Arrays & Hashing

Return true if any value appears twice in the array.

Given an integer array nums sorted in non-decreasing order, remove the duplicates [in-place](https://en.wikipedia.org/wiki/In-place_algorithm) such that each unique element appears only **on

Given an integer array nums and an integer val, remove all occurrences of val in nums [in-place](https://en.wikipedia.org/wiki/In-place_algorithm). The order of the elements may be changed

You are given an m x n integer array grid. There is a robot initially located at the top-left corner (i.e., grid[0][0]). The robot tries to move to the bottom-right corner (i.e., `grid[m

Given an array nums with n objects colored red, white, or blue, sort them [in-place](https://en.wikipedia.org/wiki/In-place_algorithm) so that objects of the same color are adjacent, with the

Given an integer array nums sorted in non-decreasing order, remove some duplicates [in-place](https://en.wikipedia.org/wiki/In-place_algorithm) such that each unique element appears **at mos

Given two integer arrays inorder and postorder where inorder is the inorder traversal of a binary tree and postorder is the postorder traversal of the same tree, construct and return _the bina

Given an array of positive integers nums and a positive integer target, return _the minimal length of a_ _subarray_ _whose sum is greater than or equal to_ target. If there is no such subarr

A city's skyline is the outer contour of the silhouette formed by all the buildings in that city when viewed from a distance. Given the locations and heights of all the buildings, return _the **sk

Given a string expression of numbers and operators, return _all possible results from computing all the different possible ways to group numbers and operators_. You may return the answer in **any or

Given an array of n integers nums and an integer target, find the number of index triplets i, j, k with 0 <= i < j < k < n that satisfy the condition `nums[i] + nums[j] + nums[k] < targe

Given an integer array nums, in which exactly two elements appear only once and all the other elements appear exactly twice. Find the two elements that appear only once. You can return the answer in

Given an array of integers citations where citations[i] is the number of citations a researcher received for their ith paper and citations is sorted in ascending order, return _the researc

You are a product manager and currently leading a team to develop a new product. Unfortunately, the latest version of your product fails the quality check. Since each version is developed based on the

Given a string num that contains only digits and an integer target, return _all possibilities to insert the binary operators_ '+'_,_ '-'_, and/or_ '*' _between the digits of_ num _so t

The abbreviation of a word is a concatenation of its first letter, the number of characters between the first and last letter, and its last letter. If a word has only two characters, then it is an

According to [Wikipedia's article](https://en.wikipedia.org/wiki/Conway%27s_Game_of_Life): "The Game of Life, also known simply as Life, is a cellular automaton devised by the British mathemat

You are given an empty 2D binary grid grid of size m x n. The grid represents a map where 0's represent water and 1's represent land. Initially, all the cells of grid are water cells (i.e.,

One way to serialize a binary tree is to use preorder traversal. When we encounter a non-null node, we record the node's value. If it is a null node, we record using a sentinel value such as '#'

Given an integer array nums, return true _if there exists a triple of indices_ (i, j, k) _such that_ i < j < k _and_ nums[i] < nums[j] < nums[k]. If no such indices exists, return false.

Given two integer arrays nums1 and nums2, return _an array of their intersection_. Each element in the result must appear as many times as it shows in both arrays and you may return the result in

Android devices have a special lock screen with a 3 x 3 grid of dots. Users can set an "unlock pattern " by connecting the dots in a specific sequence, forming a series of joined line segments where

Design a [Snake game](https://en.wikipedia.org/wiki/Snake_(video_game)) that is played on a device with screen size height x width. [Play the game online](http://patorjk.com/games/snake/) if you are

Given a string s and an integer k, rearrange s such that the same characters are at least distance k from each other. If it is not possible to rearrange the string, return an empty string

Design a logger system that receives a stream of messages along with their timestamps. Each unique message should only be printed at most every 10 seconds (i.e. a message printed at timestamp

Given a sorted integer array nums and three integers a, b and c, apply a quadratic function of the form f(x) = ax2 + bx + c to each element nums[i] in the array, and return _the array

You are given two jugs with capacities jug1Capacity and jug2Capacity liters. There is an infinite amount of water supply available. Determine whether it is possible to measure exactly `targetCapac

A wiggle sequence is a sequence where the differences between successive numbers strictly alternate between positive and negative. The first difference (if one exists) may be either positive or ne

Given an array rectangles where rectangles[i] = [xi, yi, ai, bi] represents an axis-aligned rectangle. The bottom-left point of the rectangle is (xi, yi) and the top-right point of it is `(ai, b

