Problems

100 problems

• Easy

  Reverse Integer

  Given a signed 32-bit integer x, return x with its digits reversed. …
• Easy

  Roman to Integer

  Roman numerals are represented by seven different symbols: I, V, X, L, …
• Easy

  FizzBuzz

  Given an integer n, print numbers from 1 to n, but: - …
• Easy

  Longest Common Prefix

  Write a function to find the longest common prefix string amongst an …
• Easy

  Reverse String

  Write a function that reverses a string. The input string is given …
• Easy

  Valid Parentheses

  Given a string s containing just the characters '(', ')', '{', '}', …
• Easy

  Palindrome Number

  Given an integer x, return true if x is a palindrome, and …
• Easy

  Binary Search

  Given an array of integers nums which is sorted in ascending order, …
• Easy

  Fibonacci Number

  Given an integer n, output the nth Fibonacci number. The Fibonacci sequence …
• Easy

  Prime Check

  Given an integer n, determine if it is a prime number. Output …
• Easy

  Maximum Average Subarray I

  You are given an integer array nums consisting of n elements, and …

  Sliding Window
• Easy

  Single Number

  Given a non-empty array of integers nums, every element appears twice except …

  Bit Manipulation
• Easy

  Number of 1 Bits

  Write a function that takes the binary representation of a positive integer …

  Bit Manipulation
• Easy

  Counting Bits

  Given an integer n, return an array ans of length n + …

  Bit Manipulation
• Easy

  Climbing Stairs

  You are climbing a staircase. It takes n steps to reach the …

  Math
• Easy

  Merge Sorted Array

  You are given two integer arrays nums1 and nums2, sorted in non-decreasing …

  Sorting
• Easy

  Reverse Words

  Given a sentence (a string of words separated by single spaces), reverse …
• Easy

  Pascal Triangle Row

  Given a row number n (0-indexed), return the nth row of Pascal's …
• Easy

  GCD of Two Numbers

  Given two positive integers a and b, find their Greatest Common Divisor …

  Math
• Easy

  Two Sum

  Given an array of integers nums and an integer target, return indices …

  Arrays Hashing
• Easy

  Valid Anagram

  Given two strings s and t, return true if t is an …

  Hashing Strings
• Easy

  Best Time to Buy and Sell Stock

  You are given an array prices where prices[i] is the price of …

  Arrays Dynamic Programming
• Easy

  Contains Duplicate

  Given an integer array nums, return true if any value appears at …

  Arrays Hashing
• Easy

  Move Zeroes

  Given an integer array nums, move all 0's to the end of …

  Arrays Two Pointers
• Easy

  Valid Palindrome

  Given a string s, return true if it is a palindrome, or …

  Strings Two Pointers
• Easy

  Plus One

  Given digits representing a non-negative integer, return the integer plus one.

  Arrays Math
• Easy

  Missing Number

  Given distinct numbers in [0, n], return the missing number.

  Arrays Math
• Easy

  First Unique Character in a String

  Return index of first non-repeating character, else -1.

  Hashing Strings
• Easy

  Valid Mountain Array

  Return true if arr strictly increases then strictly decreases.

  Arrays Two Pointers
• Easy

  Squares of a Sorted Array

  Return sorted squares of each number.

  Arrays Two Pointers
• Easy

  Running Sum of 1D Array

  Return running sum of nums.

  Arrays Prefix Sum
• Easy

  Richest Customer Wealth

  Return max row sum in accounts matrix.

  Arrays Matrix
• Easy

  Pascal's Triangle II

  Return the rowIndex-th row of Pascal's triangle.

  Arrays Dynamic Programming
• Easy

  Transpose Matrix

  Return the transpose of matrix.

  Matrix
• Easy

  Find Pivot Index

  Return index where left sum equals right sum, else -1.

  Arrays Prefix Sum
• Easy

  Intersection of Two Arrays

  Return unique intersection in sorted order.

  Arrays Hashing
• Easy

  Happy Number

  Determine if n is a happy number.

  Hashing Math
• Easy

  Majority Element

  Return the element that appears more than n/2 times.

  Arrays Greedy
• Easy

  Valid Perfect Square

  Return true if num is a perfect square.

  Binary Search Math
• Easy

  Monotonic Array

  Return true if array is monotone increasing or decreasing.

  Arrays
• Easy

  Min Cost Climbing Stairs

  Return minimum cost to reach the top.

  Dynamic Programming
• Easy

  Reshape the Matrix

  Reshape matrix to r x c if possible, else return original.

  Arrays Matrix
• Easy

  Kth Missing Positive Number

  Return the k-th missing positive integer.

  Arrays Binary Search
• Easy

  Intersection of Two Arrays II

  Return intersection with multiplicity in sorted order.

