Datasets:

kreimben

/

leetcode_questions

like 0

Chalkboard XOR Game

chalkboard-xor-game

You are given an array of integers nums represents the numbers written on a chalkboard. Alice and Bob take turns erasing exactly one number from the chalkboard, with Alice starting first. If erasing a number causes the bitwise XOR of all the elements of the chalkboard to become 0, then that player loses. The bitwise XOR of one element is that element itself, and the bitwise XOR of no elements is 0. Also, if any player starts their turn with the bitwise XOR of all the elements of the chalkboard equal to 0, then that player wins. Return true if and only if Alice wins the game, assuming both players play optimally.

Array,Math,Bit Manipulation,Brainteaser,Game Theory

Hard

Subdomain Visit Count

subdomain-visit-count

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.com". When we visit a domain like "discuss.leetcode.com", we will also visit the parent domains "leetcode.com" and "com" implicitly. A count-paired domain is a domain that has one of the two formats "rep d1.d2.d3" or "rep d1.d2" where rep is the number of visits to the domain and d1.d2.d3 is the domain itself. Given an array of count-paired domains cpdomains, return an array of the count-paired domains of each subdomain in the input. You may return the answer in any order.

Array,Hash Table,String,Counting

Medium

Largest Triangle Area

largest-triangle-area

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 of the actual answer will be accepted.

Array,Math,Geometry

Easy

1018

Largest Sum of Averages

largest-sum-of-averages

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 subarray. Note that the partition must use every integer in nums, and that the score is not necessarily an integer. Return the maximum score you can achieve of all the possible partitions. Answers within 10-6 of the actual answer will be accepted.

Array,Dynamic Programming

Medium

Binary Tree Pruning

binary-tree-pruning

Given the root of a binary tree, return the same tree where every subtree (of the given tree) not containing a 1 has been removed. A subtree of a node node is node plus every node that is a descendant of node.

Tree,Depth-First Search,Binary Tree

Medium

Bus Routes

bus-routes

You are given an array routes representing bus routes where routes[i] is a bus route that the ith bus repeats forever. You will start at the bus stop source (You are not on any bus initially), and you want to go to the bus stop target. You can travel between bus stops by buses only. Return the least number of buses you must take to travel from source to target. Return -1 if it is not possible.

Array,Hash Table,Breadth-First Search

Hard

Ambiguous Coordinates

ambiguous-coordinates

We had some 2-dimensional coordinates, like "(1, 3)" or "(2, 0.5)". Then, we removed all commas, decimal points, and spaces and ended up with the string s. Return a list of strings representing all possibilities for what our original coordinates could have been. Our original representation never had extraneous zeroes, so we never started with numbers like "00", "0.0", "0.00", "1.0", "001", "00.01", or any other number that can be represented with fewer digits. Also, a decimal point within a number never occurs without at least one digit occurring before it, so we never started with numbers like ".1". The final answer list can be returned in any order. All coordinates in the final answer have exactly one space between them (occurring after the comma.)

String,Backtracking

Medium

Linked List Components

linked-list-components

You are given the head of a linked list containing unique integer values and an integer array nums that is a subset of the linked list values. Return the number of connected components in nums where two values are connected if they appear consecutively in the linked list.

Hash Table,Linked List

Medium

2299

Race Car

race-car

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' (accelerate) and 'R' (reverse): For example, after commands "AAR", your car goes to positions 0 --> 1 --> 3 --> 3, and your speed goes to 1 --> 2 --> 4 --> -1. Given a target position target, return the length of the shortest sequence of instructions to get there.

Dynamic Programming

Hard

Most Common Word

most-common-word

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, and that the answer is unique. The words in paragraph are case-insensitive and the answer should be returned in lowercase.

Hash Table,String,Counting

Easy

Design Linked List

design-linked-list

Design your implementation of the linked list. You can choose to use a singly or doubly linked list. A node in a singly linked list should have two attributes: val and next. val is the value of the current node, and next is a pointer/reference to the next node. If you want to use the doubly linked list, you will need one more attribute prev to indicate the previous node in the linked list. Assume all nodes in the linked list are 0-indexed. Implement the MyLinkedList class:

Linked List,Design

Medium

1337

Short Encoding of Words

short-encoding-of-words

A valid encoding of an array of words is any reference string s and array of indices indices such that: Given an array of words, return the length of the shortest reference string s possible of any valid encoding of words.

