Coding Interview Problems by Pattern

Contains Duplicate Membership Tracking

Easy

Contains Duplicate II Index Tracking for Duplicate Detection

Easy

First Unique Character in a String Frequency Counting

Easy

Happy Number Cycle Detection via Visited Set

Easy

Isomorphic Strings One-to-One Mapping Validation

Easy

Number of Good Pairs Frequency Counting

Easy

Two Sum Complement Lookup

Easy

Valid Anagram Frequency Counting

Easy

4Sum II Complement Counting

Medium

Contiguous Array Prefix Sum with First Occurrence Tracking

Medium

Continuous Subarray Sum Prefix Sum Remainder Tracking

Medium

Copy List with Random Pointer Node Mapping for Deep Copy

Medium

Count Number of Nice Subarrays Prefix Sum Frequency Counting

Medium

Design Underground System Stateful Aggregation

Medium

Detect Squares Frequency Counting with Coordinate Grouping

Medium

Equal Row and Column Pairs Frequency Counting with Tuple Keys

Medium

Group Anagrams Grouping by Canonical Key

Medium

Longest Consecutive Sequence Set Membership for Sequence Detection

Medium

Minimum Consecutive Card Pickup Index Tracking for Duplicate Detection

Medium

Sort Characters By Frequency Frequency Counting and Sorting

Medium

Subarray Sum Equals K Prefix Sum Frequency Counting

Medium

Valid Sudoku Set Tracking for Uniqueness

Medium

Backspace String Compare Greedy Backward Traversal

Easy

Is Subsequence Linear Scan for Subsequence

Easy

Merge Strings Alternately Two-Pointer Alternating Merge

Easy

Move Zeroes Stable Partition

Easy

Remove Duplicates from Sorted Array In-Place Array Modification

Easy

Remove Element In-Place Filtering

Easy

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

Easy

Squares of a Sorted Array Greedy Contraction

Easy

Two Sum II - Input Array Is Sorted Greedy Contraction

Easy

Valid Palindrome Symmetric Character Comparison with Filtering

Easy

Valid Palindrome II Greedy with Conditional Branching

Easy

3Sum Sorted Array Pair Search

Medium

Container With Most Water Greedy Contraction

Medium

Interval List Intersections Linear Merge of Sorted Intervals

Medium

Longest Palindromic Substring Expand Around Center

Medium

Palindromic Substrings Expand Around Center

Medium

Remove Duplicates from Sorted Array II In-Place Overwrite

Medium

Remove Nth Node From End of List Fixed Gap

Medium

Sort Colors Three-Way Partitioning / Dutch National Flag

Medium

String Compression In-Place Compression

Medium

Trapping Rain Water Greedy Contraction

Hard

Maximum Average Subarray I Fixed-Size Window

Easy

Find All Anagrams in a String Fixed-Size Frequency Matching

Medium

Frequency of the Most Frequent Element Variable Size with Sum Tracking

Medium

Fruit Into Baskets Variable Size with Frequency Map

Medium

Get Equal Substrings Within Budget Variable Size Window with Sum Constraint

Medium

Grumpy Bookstore Owner Fixed-Size Window Sum Optimization

Medium

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

Medium

Longest Repeating Character Replacement Variable Size with Frequency Tracking

Medium

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

Medium

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

Medium

Longest Substring Without Repeating Characters Dynamic Window Adjustment

Medium

Max Consecutive Ones III Variable Size with Constraint

Medium

Maximize the Confusion of an Exam Variable Size with Constraint

Medium

Maximum Erasure Value Dynamic Window with Set for Uniqueness

Medium

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

Medium

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

Medium

Minimum Size Subarray Sum Variable Size Window

Medium

Permutation in String Fixed-Size Frequency Matching

Medium

Subarray Product Less Than K Variable Size with Multiplicative Constraint

Medium

Minimum Window Substring Variable Size with Frequency Counting

Hard

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

Hard

Sliding Window Maximum Monotonic Queue

Hard

Subarrays with K Different Integers Counting Subarrays with Constraints

Hard

Substring with Concatenation of All Words Fixed-Size Chunk Partitioning

Hard

Valid Parentheses Bracket Matching

Easy

Decode String State Preservation for Nested Structures

Medium

Binary Search

Easy

First Bad Version Boundary Search

Easy

Lowest Common Ancestor of a Binary Search Tree Directed Tree Traversal

Easy

Search Insert Position Lower Bound Search

Easy

Sqrt(x) Search for Integer Square Root

Easy

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

Medium

Find Minimum in Rotated Sorted Array Rotated Sorted Array

Medium

Find Peak Element Slope-Based Search for Peak

Medium

Koko Eating Bananas Search Space Reduction on Monotonic Function

Medium

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

Medium

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

Medium

Search in Rotated Sorted Array Rotated Sorted Array

Medium

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

Medium

Median of Two Sorted Arrays Partitioning Technique

Hard

Merge Two Sorted Lists Two-Pointer Merge

Easy

Reverse Linked List

Easy

Add Two Numbers Digit-by-Digit Simulation

Medium

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

Medium

Reverse Linked List II Partial Sublist

Medium

Swap Nodes in Pairs Pointer Manipulation for Node Swapping

Medium

Reverse Nodes in k-Group Interval Reversal

Hard

Balanced Binary Tree Post-Order Traversal with State Aggregation

Easy

Diameter of Binary Tree Post-Order Traversal with State Propagation

Easy

Find if Path Exists in Graph Graph Connectivity

Easy

Flood Fill Grid Connected Component Traversal

Easy

Invert Binary Tree Recursive Tree Traversal

Easy

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

Easy

Minimum Absolute Difference in BST Inorder Traversal on BST

Easy

Path Sum in Binary Tree Recursive State Exploration on Trees

Easy

Range Sum of BST Pruned Search on BST

Easy

Same Tree Recursive Tree Traversal

Easy

Subtree of Another Tree Recursive Tree Traversal and Comparison

Easy

Accounts Merge Graph Connected Components

Medium

All Paths From Source to Target Stateful Exploration with Backtracking

Medium

Binary Search Tree Iterator Controlled Inorder Traversal with Stack

Medium

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

Medium

Clone Graph Graph Cloning with Memoization

Medium

Construct Binary Tree from Preorder and Inorder Traversal Recursive Tree Construction

