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Grokking the Coding Interview Patterns

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Two Pointers

Introduction to Two Pointers Valid Palindrome Solution: Valid Palindrome 3Sum Solution: 3Sum Remove Nth Node from End of List Solution: Remove Nth Node from End of List Sort Colors Solution: Sort Colors Reverse Words in a String Solution: Reverse Words in a String Valid Word Abbreviation Solution: Valid Word Abbreviation Valid Palindrome II

Fast and Slow Pointers

Introduction to Fast and Slow Pointers Happy Number Solution: Happy Number Linked List Cycle Solution: Linked List Cycle Middle of the Linked List Solution: Middle of the Linked List Circular Array Loop Solution: Circular Array Loop Find the Duplicate Number Solution: Find the Duplicate Number Palindrome Linked List Solution: Palindrome Linked List

Sliding Window

Introduction to Sliding Window Repeated DNA Sequences Solution: Repeated DNA Sequences Sliding Window Maximum Solution: Sliding Window Maximum Minimum Window Subsequence Solution: Minimum Window Subsequence Longest Repeating Character Replacement Solution: Longest Repeating Character Replacement Minimum Window Substring Solution: Minimum Window Substring Longest Substring without Repeating Characters Solution: Longest Substring without Repeating Characters Minimum Size Subarray Sum Solution: Minimum Size Subarray Sum Best Time to Buy and Sell Stock

Merge Intervals

Introduction to Merge Intervals Merge Intervals Solution: Merge Intervals Insert Interval Solution: Insert Interval Interval List Intersections Solution: Interval List Intersections Employee Free Time Solution: Employee Free Time Task Scheduler Solution: Task Scheduler Meeting Rooms II

In-Place Manipulation of a Linked List

Introduction to In-Place Manipulation of a Linked List Reverse Linked List Solution: Reverse Linked List Reverse Nodes in k-Group Solution: Reverse Nodes in k-Group Reverse Linked List II Solution: Reverse Linked List II Reorder List Solution: Reorder List Swapping Nodes in a Linked List Solution: Swapping Nodes in a Linked List Reverse Nodes in Even Length Groups Solution: Reverse Nodes in Even Length Groups Swap Nodes in Pairs

Two Heaps

Introduction to Two Heaps IPO Solution: IPO Find Median from Data Stream Solution: Find Median from Data Stream Sliding Window Median Solution: Sliding Window Median Schedule Tasks on Minimum Machines Solution: Schedule Tasks on Minimum Machines Meeting Rooms III Solution: Meeting Rooms III

K-way merge

Introduction to K-way Merge Merge Sorted Array Solution: Merge Sorted Array Kth Smallest Number in M Sorted Lists Solution: Kth Smallest Number in M Sorted Lists Find K Pairs with Smallest Sums Solution: Find K Pairs with Smallest Sums Merge K Sorted Lists Solution: Merge K Sorted Lists Kth Smallest Element in a Sorted Matrix Solution: Kth Smallest Element in a Sorted Matrix

Top K Elements

Introduction to Top K Elements Kth Largest Element in a Stream Solution: Kth Largest Element in a Stream Reorganize String Solution: Reorganize String K Closest Points to Origin Solution: K Closest Points to Origin Top K Frequent Elements Solution: Top K Frequent Elements Kth Largest Element in an Array Solution: Kth Largest Element in an Array

Modified Binary Search

Introduction to Modified Binary Search Binary Search Solution: Binary Search Search in Rotated Sorted Array Solution: Search in Rotated Sorted Array First Bad Version Solution: First Bad Version Random Pick with Weight Solution: Random Pick with Weight Find K Closest Elements Solution: Find K Closest Elements Single Element in a Sorted Array Solution: Single Element in a Sorted Array Maximum Value at a Given Index in a Bounded Array Solution: Maximum Value at a Given Index in a Bounded Array The K Weakest Rows in a Matrix Solution: The K Weakest Rows in a Matrix Search in Rotated Sorted Array II

Subsets

Introduction to Subsets Subsets Solution: Subsets Permutations Solution: Permutations Letter Combinations of a Phone Number Solution: Letter Combinations of a Phone Number Generate Parentheses Solution: Generate Parentheses Find K-Sum Subsets

Greedy Techniques

Introduction to Greedy Techniques Jump Game Solution: Jump Game Boats to Save People Solution: Boats to Save People Gas Station Solution: Gas Station Two City Scheduling Solution: Two City Scheduling Minimum Number of Refueling Stops Solution: Minimum Number of Refueling Stops Largest Palindromic Number Solution: Largest Palindromic Number Jump Game II