  Arrays Hashing
• Medium

  Maximum Subarray

  Given an integer array nums, find the subarray with the largest sum, …
• Medium

  Container With Most Water

  You are given an integer array height of length n. There are …
• Medium

  Longest Substring Without Repeating Characters

  Given a string s, find the length of the longest substring without …
• Medium

  Group Anagrams

  Given an array of strings strs, group the anagrams together. You can …
• Medium

  Product of Array Except Self

  Given an integer array nums, return an array answer such that answer[i] …
• Medium

  Rotate Image

  You are given an n x n 2D matrix representing an image, …

  Matrix
• Medium

  Spiral Matrix

  Given an m x n matrix, return all elements of the matrix …

  Matrix Simulation
• Medium

  Set Matrix Zeroes

  Given an m x n integer matrix, if an element is 0, …

  Matrix
• Medium

  Min Stack

  Design a stack that supports push, pop, top, and retrieving the minimum …

  Stack
• Medium

  Daily Temperatures

  Given an array of integers temperatures represents the daily temperatures, return an …

  Stack
• Medium

  Minimum Size Subarray Sum

  Given an array of positive integers nums and a positive integer target, …

  Sliding Window
• Medium

  Jump Game

  You are given an integer array nums. You are initially positioned at …

  Greedy
• Medium

  Best Time to Buy and Sell Stock II

  You are given an integer array prices where prices[i] is the price …

  Greedy
• Medium

  Subsets

  Given an integer array nums of unique elements, return all possible subsets …

  Backtracking Bit Manipulation
• Medium

  Permutations

  Given an array nums of distinct integers, return all the possible permutations. …

  Backtracking
• Medium

  Combination Sum

  Given an array of distinct integers candidates and a target integer target, …

  Backtracking
• Medium

  House Robber

  You are a professional robber planning to rob houses along a street. …
• Medium

  Coin Change

  You are given an integer array coins representing coins of different denominations …
• Medium

  Unique Paths

  There is a robot on an m x n grid. The robot …

  Math
• Medium

  Sort Colors

  Given an array nums with n objects colored red, white, or blue, …

  Sorting
• Medium

  Kth Largest Element in an Array

  Given an integer array nums and an integer k, return the k-th …

  Arrays Divide and Conquer Sorting
• Medium

  Valid Sudoku

  Check if a partially filled sudoku board is valid.

  Hashing Matrix
• Medium

  Top K Frequent Elements

  Return top k frequent numbers sorted ascending by value in ties.

  Arrays Hashing Heap
• Medium

  Subarray Sum Equals K

  Count contiguous subarrays that sum to k.

  Arrays Hashing Prefix Sum
• Medium

  Longest Palindromic Substring Length

  Return the length of the longest palindromic substring.

  Dynamic Programming Strings
• Medium

  House Robber II

  Houses are in a circle. Return max amount you can rob.

  Dynamic Programming
• Medium

  Number of Islands

  Count connected components of '1' in a grid.

  BFS DFS Graphs
• Medium

  Course Schedule

  Return true if all courses can be finished.

  Graphs Topological Sort
• Medium

  Search in Rotated Sorted Array

  Return index of target in rotated sorted array, else -1.

  Arrays Binary Search
• Medium

  Find Minimum in Rotated Sorted Array

  Return minimum element in rotated sorted array.

  Arrays Binary Search
• Medium

  Target Sum Ways

  Assign + or - to each number. Return number of ways to …

  Backtracking Dynamic Programming
• Medium

  Longest Consecutive Sequence

  Return length of the longest consecutive elements sequence.

  Arrays Hashing
• Medium

  Partition Labels

  Partition string so each letter appears in at most one part; return …

  Greedy Strings
• Medium

  Decode String

  Decode strings encoded as k[encoded_string].

  Stack Strings
• Medium

  Minimum Path Sum

  Move only right or down; return min path sum to bottom-right.

  Dynamic Programming Matrix
• Medium

  Gas Station

  Return starting station index to complete circuit, else -1.

  Arrays Greedy
• Medium

  Merge Intervals

  Merge all overlapping intervals.

  Intervals Sorting
• Medium

  Insert Interval

  Insert a new interval and merge if necessary.

  Arrays Intervals
• Medium

  Non-overlapping Intervals

  Return minimum intervals to remove to eliminate overlaps.

  Greedy Intervals
• Medium

  Find K Closest Elements

  Return k closest elements to x in ascending order.

  Arrays Binary Search Two Pointers
• Medium

  Task Scheduler

  Return least intervals needed to execute tasks with cooldown n.

  Greedy Heap
• Medium

  Word Break

  Return true if s can be segmented into dictionary words.

  Dynamic Programming Strings
• Medium

  Decode Ways

  Return number of ways to decode the string.

  Dynamic Programming Strings
• Medium

  Unique Binary Search Trees

  Return number of structurally unique BSTs that store values 1..n.

  Dynamic Programming Math
• Medium

  Asteroid Collision

  Simulate asteroid collisions and return final state.

  Simulation Stack
• Medium

  Remove K Digits

  Remove k digits to make the smallest possible number string.

  Greedy Stack
• Hard

  Merge k Sorted Lists

  You are given an array of k linked-lists lists, each linked-list is …
• Hard

  Trapping Rain Water

  Given n non-negative integers representing an elevation map where the width of …
• Hard

  Longest Valid Parentheses

  Given a string containing just the characters '(' and ')', return the …
• Hard

  Median of Two Sorted Arrays

  Return median of two sorted arrays.

  Arrays Binary Search
• Hard

  Minimum Window Substring

  Return the minimum window in s that contains all chars of t.

  Sliding Window Strings
• Hard

  Largest Rectangle in Histogram

  Return area of largest rectangle in histogram.

  Monotonic Stack Stack
• Hard

  Edit Distance

  Return minimum number of operations to convert word1 to word2.

  Dynamic Programming Strings
• Hard

  Sliding Window Maximum

  Return max value in each sliding window of size k.

  Arrays Deque Sliding Window
• Hard

  Distinct Subsequences

  Return number of distinct subsequences of s that equal t.

  Dynamic Programming Strings
• Hard

  Regular Expression Matching

  Implement regex matching with support for '.' and '*'.

  Dynamic Programming Strings