Medium

Count Good Nodes in Binary Tree Stateful Exploration

Medium

Course Schedule Cycle Detection in Directed Graph

Medium

Course Schedule II Cycle Detection and Topological Sort

Medium

Evaluate Division Graph Traversal with Accumulated State

Medium

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

Medium

Flatten Binary Tree to Linked List Preorder Traversal with Postprocessing

Medium

Graph Valid Tree Cycle Detection and Connectivity Check

Medium

House Robber III Stateful Exploration with Dual State Tracking

Medium

Keys and Rooms Graph Reachability

Medium

Kth Smallest Element in a BST Inorder Traversal with Early Termination

Medium

Max Area of Island Connected Components in Grid

Medium

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

Medium

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

Medium

Number of Islands Connected Components in Grid

Medium

Number of Provinces Connected Components in Graph

Medium

Pacific Atlantic Water Flow Reverse Reachability in Grid

Medium

Path Sum III Prefix Sum with Backtracking

Medium

Serialize and Deserialize Binary Tree Preorder Traversal with State Preservation

Medium

Sum Root to Leaf Numbers Stateful Exploration

Medium

Surrounded Regions Boundary-Connected Region Marking

Medium

Validate Binary Search Tree Boundary-Constrained Validation

Medium

Word Search Stateful Exploration with Backtracking

Medium

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

Hard

01 Matrix Multi-Source Traversal

Medium

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

Medium

As Far from Land as Possible Multi-Source Traversal

Medium

Binary Tree Level Order Traversal Level Order Traversal on Tree

Medium

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

Medium

Binary Tree Vertical Order Traversal Layered Graph Traversal with State Tracking

Medium

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

Medium

Even Odd Tree Level-Order Traversal with Conditional Validation

Medium

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

Medium

Nearest Exit from Entrance in Maze Shortest Path on Unweighted Grid

Medium

Open the Lock Shortest Path on Unweighted Graph

Medium

Rotting Oranges Shortest Path on Unweighted Grid

Medium

Shortest Path in Binary Matrix Shortest Path on Unweighted Graph

Medium

Walls and Gates Multi-Source Traversal

Medium

Bus Routes Shortest Path on Unweighted Graph

Hard

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

Hard

Sliding Puzzle Shortest Path on Unweighted Graph

Hard

Word Ladder Shortest Path on Unweighted Graph

Hard

Last Stone Weight Max Heap Simulation

Easy

Design Twitter K-Way Merge of Sorted Lists

Medium

Hand of Straights Greedy Sequential Grouping

Medium

K Closest Points to Origin Partial Sorting

Medium

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

Medium

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

Medium

Maximum Subsequence Score Sorted Greedy with Min-Heap over Values

Medium

Meeting Rooms II Interval Scheduling Resource Allocation

Medium

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

Medium

Minimum Interval to Include Each Query Dynamic Candidate Filtering

Medium

Minimum Operations to Halve Array Sum Greedy Maximum Extraction

Medium

Reorganize String Greedy Frequency Scheduling

Medium

Top K Frequent Elements Top K Elements

Medium

Top K Frequent Words Frequency Sorting with Custom Comparator

Medium

Total Cost to Hire K Workers Dynamic Candidate Pool Management

Medium

Find Median from Data Stream Dual Heap Median Maintenance

Hard

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

Hard

Swim in Rising Water Best-First Search on Grid

Hard

Combination Sum State Restoration