You are given an array of variable pairs equations and an array of real numbers values, where equations[i] = [Ai, Bi] and values[i] represent the equation Ai / Bi = values[i]. Each Ai or `

A binary watch has 4 LEDs on the top to represent the hours (0-11), and 6 LEDs on the bottom to represent the minutes (0-59). Each LED represents a zero or one, with the least significant bit on the r

You are given an array of people, people, which are the attributes of some people in a queue (not necessarily in order). Each people[i] = [hi, ki] represents the ith person of height hi with *

A string can be abbreviated by replacing any number of non-adjacent substrings with their lengths. For example, a string such as "substitution " could be abbreviated as (but not limited to):

Given an array of unique strings words, return _all the_ [word squares](https://en.wikipedia.org/wiki/Word_square) _you can build from_ words. The same word from words can be used **mult

Serialization is the process of converting a data structure or object into a sequence of bits so that it can be stored in a file or memory buffer, or transmitted across a network connection link to be

You have n coins and you want to build a staircase with these coins. The staircase consists of k rows where the ith row has exactly i coins. The last row of the staircase may be incomplete

Given an integer array nums of length n where all the integers of nums are in the range [1, n] and each integer appears once or twice, return _an array of all the integers that appears

You are given an integer array nums of length n where nums is a permutation of the integers in the range [1, n]. You are also given a 2D integer array sequences where sequences[i] is a sub

You are given n points in the plane that are all distinct, where points[i] = [xi, yi]. A boomerang is a tuple of points (i, j, k) such that the distance between i and j equals the

Serialization is converting a data structure or object into a sequence of bits so that it can be stored in a file or memory buffer, or transmitted across a network connection link to be reconstructed

There are some spherical balloons taped onto a flat wall that represents the XY-plane. The balloons are represented as a 2D integer array points where points[i] = [xstart, xend] denotes a balloon

You are given an integer array matchsticks where matchsticks[i] is the length of the ith matchstick. You want to use all the matchsticks to make one square. You should not break any stic

You are given a license key represented as a string s that consists of only alphanumeric characters and dashes. The string is separated into n + 1 groups by n dashes. You are also given an integ

There is a ball in a maze with empty spaces (represented as 0) and walls (represented as 1). The ball can go through the empty spaces by rolling up, down, left or right, but it won't stop ro

A web developer needs to know how to design a web page's size. So, given a specific rectangular web page's area, your job by now is to design a rectangular web page, whose length L and width W satisfy

Our hero Teemo is attacking an enemy Ashe with poison attacks! When Teemo attacks Ashe, Ashe gets poisoned for a exactly duration seconds. More formally, an attack at second t will mean Ashe is po

You are given an array of non-overlapping axis-aligned rectangles rects where rects[i] = [ai, bi, xi, yi] indicates that (ai, bi) is the bottom-left corner point of the ith rectangle and `(xi,

There is a ball in a maze with empty spaces (represented as 0) and walls (represented as 1). The ball can go through the empty spaces by rolling up, down, left or right, but it won't stop ro

Suppose LeetCode will start its IPO soon. In order to sell a good price of its shares to Venture Capital, LeetCode would like to work on some projects to increase its capital before the IPO. S

There is a ball in a maze with empty spaces (represented as 0) and walls (represented as 1). The ball can go through the empty spaces by rolling up, down, left or right, but it won't stop ro

A [perfect number](https://en.wikipedia.org/wiki/Perfect_number) is a positive integer that is equal to the sum of its positive divisors, excluding the number itself. A divisor of an i

The Fibonacci numbers, commonly denoted F(n) form a sequence, called the Fibonacci sequence, such that each number is the sum of the two preceding ones, starting from 0 and 1. That is,

In the video game Fallout 4, the quest "Road to Freedom " requires players to reach a metal dial called the "Freedom Trail Ring " and use the dial to spell a specific keyword to open the door.

