Medium

Problem	Pattern	Lists	Description
3Sum	Array, Two Pointers, Sorting	Blind 75, Grind 75, Grind 169, NeetCode 150, Google Top 50	Given an integer array nums, return all the triplets [nums[i], nums[j], nums[k]] such that i != j, i != k, and j != k,
Add Two Numbers	Linked List, Math, Recursion	Grind 169, NeetCode 150, Google Top 50	You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse
Binary Tree Level Order Traversal	Tree, Breadth-First Search, Binary Tree	Blind 75, Grind 75, Grind 169, NeetCode 150	Given the root of a binary tree, return the level order traversal of its nodes’ values. (i.e., from left to right,
Binary Tree Right Side View	Tree, Depth-First Search, Breadth-First Search, Binary Tree	Grind 75, Grind 169, NeetCode 150	Given the root of a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you
Binary Tree Zigzag Level Order Traversal	Tree, Breadth-First Search, Binary Tree	Grind 169	Given the root of a binary tree, return the zigzag level order traversal of its nodes’ values. (i.e., from left to
Bitwise And Of Numbers Range	Bit Manipulation		Given two integers left and right that represent the range [left, right], return the bitwise AND of all numbers in
Car Fleet	Array, Stack, Sorting, Monotonic Stack	NeetCode 150	There are n cars at given miles away from the starting mile 0, traveling to reach the mile target. You are given two
Cheapest Flights Within K Stops	Dynamic Programming, Depth-First Search, Breadth-First Search, Graph Theory, Heap (Priority Queue)	Grind 169, NeetCode 150	There are n cities connected by some number of flights. You are given an array flights where flights[i] = [from
Clone Graph	Hash Table, Depth-First Search, Breadth-First Search, Graph Theory	Blind 75, Grind 75, Grind 169, NeetCode 150	Given a reference of a node in a connected
Combination Sum	Array, Backtracking	Blind 75, Grind 75, Grind 169, NeetCode 150	Given an array of distinct integers candidates and a target integer target, return a list of all unique combinations of
Combination Sum II	Array, Backtracking	NeetCode 150	Given a collection of candidate numbers (candidates) and a target number (target), find all unique combinations in
Construct Binary Tree from Preorder and Inorder Traversal	Array, Hash Table, Divide and Conquer, Tree, Binary Tree	Blind 75, Grind 75, Grind 169, NeetCode 150	Given two integer arrays preorder and inorder where preorder is the preorder traversal of a binary tree and inorder is
Container With Most Water	Array, Two Pointers, Greedy	Blind 75, Grind 75, Grind 169, NeetCode 150	You are given an integer array height of length n. There are n vertical lines drawn such that the two endpoints of the
Copy List With Random Pointer	Hash Table, Linked List	NeetCode 150, Amazon Top 50	A linked list of length n is given such that each node contains an additional random pointer, which could point to any
Count And Say	String		The count-and-say sequence is a sequence of digit strings defined by the recursive formula: - countAndSay(1) = “1” -
Count Good Nodes in Binary Tree	Tree, Depth-First Search, Breadth-First Search, Binary Tree	NeetCode 150	Given a binary tree root, a node X in the tree is named good if in the path from root to X there are no nodes with a
Count Primes	Array, Math, Enumeration, Number Theory		Given an integer n, return the number of prime numbers that are strictly less than n
Course Schedule	Depth-First Search, Breadth-First Search, Graph Theory, Topological Sort	Blind 75, Grind 75, Grind 169, NeetCode 150, Amazon Top 50	There are a total of numCourses courses you have to take, labeled from 0 to numCourses - 1. You are given an array
Course Schedule II	Depth-First Search, Breadth-First Search, Graph Theory, Topological Sort	Grind 169, NeetCode 150, Google Top 50	There are a total of numCourses courses you have to take, labeled from 0 to numCourses - 1. You are given an array
Daily Temperatures	Array, Stack, Monotonic Stack	Grind 169, NeetCode 150	Given an array of integers temperatures represents the daily temperatures, return an array answer such that answer[i]
Decode Ways	String, Dynamic Programming	Blind 75, Grind 169, NeetCode 150	You have intercepted a secret message encoded as a string of numbers. The message is decoded via the following mapping:
Delete Node In A Linked List	Linked List		There is a singly-linked list head and we want to delete a node node in it. You are given the node to be deleted node
