Coders Crushby Napplied AI

Data Structures & Algorithms

100+ curated problems from top tech interviews

All Arrays & Hashing Two Pointers Sliding Window Stack & Queue Binary Search Linked Lists Trees Tries Heap / Priority Queue Graphs Dynamic Programming Backtracking Greedy Intervals Math & Geometry Bit Manipulation

129 problems found

Level Order Traversal

BFS level by level...

O(V + E) / O(V)

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System Design

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EasyMath & Geometry

Check if number is happy...

O(log n) / O(1)

Google, Facebook

Is Palindrome II

EasyTwo Pointers

Check palindrome with alphanumeric only...

Facebook, Apple

Majority Element

EasyArrays & Hashing

Find element appearing > n/2 times...

Facebook, Microsoft

Number of 1 Bits

EasyBit Manipulation

Count 1 bits in integer...

Google, Microsoft

Add Two Numbers

MediumLinked Lists

Add two numbers represented by lists...

O(max(m,n)) / O(max(m,n))

Reorder List

MediumLinked Lists

Reorder list in alternating pattern...

Google, Facebook

Number of Recent Calls

EasyStack & Queue

Recent request count in range...

Path Sum

Check if root-leaf path equals target...

Google, Microsoft

Reverse Bits

EasyBit Manipulation

Reverse bits of integer...

Microsoft, Amazon

Reverse String

EasyTwo Pointers

Reverse character array in-place...

Search Insert Position

EasyBinary Search

Find position to insert target...

O(log n) / O(1)

Google, Facebook

Single Number

EasyBit Manipulation

Find single number in array...

Network Delay Time

Time for all nodes to receive signal...

O(n*log n + e) / O(n)

Sqrt(x)

EasyMath & Geometry

Integer square root...

O(log n) / O(1)

Autocomplete System

Implement autocomplete...

O(n*log n) / O(n)

Binary Tree Maximum Path Sum

Find max sum path in tree...

Remove Nth Node From End of List

MediumTwo Pointers

Remove nth node from linked list...

Longest Repeating Character Replacement

MediumSliding Window

Max length after k character replacements...

Find First and Last Position

MediumBinary Search

Find range of target in sorted array...

O(log n) / O(1)

Google, Microsoft

Time Based Key-Value Store

MediumBinary Search

Query key value at timestamp...

O(log n) / O(n)

Max Consecutive Ones III

MediumSliding Window

Max ones after flipping k zeros...

Facebook, Google

Substring with Concatenation of All Words

HardSliding Window

Find all substrings with concatenated words...

4Sum

MediumTwo Pointers

Find all unique quadruplets...

Google, Facebook

Koko Eating Bananas

MediumBinary Search

Find min eating speed for h hours...

O(n*log m) / O(1)

Google, Facebook

Implement Trie

Implement prefix tree...

Fruit Into Baskets

MediumSliding Window

Max fruit with at most 2 types...

Minimum Size Subarray Sum

MediumSliding Window

Find minimum length subarray with sum >= target...

Graph Valid Tree

Check if graph is valid tree...

Number of Connected Components

Count connected components...

Google, Facebook

Evaluate Reverse Polish Notation

MediumStack & Queue

Evaluate RPN expression...

Largest Rectangle in Histogram

HardStack & Queue

Largest rectangle in histogram...

Google, Microsoft

Simplify Path

MediumStack & Queue

Simplify Unix file path...

Google, Facebook

Word Search

MediumBacktracking

Search word in 2D grid...

O(m*n*4^L) / O(L)

Palindrome Partitioning

MediumBacktracking

Partition string into palindromes...

O(n*2^n) / O(n)

Google, Facebook

Subset Sum

MediumBacktracking

Check if subset sum exists...

Gas Station

Find start gas station...

Bitwise AND of Range

MediumBit Manipulation

Bitwise AND of range...

O(log n) / O(1)

Google, Facebook

Pow(x, n)

MediumMath & Geometry

Calculate x^n...

O(log n) / O(log n)

Fraction to Recurring Decimal

MediumMath & Geometry

Convert fraction to decimal...

Alien Dictionary

Order characters from alien dictionary...

Google, Facebook

Daily Temperatures

MediumStack & Queue

Days until warmer temperature...

Replace Words

Replace words with dictionary root...

O(n*m*k) / O(t)

Google, Facebook

Kth Largest Element

MediumHeap / Priority Queue

Find kth largest element...

O(n*log k) / O(k)

Reorganize String

MediumHeap / Priority Queue

Reorganize string by frequency...

O(n*log n) / O(n)

Meeting Rooms II

MediumIntervals

Minimum conference rooms needed...

O(n*log n) / O(n)

Longest Substring Without Repeating

MediumSliding Window

Find longest substring without repeating...