Medium

Combination Sum II State Restoration with Duplicate Pruning

Medium

Combinations State Restoration

Medium

Generate Parentheses State Restoration

Medium

Letter Combinations of a Phone Number State Restoration

Medium

Palindrome Partitioning State Restoration

Medium

Permutations State Restoration

Medium

Reconstruct Itinerary State Restoration with Lexicographical Ordering

Medium

Restore IP Addresses State Restoration

Medium

Subsets State Restoration

Medium

Subsets II State Restoration with Duplicate Skipping

Medium

N-Queens State Restoration

Hard

Design Add and Search Words Data Structure Prefix Tree

Medium

Implement Trie (Prefix Tree) Prefix Tree

Medium

Replace Words Prefix Tree for Efficient Prefix Matching

Medium

Search Suggestions System Prefix Tree with Stored Suggestions

Medium

Word Search II Prefix Tree

Hard

Number of Connected Components in an Undirected Graph Disjoint Set Union

Medium

Redundant Connection Cycle Detection in Undirected Graph

Medium

Employee Free Time Flatten and Merge

Medium

Insert Interval Linear Scan with Boundary Updates

Medium

Merge Intervals Sorting and Linear Merge

Medium

Find Pivot Index

Easy

Binary Subarrays With Sum Frequency Counting

Medium

Car Pooling Difference Array

Medium

Corporate Flight Bookings Difference Array for Range Updates

Medium

Product of Array Except Self Prefix and Postfix Product Arrays

Medium

Ways to Split Array Range Sum Queries

Medium

Assign Cookies Resource Allocation

Easy

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

Easy

Best Time to Buy and Sell Stock II Incremental Profit Accumulation

Easy

Majority Element Boyer-Moore Voting

Easy

Meeting Rooms Interval Scheduling Overlap Detection

Easy

Asteroids Destroyed Incremental Accumulation

Medium

Best Sightseeing Pair Prefix Tracking

Medium

Car Fleet Monotonic Stack for Grouping

Medium

Gas Station Single Pass Feasibility and Reset

Medium

Increasing Triplet Subsequence State Compression for Subsequence Detection

Medium

Jump Game Backward Reachability

Medium

Jump Game II Range Expansion

Medium

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

Medium

Merge Triplets to Form Target Selective Filtering and Coverage

Medium

Minimum Number of Arrows to Burst Balloons Interval Scheduling

Medium

Non-overlapping Intervals Interval Scheduling

Medium

Partition Labels Interval Partitioning

Medium

Remove Covered Intervals Interval Coverage via Sorting

Medium

Task Scheduler Priority Scheduling with Cooldown

Medium

Valid Parenthesis String Range Tracking

Medium

Candy Two Pass Local Constraint Enforcement

Hard

Set Intersection Size At Least Two Interval Sorting

Hard

Climbing Stairs Top-Down Memoization

Easy

Counting Bits Bit Manipulation

Easy

Maximum Subarray Kadane's Algorithm

Easy

Min Cost Climbing Stairs Top-Down Memoization

Easy

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

Medium

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

Medium

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

Medium

Coin Change Top-Down Memoization

Medium

Coin Change 2 Top-Down Memoization for Combinatorial Counting

Medium

Decode Ways Top-Down Memoization

Medium

Delete and Earn House Robber Variant

Medium

Distinct Subsequences Top-Down Memoization for Counting Subsequences

Medium

House Robber Top-Down Memoization

Medium

House Robber II Linear Subproblem Decomposition

Medium

Interleaving String Top-Down Memoization for Sequence Interleaving

Medium

Length of Longest Fibonacci Subsequence Pair-State Tracking

Medium

Longest Common Subsequence Top-Down Memoization

Medium

Longest Increasing Subsequence Top-Down Memoization

Medium