Design Add and Search Words	String, Depth-First Search, Design, Trie	Blind 75, Grind 169, NeetCode 150	Design a data structure that supports adding new words and finding if a string matches any previously added string
Divide Two Integers	Math, Bit Manipulation		Given two integers dividend and divisor, divide two integers without using multiplication, division, and mod operator
Edit Distance	String, Dynamic Programming	NeetCode 150	Given two strings word1 and word2, return the minimum number of operations required to convert word1 to word2. You
Encode and Decode Strings	Array, String, Design	Blind 75, Grind 169, NeetCode 150
Evaluate Reverse Polish Notation	Array, Math, Stack	Grind 75, Grind 169, NeetCode 150, Google Top 50	You are given an array of strings tokens that represents an arithmetic expression in a Reverse Polish Notation
Find First And Last Position Of Element In Sorted Array	Array, Binary Search		Given an array of integers nums sorted in non-decreasing order, find the starting and ending position of a given target
Find Minimum in Rotated Sorted Array	Array, Binary Search	Blind 75, Grind 169, NeetCode 150	Suppose an array of length n sorted in ascending order is rotated between 1 and n times. For example, the array nums =
Find the Duplicate Number	Array, Two Pointers, Binary Search, Bit Manipulation	Grind 169, NeetCode 150	Given an array of integers nums containing n + 1 integers where each integer is in the range [1, n] inclusive. There is
Game Of Life	Array, Matrix, Simulation		According to Wikipedia’s article <https://en.wikipedia.org/wiki/Conway%27s_Game_of_Life>__: “The Game of Life, also
Gas Station	Array, Greedy	Grind 169, NeetCode 150	There are n gas stations along a circular route, where the amount of gas at the i :sup:th station is gas[i]. You
Generate Parentheses	String, Dynamic Programming, Backtracking	Grind 169, NeetCode 150	Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses
Group Anagrams	Array, Hash Table, String, Sorting	Blind 75, Grind 169, NeetCode 150, Amazon Top 50	Given an array of strings strs, group the anagrams together. You can return the answer in any order
H-Index	Array, Sorting, Counting Sort	Google Top 50	Given an array of integers citations where citations[i] is the number of citations a researcher received for their i
House Robber	Array, Dynamic Programming	Blind 75, Grind 169, NeetCode 150	You are a professional robber planning to rob houses along a street. Each house has a certain amount of money stashed,
House Robber II	Array, Dynamic Programming	Blind 75, NeetCode 150, Google Top 50	You are a professional robber planning to rob houses along a street. Each house has a certain amount of money stashed
Implement Trie (Prefix Tree)	Hash Table, String, Design, Trie	Blind 75, Grind 75, Grind 169, NeetCode 150, Google Top 50	A trie <https://en.wikipedia.org/wiki/Trie>__ (pronounced as “try”) or prefix tree is a tree data structure used to
Insert Delete GetRandom O(1)	Array, Hash Table, Math, Design, Randomized	Grind 169	Implement the RandomizedSet class: - RandomizedSet() Initializes the RandomizedSet object. - bool insert(int val)
Integer to Roman	Hash Table, Math, String		Seven different symbols represent Roman numerals with the following values: ====== ===== Symbol Value ====== ===== I
Jump Game	Array, Dynamic Programming, Greedy	Blind 75, Grind 169, NeetCode 150	You are given an integer array nums. You are initially positioned at the array’s first index, and each element in the
Koko Eating Bananas	Array, Binary Search	NeetCode 150	Koko loves to eat bananas. There are n piles of bananas, the i :sup:th pile has piles[i] bananas. The guards have
Kth Smallest Element in a BST	Tree, Depth-First Search, Binary Search Tree, Binary Tree	Blind 75, Grind 75, Grind 169, NeetCode 150	Given the root of a binary search tree, and an integer k, return the k :sup:th smallest value ( 1-indexed ) of
LRU Cache	Hash Table, Linked List, Design, Doubly-Linked List	Grind 75, Grind 169, NeetCode 150, Amazon Top 50, Google Top 50	Design a data structure that follows the constraints of a Least Recently Used (LRU) cache
Letter Combinations of a Phone Number	Hash Table, String, Backtracking	Grind 75, Grind 169, NeetCode 150	Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could
Longest Common Subsequence	String, Dynamic Programming	Blind 75, NeetCode 150	Given two strings text1 and text2, return the length of their longest common subsequence . If there is no common