Backtracking

Introduction to Backtracking N-Queens Solution: N-Queens Word Search Solution: Word Search House Robber III Solution: House Robber III Restore IP Addresses Solution: Restore IP Addresses Flood Fill Solution: Flood Fill Minimum Moves to Spread Stones Over Grid Solution: Minimum Moves to Spread Stones Over Grid Sudoku Solver Matchsticks to Square

Dynamic Programming

Introduction to Dynamic Programming 0/1 Knapsack Solution: 0/1 Knapsack Coin Change Solution: Coin Change N-th Tribonacci Number Solution: N-th Tribonacci Number Partition Equal Subset Sum Solution: Partition Equal Subset Sum Counting Bits Solution: Counting Bits 01 Matrix Solution: 01 Matrix House Robber II Solution: House Robber II Maximum Product Subarray Solution: Maximum Product Subarray Combination Sum Solution: Combination Sum Word Break Solution: Word Break Palindromic Substrings Solution: Palindromic Substrings Longest Common Subsequence Solution: Longest Common Subsequence Word Break II Solution: Word Break II Decode Ways Solution: Decode Ways Count the Number of Good Subsequences Solution: Count the Number of Good Subsequences Climbing Stairs

Cyclic Sort

Introduction to Cyclic Sort Missing Number Solution: Missing Number First Missing Positive Solution: First Missing Positive Find the Corrupt Pair Solution: Find the Corrupt Pair Find the First K Missing Positive Numbers

Topological Sort

Introduction to Topological Sort Compilation Order Solution: Compilation Order Alien Dictionary Solution: Alien Dictionary Verifying an Alien Dictionary Solution: Verifying an Alien Dictionary Course Schedule II Solution: Course Schedule II Course Schedule Solution: Course Schedule Find All Possible Recipes from Given Supplies

Matrices

Introduction to Matrices Set Matrix Zeroes Solution: Set Matrix Zeroes Rotate Image Solution: Rotate Image Spiral Matrix Solution: Spiral Matrix Where Will the Ball Fall Solution: Where Will the Ball Fall Transpose Matrix Solution: Transpose Matrix

Stacks

Introduction to Stacks Basic Calculator Solution: Basic Calculator Remove All Adjacent Duplicates In String Solution: Remove All Adjacent Duplicates In String Minimum Remove to Make Valid Parentheses Solution: Minimum Remove to Make Valid Parentheses Exclusive Time of Functions Solution: Exclusive Time of Functions Flatten Nested List Iterator Solution: Flatten Nested List Iterator Implement Queue Using Stacks Solution: Implement Queue Using Stacks Valid Parentheses

Graphs

Introduction to Graphs Network Delay Time Solution: Network Delay Time Paths in Maze That Lead to Same Room Solution: Paths in Maze That Lead to Same Room Clone Graph Solution: Clone Graph Graph Valid Tree Solution: Graph Valid Tree Bus Routes Solution: Bus Routes Reconstruct Itinerary Solution: Reconstruct Itinerary

Tree Depth-First Search

Introduction to Tree Depth-First Search Flatten Binary Tree to Linked List Solution: Flatten Binary Tree to Linked List Diameter of Binary Tree Solution: Diameter of Binary Tree Serialize and Deserialize Binary Tree Solution: Serialize and Deserialize Binary Tree Invert Binary Tree Solution: Invert Binary Tree Binary Tree Maximum Path Sum Solution: Binary Tree Maximum Path Sum Convert Sorted Array to Binary Search Tree Solution: Convert Sorted Array to Binary Search Tree Build Binary Tree from Preorder and Inorder Traversal Solution: Build Binary Tree from Preorder and Inorder Traversal Binary Tree Right Side View Solution: Binary Tree Right Side View Lowest Common Ancestor of a Binary Tree Solution: Lowest Common Ancestor of a Binary Tree Validate Binary Search Tree Solution: Validate Binary Search Tree Nested List Weight Sum II Solution: Nested List Weight Sum II Inorder Successor in BST Solution: Inorder Successor in BST Height of Binary Tree After Subtree Removal Queries Solution: Height of Binary Tree After Subtree Removal Queries Maximum Depth of Binary Tree Kth Smallest Element in a BST

Tree Breadth-First Search

Introduction to Tree Breadth-First Search Binary Tree Level Order Traversal Solution: Binary Tree Level Order Traversal Binary Tree Zigzag Level Order Traversal Solution: Binary Tree Zigzag Level Order Traversal Populating Next Right Pointers in Each Node Solution: Populating Next Right Pointers in Each Node Vertical Order Traversal of a Binary Tree Solution: Vertical Order Traversal of a Binary Tree Symmetric Tree Solution: Symmetric Tree Word Ladder Solution: Word Ladder Connect All Siblings of a Binary Tree