Longest Palindromic Subsequence Top-Down Memoized Recursion

Medium

Maximum Product Subarray Tracking Dual States

Medium

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

Medium

Minimum Cost For Tickets Interval Coverage

Medium

Partition Equal Subset Sum Top-Down Memoized Subset Sum

Medium

Target Sum Top-Down Memoization

Medium

Unique Paths Top-Down Memoization

Medium

Unique Paths II Grid Path Counting with Obstacles

Medium

Word Break Top-Down Memoization

Medium

Best Time to Buy and Sell Stock IV State Machine

Hard

Burst Balloons Interval DP

Hard

Dungeon Game Grid Backward State Propagation

Hard

Edit Distance Top-Down Memoization for Sequence Alignment

Hard

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

Hard

Regular Expression Matching Top-Down Memoization for Pattern Matching

Hard

Find the Town Judge or Counting Arrays

Easy

Game of Life In-Place State Encoding

Medium

Rotate Image Layer-by-Layer In-Place Transformation

Medium

Set Matrix Zeroes In-Place State Encoding

Medium

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

Medium

Parallel Courses III Dependency Resolution with DP

Medium

Alien Dictionary Cycle Detection in Directed Graph

Hard

Network Delay Time Single-Source Shortest Path

Medium

Merge k Sorted Lists Pairwise Merging

Hard

Linked List Cycle Detection Cycle Detection in Linked List

Easy

Middle of the Linked List Tortoise and Hare

Easy

Palindrome Linked List Midpoint Detection and In-Place Reversal

Easy

Find the Duplicate Number Cycle Detection in Array as Linked List

Medium

Linked List Cycle II Cycle Detection and Entry Point Identification

Medium

Find All Duplicates in an Array In-Place Index Marking

Medium

Min Stack Auxiliary Stack for State Tracking

Easy

Next Greater Element I Monotonic Stack for Next Greater Element

Easy

Remove All Adjacent Duplicates In String Simulated State Reduction

Easy

Asteroid Collision Collision Simulation

Medium

Basic Calculator II Operator Precedence Handling

Medium

Daily Temperatures Next Greater Element

Medium

Evaluate Reverse Polish Notation Expression Evaluation

Medium

Minimum Remove to Make Valid Parentheses Parentheses Matching

Medium

Online Stock Span Compress Consecutive Smaller Values

Medium

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

Medium

Simplify Path Path Simplification

Medium

Sum of Subarray Minimums Counting Subarray Contributions

Medium

Largest Rectangle in Histogram Nearest Smaller Elements for Range Queries

Hard

Longest Valid Parentheses Index Tracking for Valid Parentheses

Hard

Maximum Frequency Stack Frequency Grouping with Max Frequency Tracking

Hard

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

Easy

Reverse Bits Bitwise Position Reversal

Easy

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

Medium

Single Number Pair Cancellation

Easy

Greatest Common Divisor of Strings String Divisibility via GCD

Easy

Missing Number Arithmetic Series Summation

Easy

Plus One Digit-by-Digit Simulation

Easy

Reverse Integer Digit-by-Digit Integer Manipulation

Easy

Sum of All Odd Length Subarrays Combinatorial Counting

Easy

Pow(x, n) Exponentiation by Squaring

Medium

Kids With the Greatest Number of Candies Direct State Evaluation

Easy

Design Browser History Stateful Stack-Based History Management

Medium

Design Snake Game Stateful Grid Movement with Collision Detection

Medium

Design Tic-Tac-Toe Incremental State Tracking

Medium

Encode and Decode Strings Length-Prefixed Encoding

Medium

Multiply Strings Manual Digit-by-Digit Multiplication

Medium

Spiral Matrix Boundary Shrinking

Medium

Zigzag Conversion Stateful Traversal

Medium

LRU Cache Hash Map + Doubly Linked List

Medium