Longest Consecutive Sequence	Array, Hash Table, Union-Find	Blind 75, Grind 169, NeetCode 150	Given an unsorted array of integers nums, return the length of the longest consecutive elements sequence. You must
Longest Palindromic Substring	Two Pointers, String, Dynamic Programming	Blind 75, Grind 75, Grind 169, NeetCode 150, Google Top 50	Given a string s, return the longest palindromic substring in s
Longest Substring Without Repeating Characters	Hash Table, String, Sliding Window	Blind 75, Grind 75, Grind 169, NeetCode 150, Google Top 50	Given a string s, find the length of the longest substring without duplicate characters
Lowest Common Ancestor of a BST	Tree, Depth-First Search, Binary Search Tree, Binary Tree	Blind 75, Grind 75, Grind 169, NeetCode 150	Given a binary search tree (BST), find the lowest common ancestor (LCA) node of two given nodes in the BST. According
Max Area of Island	Array, Depth-First Search, Breadth-First Search, Union-Find, Matrix	NeetCode 150, Google Top 50	You are given an m x n binary matrix grid. An island is a group of 1’s (representing land) connected 4-directionally
Maximum Product Subarray	Array, Dynamic Programming	Blind 75, Grind 169, NeetCode 150	Given an integer array nums, find a subarray that has the largest product, and return the product. The test cases are
Maximum Subarray	Array, Divide and Conquer, Dynamic Programming	Blind 75, Grind 75, Grind 169, NeetCode 150, Google Top 50	Given an integer array nums, find the subarray with the largest sum, and return its sum
Merge Intervals	Array, Sorting	Blind 75, Grind 75, Grind 169, NeetCode 150, Google Top 50	Given an array of intervals where intervals[i] = [start :sub:i , end :sub:i ], merge all overlapping intervals,
Min Cost to Connect All Points	Array, Union-Find, Graph Theory, Minimum Spanning Tree	NeetCode 150	You are given an array points representing integer coordinates of some points on a 2D-plane, where points[i] = [x
Min Stack	Stack, Design	Grind 75, Grind 169, NeetCode 150	Design a stack that supports push, pop, top, and retrieving the minimum element in constant time. Implement the
Network Delay Time	Depth-First Search, Breadth-First Search, Graph Theory, Heap (Priority Queue), Shortest Path	NeetCode 150	You are given a network of n nodes, labeled from 1 to n. You are also given times, a list of travel times as directed
Number of Islands	Array, Depth-First Search, Breadth-First Search, Union-Find, Matrix	Blind 75, Grind 75, Grind 169, NeetCode 150, Amazon Top 50, Google Top 50	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
Palindrome Partitioning	String, Dynamic Programming, Backtracking	NeetCode 150	Given a string s, partition s such that every substring of the partition is a palindrome. Return all possible
Palindromic Substrings	Two Pointers, String, Dynamic Programming	Blind 75, NeetCode 150	Given a string s, return the number of palindromic substrings in it. A string is a palindrome when it reads the same
Partition Equal Subset Sum	Array, Dynamic Programming	Grind 75, Grind 169, NeetCode 150	Given an integer array nums, return true if you can partition the array into two subsets such that the sum of the
Permutation in String	Hash Table, Two Pointers, String, Sliding Window	NeetCode 150	Given two strings s1 and s2, return true if s2 contains a permutation of s1, or false otherwise. In other words, return
Permutations	Array, Backtracking	Grind 75, Grind 169, NeetCode 150	Given an array nums of distinct integers, return all the possible permutations. You can return the answer in any order
Pow(x, n)	Math, Recursion	Grind 169, NeetCode 150	Implement pow(x, n) <http://www.cplusplus.com/reference/valarray/pow/>__, which calculates x raised to the power n
Product of Array Except Self	Array, Prefix Sum	Blind 75, Grind 75, Grind 169, NeetCode 150	Given an integer array nums, return an array answer such that answer[i] is equal to the product of all the elements
Remove Duplicates From Sorted Array Ii	Array, Two Pointers		Given an integer array nums sorted in non-decreasing order, remove some duplicates in-place
Remove Nth Node From End of List	Linked List, Two Pointers	Blind 75, Grind 169, NeetCode 150	Given the head of a linked list, remove the n :sup:th node from the end of the list and return its head
Reorder List	Linked List, Two Pointers, Stack, Recursion	Blind 75, Grind 169, NeetCode 150	You are given the head of a singly linked-list. The list can be represented as: :: L0 → L1 → … → Ln - 1 → Ln Reorder