Array,Hash Table,String,Trie

Medium

Shortest Distance to a Character

shortest-distance-to-a-character

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 closest occurrence of character c in s. The distance between two indices i and j is abs(i - j), where abs is the absolute value function.

Array,Two Pointers,String

Easy

Card Flipping Game

card-flipping-game

You are given n cards, with a positive integer printed on the front and back of each card (possibly different). You can flip any number of cards (possibly zero). After choosing the front and the back of each card, you will pick each card, and if the integer printed on the back of this card is not printed on the front of any other card, then this integer is good. You are given two integer array fronts and backs where fronts[i] and backs[i] are the integers printer on the front and the back of the ith card respectively. Return the smallest good and integer after flipping the cards. If there are no good integers, return 0. Note that a flip swaps the front and back numbers, so the value on the front is now on the back and vice versa.

Array,Hash Table

Medium

Binary Trees With Factors

binary-trees-with-factors

Given an array of unique integers, arr, where each integer arr[i] is strictly greater than 1. We make a binary tree using these integers, and each number may be used for any number of times. Each non-leaf node's value should be equal to the product of the values of its children. Return the number of binary trees we can make. The answer may be too large so return the answer modulo 109 + 7.

Array,Hash Table,Dynamic Programming

Medium

Insert into a Sorted Circular Linked List

insert-into-a-sorted-circular-linked-list

Linked List

Medium

147

Goat Latin

goat-latin

You are given a string sentence that consist of words separated by spaces. Each word consists of lowercase and uppercase letters only. We would like to convert the sentence to "Goat Latin" (a made-up language similar to Pig Latin.) The rules of Goat Latin are as follows: Return the final sentence representing the conversion from sentence to Goat Latin.

String

Easy

Friends Of Appropriate Ages

friends-of-appropriate-ages

There are n persons on a social media website. You are given an integer array ages where ages[i] is the age of the ith person. A Person x will not send a friend request to a person y (x != y) if any of the following conditions is true: Otherwise, x will send a friend request to y. Note that if x sends a request to y, y will not necessarily send a request to x. Also, a person will not send a friend request to themself. Return the total number of friend requests made.

Array,Two Pointers,Binary Search,Sorting

Medium

Most Profit Assigning Work

most-profit-assigning-work

You have n jobs and m workers. You are given three arrays: difficulty, profit, and worker where: Every worker can be assigned at most one job, but one job can be completed multiple times. Return the maximum profit we can achieve after assigning the workers to the jobs.

Array,Two Pointers,Binary Search,Greedy,Sorting

Medium

2180

Making A Large Island

making-a-large-island

You are given an n x n binary matrix grid. You are allowed to change at most one 0 to be 1. Return the size of the largest island in grid after applying this operation. An island is a 4-directionally connected group of 1s.

Array,Depth-First Search,Breadth-First Search,Union Find,Matrix

Hard

Count Unique Characters of All Substrings of a Given String

count-unique-characters-of-all-substrings-of-a-given-string

Let's define a function countUniqueChars(s) that returns the number of unique characters on s. Given a string s, return the sum of countUniqueChars(t) where t is a substring of s. Notice that some substrings can be repeated so in this case you have to count the repeated ones too.

String,Dynamic Programming

Hard

Consecutive Numbers Sum

consecutive-numbers-sum

Given an integer n, return the number of ways you can write n as the sum of consecutive positive integers.

Math,Enumeration

Hard

Positions of Large Groups

positions-of-large-groups

In a string s of lowercase letters, these letters form consecutive groups of the same character. For example, a string like s = "abbxxxxzyy" has the groups "a", "bb", "xxxx", "z", and "yy". A group is identified by an interval [start, end], where start and end denote the start and end indices (inclusive) of the group. In the above example, "xxxx" has the interval [3,6]. A group is considered large if it has 3 or more characters. Return the intervals of every large group sorted in increasing order by start index.