Given a string s and a string array dictionary, return _the longest string in the dictionary that can be formed by deleting some of the given string characters_. If there is more than one possible

> Note: This is a companion problem to the [System Design](https://leetcode.com/discuss/interview-question/system-design/) problem: [Design TinyURL](https://leetcode.com/discuss/interview-question/124

During the NBA playoffs, we always set the rather strong team to play with the rather weak team, like make the rank 1 team play with the rank nth team, which is a good strategy to make the contest

You are given a string s representing an attendance record for a student where each character signifies whether the student was absent, late, or present on that day. The record only contains the fol

An attendance record for a student can be represented as a string where each character signifies whether the student was absent, late, or present on that day. The record only contains the following th

There is a rectangular brick wall in front of you with n rows of bricks. The ith row has some number of bricks each of the same height (i.e., one unit) but they can be of different widths. The tot

Given an integer array nums of 2n integers, group these integers into n pairs (a1, b1), (a2, b2), ..., (an, bn) such that the sum of min(ai, bi) for all i is maximized. Return _the max

LeetCode wants to give one of its best employees the option to travel among n cities to collect algorithm problems. But all work and no play makes Jack a dull boy, you could take vacations in some p

You have n processes forming a rooted tree structure. You are given two integer arrays pid and ppid, where pid[i] is the ID of the ith process and ppid[i] is the ID of the ith process's

You are given an m x n matrix M initialized with all 0's and an array of operations ops, where ops[i] = [ai, bi] means M[x][y] should be incremented by one for all 0 <= x < ai and `0 <=

Design and implement a data structure for a compressed string iterator. The given compressed string will be in the form of each letter followed by a positive integer representing the number of this le

Given a list paths of directory info, including the directory path, and all the files with contents in this directory, return _all the duplicate files in the file system in terms of their paths_. Yo

There are n different online courses numbered from 1 to n. You are given an array courses where courses[i] = [durationi, lastDayi] indicate that the ith course should be taken **continuous

You are given several logs, where each log contains a unique ID and timestamp. Timestamp is a string that has the following format: Year:Month:Day:Hour:Minute:Second, for example, `2017:01:01:23:59:

In LeetCode Store, there are n items to sell. Each item has a price. However, there are some special offers, and a special offer consists of one or more different kinds of items with a sale price.

Solve a given equation and return the value of 'x' in the form of a string "x=#value ". The equation contains only '+', '-' operation, the variable 'x' and its coefficient. You should return

You have a set of integers s, which originally contains all the numbers from 1 to n. Unfortunately, due to some error, one of the numbers in s got duplicated to another number in the set, whic

In English, we have a concept called root, which can be followed by some other word to form another longer word - let's call this word successor. For example, when the root "an " is foll

Given the root of a binary tree, construct a 0-indexed m x n string matrix res that represents a formatted layout of the tree. The formatted layout matrix should be constructed using the

You are given an integer array coins (1-indexed) of length n and an integer maxJump. You can jump to any index i of the array coins if coins[i] != -1 and you have to pay coins[i] whe

Design a data structure that is initialized with a list of different words. Provided a string, you should determine if you can change exactly one character in this string to match any word in the

You are given an integer array cards of length 4. You have four cards, each containing a number in the range [1, 9]. You should arrange the numbers on these cards in a mathematical expression us

In this problem, a rooted tree is a directed graph such that, there is exactly one node (the root) for which all other nodes are descendants of this node, plus every node has exactly one parent, e

Given two strings a and b, return _the minimum number of times you should repeat string_ a _so that string_ b _is a substring of it_. If it is impossible for b​​​​​​ to be a substring of a

You are given an integer n and an array of unique integers blacklist. Design an algorithm to pick a random integer in the range [0, n - 1] that is not in blacklist. Any integer that is

Given a list of accounts where each element accounts[i] is a list of strings, where the first element accounts[i][0] is a name, and the rest of the elements are emails representing emails of

Given a C++ program, remove comments from it. The program source is an array of strings source where source[i] is the ith line of the source code. This represents the result of splitting the ori

You have a lock in front of you with 4 circular wheels. Each wheel has 10 slots: '0', '1', '2', '3', '4', '5', '6', '7', '8', '9'. The wheels can rotate freely and wrap around: for example we can tu

You are given an elevation map represents as an integer array heights where heights[i] representing the height of the terrain at index i. The width at each index is 1. You are also given two i

You are stacking blocks to form a pyramid. Each block has a color, which is represented by a single letter. Each row of blocks contains one less block than the row beneath it and is centered on to

You are given an integer n. You have an n x n binary grid grid with all values initially 1's except for some indices given in the array mines. The ith element of the array mines is defin

Given an expression such as expression = "e + 8 - a + 5 " and an evaluation map such as { "e ": 1} (given in terms of evalvars = [ "e "] and evalints = [1]), return a list of tokens representi