Reverse Integer	Math	Grind 169, NeetCode 150	Given a signed 32-bit integer x, return x with its digits reversed. If reversing x causes the value to go outside the
Reverse Words In A String	Two Pointers, String		Given an input string s, reverse the order of the words. A word is defined as a sequence of non-space characters. The
Rotate Image	Array, Math, Matrix	Blind 75, Grind 169, NeetCode 150	You are given an n x n 2D matrix representing an image, rotate the image by 90 degrees (clockwise). You have to rotate
Rotting Oranges	Array, Breadth-First Search, Matrix	Grind 75, Grind 169, NeetCode 150, Amazon Top 50	You are given an m x n grid where each cell can have one of three values: - 0 representing an empty cell, - 1
Search a 2D Matrix	Array, Binary Search, Matrix	Grind 169, NeetCode 150	You are given an m x n integer matrix matrix with the following two properties: - Each row is sorted in non-decreasing
Search in Rotated Sorted Array	Array, Binary Search	Blind 75, Grind 75, Grind 169, NeetCode 150	There is an integer array nums sorted in ascending order (with distinct values). Prior to being passed to your
Set Matrix Zeroes	Array, Hash Table, Matrix	Blind 75, Grind 169, NeetCode 150	Given an m x n integer matrix matrix, if an element is 0, set its entire row and column to 0’s. You must do it in
Single Number Ii	Array, Bit Manipulation		Given an integer array nums where every element appears three times except for one, which appears exactly once. Find
Sort List	Linked List, Two Pointers, Divide and Conquer, Sorting, Merge Sort	Grind 169	Given the head of a linked list, return the list after sorting it in ascending order
String to Integer (atoi)	String	Grind 75, Grind 169	Implement the myAtoi(string s) function, which converts a string to a 32-bit signed integer. The algorithm for
Subsets	Array, Backtracking, Bit Manipulation	Grind 75, Grind 169, NeetCode 150	Given an integer array nums of unique elements, return all possible subsets (the power set). The solution set must
Subsets II	Array, Backtracking, Bit Manipulation	NeetCode 150	Given an integer array nums that may contain duplicates, return all possible subsets (the power set). The solution
Target Sum	Array, Dynamic Programming, Backtracking	NeetCode 150	You are given an integer array nums and an integer target. You want to build an expression out of nums by adding one of
Time Based Key-Value Store	Hash Table, String, Binary Search, Design	Grind 75, Grind 169, NeetCode 150	Design a time-based key-value data structure that can store multiple values for the same key at different time stamps
Top K Frequent Elements	Array, Hash Table, Divide and Conquer, Sorting, Heap (Priority Queue)	Blind 75, NeetCode 150, Google Top 50	Given an integer array nums and an integer k, return the k most frequent elements. You may return the answer in any
Tries	Trie
Two Sum II	Array, Two Pointers, Binary Search	NeetCode 150	Given a 1-indexed array of integers numbers that is already sorted in non-decreasing order, find two numbers such that
Unique Paths	Math, Dynamic Programming, Combinatorics	Blind 75, Grind 75, Grind 169, NeetCode 150, Google Top 50	There is a robot on an m x n grid. The robot is initially located at the top-left corner (i.e., grid[0][0]). The robot
Valid Sudoku	Array, Hash Table, Matrix	Grind 169, NeetCode 150	Determine if a 9 x 9 Sudoku board is valid. Only the filled cells need to be validated according to the following
Validate Binary Search Tree	Tree, Depth-First Search, Binary Search Tree, Binary Tree	Blind 75, Grind 75, Grind 169, NeetCode 150	Given the root of a binary tree, determine if it is a valid binary search tree (BST). A valid BST is defined as
Walls and Gates	Array, Breadth-First Search, Matrix	NeetCode 150
Word Break	Array, Hash Table, String, Dynamic Programming, Trie	Blind 75, Grind 75, Grind 169, NeetCode 150	Given a string s and a dictionary of strings wordDict, return true if s can be segmented into a space-separated
Word Search	Array, String, Backtracking, Depth-First Search, Matrix	Blind 75, Grind 75, Grind 169, NeetCode 150	Given an m x n grid of characters board and a string word, return true if word exists in the grid. The word can be
Zigzag Conversion	String		The string “PAYPALISHIRING” is written in a zigzag pattern on a given number of rows like this: (you may want to