Trie

Introduction to Trie Implement Trie (Prefix Tree)Solution: Implement Trie (Prefix Tree)Search Suggestions System Solution: Search Suggestions System Replace Words Solution: Replace Words Design Add and Search Words Data Structure Solution: Design Add and Search Words Data Structure Word Search II Solution: Word Search II Top K Frequent Words Solution: Top K Frequent Words Longest Common Prefix Solution: Longest Common Prefix Lexicographical Numbers

Hash Maps

Introduction to Hash Maps Design HashMap Solution: Design HashMap Fraction to Recurring Decimal Solution: Fraction to Recurring Decimal Logger Rate Limiter Solution: Logger Rate Limiter Next Greater Element I Solution: Next Greater Element I Isomorphic Strings Solution: Isomorphic Strings Find Duplicate File in System Solution: Find Duplicate File in System Dot Product of Two Sparse Vectors Solution: Dot Product of Two Sparse Vectors High Five Solution: High Five Longest Palindrome

Knowing What to Track

Introduction to Knowing What to Track Palindrome Permutation Solution: Palindrome Permutation Valid Anagram Solution: Valid Anagram Design Tic-Tac-Toe Solution: Design Tic-Tac-Toe Group Anagrams Solution: Group Anagrams Maximum Frequency Stack Solution: Maximum Frequency Stack First Unique Character in a String Solution: First Unique Character in a String Find All Anagrams in a String Solution: Find All Anagrams in a String Longest Palindrome by Concatenating Two-Letter Words Solution: Longest Palindrome by Concatenating Two-Letter Words Ransom Note

Union Find

Introduction to Union Find Redundant Connection Solution: Redundant Connection Number of Islands Solution: Number of Islands Most Stones Removed with Same Row or Column Solution: Most Stones Removed with Same Row or Column Longest Consecutive Sequence Solution: Longest Consecutive Sequence Last Day Where You Can Still Cross Solution: Last Day Where You Can Still Cross Regions Cut by Slashes Solution: Regions Cut by Slashes Accounts Merge Solution: Accounts Merge Minimize Malware Spread Solution: Minimize Malware Spread Evaluate Division

Custom Data Structures

Introduction to Custom Data Structures Snapshot Array Solution: Snapshot Array Time Based Key-Value Store Solution: Time Based Key-Value Store LRU Cache Solution: LRU Cache Insert Delete GetRandom O(1)Solution: Insert Delete GetRandom O(1)Min Stack Solution: Min Stack Range Module Solution: Range Module Shortest Word Distance II Solution: Shortest Word Distance II Max Stack Solution: Max Stack LFU Cache

Bitwise Manipulation

Introduction to Bitwise Manipulation Find the Difference Solution: Find the Difference Complement of Base 10 Integer Solution: Complement of Base 10 Integer Flipping an Image Solution: Flipping an Image Single Number Solution: Single Number Single Number II Solution: Single Number II Encode and Decode Strings Solution: Encode and Decode Strings Sum of All Subset XOR Totals Solution: Sum of All Subset XOR Totals Reverse Bits

Challenge Yourself

Challenge Yourself: Introduction

Shortest Bridge

Number of Connected Components in an Undirected Graph

Median of Two Sorted Arrays

Pacific Atlantic Water Flow

Contains Duplicate

Maximum Subarray

Find Minimum in Rotated Sorted Array

Non-overlapping Intervals

Largest Rectangle in Histogram

Subtree of Another Tree

Number of 1 Bits

Container with Most Water

Evaluate Reverse Polish Notation

Product of Array Except Self

Longest Increasing Subsequence

Sum of Two Integers

Majority Element

Longest Palindromic Substring

Permutations II

Number of Provinces

Linked List Cycle II

Minimum Flips to Make the Binary String Alternate

Lemonade Change

Find All Numbers Disappeared in an Array

Find All Duplicates in an Array

Design In-Memory File System

Design File System

Asteroid Collision

Rotting Oranges

Multiply Strings

Conclusion

Where to Go from Here?

4Sum

Try to solve the 4Sum problem.

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Statement
Examples
Understand the problem
Try it yourself

Statement

Given an array nums of $n$ integers, return an array of all the unique quadruplets, [nums[a], nums[b], nums[c], nums[d]] such that:

$0 \leq$ a, b, c, d $< n$
a, b, c, and d are distinct.
nums[a] + nums[b] + nums[c] + nums[d] $=$ target

You may return the answer in any order.

Constraints:

$1 \leq$ nums.length $\leq 200$
$-10^9 \leq$ nums[i] $\leq 10^9$
$-10^9 \leq$ target $\leq 10^9$

Examples

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