You're given strings jewels representing the types of stones that are jewels, and stones representing the stones you have. Each character in stones is a type of stone you have. You want to know

Given the root of a binary search tree (BST) and an integer target, split the tree into two subtrees where one subtree has nodes that are all smaller or equal to the target value, while the other

In a string composed of 'L', 'R', and 'X' characters, like "RXXLRXRXL ", a move consists of either replacing one occurrence of "XL " with "LX ", or replacing one occurrence of "RX " with

We build a table of n rows (1-indexed). We start by writing 0 in the 1st row. Now in every subsequent row, we look at the previous row and replace each occurrence of 0 with 01, and each

There is a forest with an unknown number of rabbits. We asked n rabbits "How many rabbits have the same color as you? " and collected the answers in an integer array answers where answers[i] i

There is an undirected graph with n nodes, where each node is numbered between 0 and n - 1. You are given a 2D array graph, where graph[u] is an array of nodes that node u is adjacent

An integer x is a good if after rotating each digit individually by 180 degrees, we get a valid number that is different from x. Each digit must be rotated - we cannot choose to leave it alone

You are playing a simplified PAC-MAN game on an infinite 2-D grid. You start at the point [0, 0], and you are given a destination point target = [xtarget, ytarget] that you are trying to get to. T

Given an integer array nums and two integers left and right, return _the number of contiguous non-empty subarrays such that the value of the maximum array element in that subarray is in the

You are given an array nums. You can rotate it by a non-negative integer k so that the array becomes [nums[k], nums[k + 1], ... nums[nums.length - 1], nums[0], nums[1], ..., nums[k-1]]. Afterwar

There is a directed graph of n nodes with each node labeled from 0 to n - 1. The graph is represented by a 0-indexed 2D integer array graph where graph[i] is an integer array of nodes ad

You are given a string s of lowercase English letters and an array widths denoting how many pixels wide each lowercase English letter is. Specifically, widths[0] is the width of 'a', `widt

A website domain "discuss.leetcode.com " consists of various subdomains. At the top level, we have "com ", at the next level, we have "leetcode.com " and at the lowest level, `"discuss.leetcode.

Given an array of points on the X-Y plane points where points[i] = [xi, yi], return _the area of the largest triangle that can be formed by any three different points_. Answers within 10-5 o

You are given an integer array nums and an integer k. You can partition the array into at most k non-empty adjacent subarrays. The score of a partition is the sum of the averages of each

Your car starts at position 0 and speed +1 on an infinite number line. Your car can go into negative positions. Your car drives automatically according to a sequence of instructions 'A' (acceler

Given a string paragraph and a string array of the banned words banned, return _the most frequent word that is not banned_. It is guaranteed there is at least one word that is not banned,

Given a string s and a character c that occurs in s, return _an array of integers_ answer _where_ answer.length == s.length _and_ answer[i] _is the distance from index_ i _to the **c

You are given two 0-indexed integer arrays fronts and backs of length n, where the ith card has the positive integer fronts[i] printed on the front and backs[i] printed on the back. In

You are given a 0-indexed string s that you must perform k replacement operations on. The replacement operations are given as three 0-indexed parallel arrays, indices, sources, and `ta

There are n rooms labeled from 0 to n - 1 and all the rooms are locked except for room 0. Your goal is to visit all the rooms. However, you cannot enter a locked room without having its key.

You are given a 2D array of axis-aligned rectangles. Each rectangle[i] = [xi1, yi1, xi2, yi2] denotes the ith rectangle where (xi1, yi1) are the coordinates of the bottom-left corner, and

There are n workers. You are given two integer arrays quality and wage where quality[i] is the quality of the ith worker and wage[i] is the minimum wage expectation for the ith worker.

At a lemonade stand, each lemonade costs $5. Customers are standing in a queue to buy from you and order one at a time (in the order specified by bills). Each customer will only buy one lemonade and

You are given an array people where people[i] is the weight of the ith person, and an infinite number of boats where each boat can carry a maximum weight of limit. Each boat carries at mos

You are given an undirected graph (the "original graph ") with n nodes labeled from 0 to n - 1. You decide to subdivide each edge in the graph into a chain of nodes, with the number of n

You start at the cell (rStart, cStart) of an rows x cols grid facing east. The northwest corner is at the first row and column in the grid, and the southeast corner is at the last row and column.

Alice and Bob have a different total number of candies. You are given two integer arrays aliceSizes and bobSizes where aliceSizes[i] is the number of candies of the ith box of candy that Alice