Balloons Interval DP

Dungeon Game Grid Backward State Propagation

Edit Distance Top-Down Memoization for Sequence Alignment

Longest Increasing Path in a Matrix Top-Down Memoization with DFS

Regular Expression Matching Top-Down Memoization for Pattern Matching

Find the Town Judge or Counting Arrays

Graphs

7 problems·1 Easy·5 Medium·1 Hard

Minimum Number of Vertices to Reach All Nodes Indegree Analysis for Reachability

Network Delay Time Single-Source Shortest Path

Number of Connected Components in an Undirected Graph Disjoint Set Union

Parallel Courses III Dependency Resolution with DP

Redundant Connection Cycle Detection in Undirected Graph

Alien Dictionary Cycle Detection in Directed Graph

Design Add and Search Words Data Structure Prefix Tree

Tries

5 problems·4 Medium·1 Hard

Implement Trie (Prefix Tree) Prefix Tree

Replace Words Prefix Tree for Efficient Prefix Matching

Search Suggestions System Prefix Tree with Stored Suggestions

Word Search II Prefix Tree

Game of Life In-Place State Encoding

Matrix

4 problems·4 Medium

Rotate Image Layer-by-Layer In-Place Transformation

Set Matrix Zeroes In-Place State Encoding

Sparse Matrix Multiplication Sparse Optimization

Number of 1 Bits Brian Kernighan’s Algorithm for Counting Set Bits

Bit Manipulation

4 problems·3 Easy·1 Medium

Reverse Bits Bitwise Position Reversal

Single Number Pair Cancellation

Sum of Two Integers Without Using '+' Operator Iterative Carry Propagation

Greatest Common Divisor of Strings String Divisibility via GCD

Math

6 problems·5 Easy·1 Medium

Missing Number Arithmetic Series Summation

Plus One Digit-by-Digit Simulation

Reverse Integer Digit-by-Digit Integer Manipulation

Sum of All Odd Length Subarrays Combinatorial Counting

Pow(x, n) Exponentiation by Squaring

Kids With the Greatest Number of Candies Direct State Evaluation

Simulation

11 problems·3 Easy·8 Medium

Reverse Words in a String III Word-by-Word Transformation

Roman to Integer Stateful Iteration with Relative Comparison

Design Browser History Stateful Stack-Based History Management

Design Snake Game Stateful Grid Movement with Collision Detection

Design Tic-Tac-Toe Incremental State Tracking

Encode and Decode Strings Length-Prefixed Encoding

Multiply Strings Manual Digit-by-Digit Multiplication

Partition Array According to Given Pivot Stable Partitioning

Spiral Matrix Boundary Shrinking

Zigzag Conversion Stateful Traversal

LRU Cache Hash Map + Doubly Linked List

System Design

1 problems·1 Medium