O(n) / O(min(m, n))

Reverse Linked List

EasyLinked Lists

Reverse singly linked list...

First Bad Version

EasyBinary Search

Find first bad version...

O(log n) / O(1)

Google, Facebook

Candy

Minimum candies for children...

Find Median from Data Stream

HardHeap / Priority Queue

Find median with streaming numbers...

O(log n) / O(n)

Word Search II

Search words in grid using trie...

O(m*n*4^L) / O(t)

Palindrome Linked List

EasyLinked Lists

Check if linked list is palindrome...

Google, Facebook

Valid Parentheses

EasyStack & Queue

Check if parentheses are valid...

Inorder Traversal

Inorder tree traversal...

Maximum Depth of Binary Tree

Find max depth of tree...

Same Tree

Check if two trees are same...

O(min(m,n)) / O(min(h1,h2))

Generate Parentheses

MediumBacktracking

Generate valid parentheses combinations...

O(4^n/√n) / O(n)

Best Time to Buy and Sell Stock

EasySliding Window

Find max profit from one transaction...

Jump Game

MediumDynamic Programming

Can reach last index...

Jump Game II

Minimum jumps to reach end...

Permutation in String

MediumSliding Window

Check if s1 permutation in s2...

Combinations

MediumBacktracking

Generate all k-combinations...

Lowest Common Ancestor of BST

Find LCA in binary search tree...

Binary Tree Right Side View

Get rightmost nodes at each level...

Letter Combinations Phone Number

MediumBacktracking

Letter combinations of phone number...

Validate Binary Search Tree

Check if tree is valid BST...

O(log n) / O(1)

Interval Scheduling

Maximum non-overlapping intervals...

O(n log n) / O(1)

Non-overlapping Intervals

MediumIntervals

Remove minimum intervals for non-overlap...

O(n log n) / O(1)

Google, Facebook

Implement Queue using Stacks

EasyStack & Queue

Queue using two stacks...

Google, Microsoft

Min Stack

MediumStack & Queue

Stack supporting min queries...

Two Sum II Input Array Sorted

EasyTwo Pointers

Find two numbers in sorted array...

O(n log n) / O(n)

Merge Sorted Array

EasyTwo Pointers

Merge two sorted arrays in-place...

O(n log n) / O(n)

Google, Microsoft

Search in Rotated Sorted Array

MediumBinary Search

Find target in rotated array...

O(n log n) / O(n)

Google, Microsoft

Median of Two Sorted Arrays

HardBinary Search

Find median of two sorted arrays...

O(n log n) / O(n)

Google, Microsoft

Serialize and Deserialize Binary Tree

Convert tree to/from string...

Construct Binary Tree from Preorder and Inorder

Build tree from traversals...

Merge Two Sorted Lists

EasyLinked Lists

Merge two sorted linked lists...

O(n log n) / O(n)

Remove Duplicates from Sorted List

EasyLinked Lists

Remove duplicates in sorted list...

O(n log n) / O(n)

Google, Microsoft

Merge k Sorted Lists

HardHeap / Priority Queue

Merge k sorted linked lists...

O(n log n) / O(n)

Insert Interval

Insert interval into sorted list...

O(n log n) / O(1)

Longest Consecutive

MediumArrays & Hashing

Find longest consecutive sequence...

Google, Facebook

Valid Sudoku

MediumArrays & Hashing

Validate sudoku board...

Encode and Decode Strings

MediumArrays & Hashing

Encode/decode list of strings...

House Robber

MediumDynamic Programming

Maximum sum without adjacent elements...

Coin Change II

MediumDynamic Programming

Number of ways to make amount...

Spiral Matrix

MediumArrays & Hashing

Traverse matrix in spiral order...

Copy List with Random Pointer

MediumLinked Lists

Deep copy list with random pointers...

Unique Paths

MediumDynamic Programming

Number of paths in grid...

Decode Ways

MediumDynamic Programming

Number of ways to decode string...

Maximum Subarray

MediumDynamic Programming

Find max sum contiguous subarray...

Burst Balloons

HardDynamic Programming

Maximum coins from bursting balloons...

O(n³) / O(n²)

Clone Graph

Deep copy undirected graph...

Cheapest Flights Within K Stops

Shortest path with at most k stops...

Surrounded Regions

Flip surrounded regions...

O(m*n) / O(m*n)

Google, Facebook

Set Matrix Zeroes

MediumArrays & Hashing

Set entire row/col to 0 if element is 0...

Google, Microsoft

3Sum

MediumTwo Pointers

Find all unique triplets summing to 0...

Google, Facebook

Merge Intervals

MediumIntervals

Given an array of intervals where intervals[i] = [starti, endi], merge all overlapping intervals, and return an array of the non-overlapping intervals...

O(n log n) / O(1)

Word Ladder

A transformation sequence from beginWord to endWord using dictionary wordList is a sequence where every adjacent pair of words differs by a single let...

O(M² * N) / O(M * N)

Number of Islands

Given an m x n 2D binary grid grid which represents a map of '1's (land) and '0's (water), return the number of islands. An island is surrounded by wa...

O(V + E) / O(V)

LRU Cache

MediumStack & Queue

Design a data structure that follows the constraints of a Least Recently Used (LRU) cache. Implement the LRUCache class with get and put operations in...

O(1) / O(capacity)

Course Schedule

There are a total of numCourses courses you have to take. Some courses may have prerequisites. Given the total number of courses and a list of prerequ...

O(V + E) / O(V + E)

N-Queens

HardBacktracking

The n-queens puzzle is the problem of placing n queens on an n x n chessboard such that no two queens attack each other. Given an integer n, return al...

Permutations

MediumBacktracking

Given an array nums of distinct integers, return all the possible permutations. You can return the answer in any order....

Subsets

MediumBacktracking

Given an integer array nums of unique elements, return all possible subsets (the power set). The solution set must not contain duplicate subsets....

O(n * 2^n) / O(n)

Binary Tree Maximum Path Sum

A path in a binary tree is a sequence of nodes where each pair of adjacent nodes in the sequence has an edge connecting them. The path sum of a path i...

Search in Rotated Sorted Array

MediumBinary Search

There is an integer array nums sorted in ascending order (with distinct values). Prior to being passed to your function, nums is possibly rotated at a...

O(n log n) / O(n)

Lowest Common Ancestor of BST

Given a binary search tree (BST), find the lowest common ancestor (LCA) node of two given nodes in the BST....

Edit Distance

MediumDynamic Programming

Given two strings word1 and word2, return the minimum number of operations required to convert word1 to word2. You have the following three operations...

Climbing Stairs

EasyDynamic Programming

You are climbing a staircase. It takes n steps to reach the top. Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb ...

Coin Change

MediumDynamic Programming

You are given an integer array coins representing coins of different denominations and an integer amount representing a total amount of money. Return ...

O(amount * n) / O(amount)

Word Break

MediumDynamic Programming

Given a string s and a dictionary of strings wordDict, return true if s can be segmented into a space-separated sequence of one or more dictionary wor...

O(n² * m) / O(n)

Longest Increasing Subsequence

MediumDynamic Programming

Given an integer array nums, return the length of the longest strictly increasing subsequence....

O(n log n) / O(n)

Invert Binary Tree

Given the root of a binary tree, invert the tree, and return its root....

Binary Search

EasyBinary Search

Given an array of integers nums which is sorted in ascending order, and an integer target, write a function to search target in nums. If target exists...

O(log n) / O(1)

Longest Substring Without Repeating Characters

MediumSliding Window

Given a string s, find the length of the longest substring without repeating characters....

O(n) / O(min(m, n))

Valid Palindrome

EasyTwo Pointers

A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the ...

3Sum

MediumTwo Pointers

Given an integer array nums, return all the triplets [nums[i], nums[j], nums[k]] such that i != j, i != k, and j != k, and nums[i] + nums[j] + nums[k]...

Container With Most Water

MediumTwo Pointers

You are given an integer array height of length n. There are n vertical lines drawn such that the two endpoints of the ith line are (i, 0) and (i, hei...

Minimum Window Substring

HardSliding Window

Given two strings s and t of lengths m and n respectively, return the minimum window substring of s such that every character in t (including duplicat...

O(m + n) / O(m + n)

Trapping Rain Water

HardTwo Pointers

Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it can trap after raining....

Sliding Window Maximum

HardSliding Window

You are given an array of integers nums, there is a sliding window of size k which is moving from the very left of the array to the very right. You ca...

O(n) / O(min(m, n))

Group Anagrams

MediumArrays & Hashing

Given an array of strings strs, group the anagrams together. You can return the answer in any order....

Contains Duplicate

EasyArrays & Hashing

Given an integer array nums, return true if any value appears at least twice in the array, and return false if every element is distinct....

Valid Anagram

EasyArrays & Hashing

Given two strings s and t, return true if t is an anagram of s, and false otherwise. An Anagram is a word or phrase formed by rearranging the letters ...

Top K Frequent Elements

MediumArrays & Hashing

Given an integer array nums and an integer k, return the k most frequent elements. You may return the answer in any order....

Two Sum

EasyArrays & Hashing

Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target. You may assume that each inp...

Longest Consecutive Sequence

MediumArrays & Hashing

Given an unsorted array of integers nums, return the length of the longest consecutive elements sequence. You must write an algorithm that runs in O(n...

Product of Array Except Self

MediumArrays & Hashing

Given an integer array nums, return an array answer such that answer[i] is equal to the product of all the elements of nums except nums[i]. You must w...