String

Easy

2260

Masking Personal Information

masking-personal-information

You are given a personal information string s, representing either an email address or a phone number. Return the masked personal information using the below rules. Email address: An email address is: To mask an email: Phone number: A phone number is formatted as follows: To mask a phone number:

String

Medium

Design Circular Deque

design-circular-deque

Design your implementation of the circular double-ended queue (deque). Implement the MyCircularDeque class:

Array,Linked List,Design,Queue

Medium

860,1767

Design Circular Queue

design-circular-queue

Design your implementation of the circular queue. The circular queue is a linear data structure in which the operations are performed based on FIFO (First In First Out) principle and the last position is connected back to the first position to make a circle. It is also called "Ring Buffer". One of the benefits of the circular queue is that we can make use of the spaces in front of the queue. In a normal queue, once the queue becomes full, we cannot insert the next element even if there is a space in front of the queue. But using the circular queue, we can use the space to store new values. Implementation the MyCircularQueue class: You must solve the problem without using the built-in queue data structure in your programming language.

Array,Linked List,Design,Queue

Medium

859,1767

Flipping an Image

flipping-an-image

Given an n x n binary matrix image, flip the image horizontally, then invert it, and return the resulting image. To flip an image horizontally means that each row of the image is reversed. To invert an image means that each 0 is replaced by 1, and each 1 is replaced by 0.

Array,Two Pointers,Matrix,Simulation

Easy

Find And Replace in String

find-and-replace-in-string

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 targets, all of length k. To complete the ith replacement operation: For example, if s = "abcd", indices[i] = 0, sources[i] = "ab", and targets[i] = "eee", then the result of this replacement will be "eeecd". All replacement operations must occur simultaneously, meaning the replacement operations should not affect the indexing of each other. The testcases will be generated such that the replacements will not overlap. Return the resulting string after performing all replacement operations on s. A substring is a contiguous sequence of characters in a string.

Array,String,Sorting

Medium

Sum of Distances in Tree

sum-of-distances-in-tree

There is an undirected connected tree with n nodes labeled from 0 to n - 1 and n - 1 edges. You are given the integer n and the array edges where edges[i] = [ai, bi] indicates that there is an edge between nodes ai and bi in the tree. Return an array answer of length n where answer[i] is the sum of the distances between the ith node in the tree and all other nodes.

Dynamic Programming,Tree,Depth-First Search,Graph

Hard

1021,2175

Image Overlap

image-overlap

You are given two images, img1 and img2, represented as binary, square matrices of size n x n. A binary matrix has only 0s and 1s as values. We translate one image however we choose by sliding all the 1 bits left, right, up, and/or down any number of units. We then place it on top of the other image. We can then calculate the overlap by counting the number of positions that have a 1 in both images. Note also that a translation does not include any kind of rotation. Any 1 bits that are translated outside of the matrix borders are erased. Return the largest possible overlap.

Array,Matrix

Medium

Robot Room Cleaner

robot-room-cleaner

Backtracking,Interactive

Hard

286,1931,1959,2203

Rectangle Overlap

rectangle-overlap

An axis-aligned rectangle is represented as a list [x1, y1, x2, y2], where (x1, y1) is the coordinate of its bottom-left corner, and (x2, y2) is the coordinate of its top-right corner. Its top and bottom edges are parallel to the X-axis, and its left and right edges are parallel to the Y-axis. Two rectangles overlap if the area of their intersection is positive. To be clear, two rectangles that only touch at the corner or edges do not overlap. Given two axis-aligned rectangles rec1 and rec2, return true if they overlap, otherwise return false.

Math,Geometry

Easy

223

New 21 Game

new-21-game

Alice plays the following game, loosely based on the card game "21". Alice starts with 0 points and draws numbers while she has less than k points. During each draw, she gains an integer number of points randomly from the range [1, maxPts], where maxPts is an integer. Each draw is independent and the outcomes have equal probabilities. Alice stops drawing numbers when she gets k or more points. Return the probability that Alice has n or fewer points. Answers within 10-5 of the actual answer are considered accepted.

Math,Dynamic Programming,Sliding Window,Probability and Statistics

Medium

Push Dominoes

push-dominoes

There are n dominoes in a line, and we place each domino vertically upright. In the beginning, we simultaneously push some of the dominoes either to the left or to the right. After each second, each domino that is falling to the left pushes the adjacent domino on the left. Similarly, the dominoes falling to the right push their adjacent dominoes standing on the right. When a vertical domino has dominoes falling on it from both sides, it stays still due to the balance of the forces. For the purposes of this question, we will consider that a falling domino expends no additional force to a falling or already fallen domino. You are given a string dominoes representing the initial state where: Return a string representing the final state.

Two Pointers,String,Dynamic Programming

Medium

Similar String Groups

similar-string-groups

Two strings X and Y are similar if we can swap two letters (in different positions) of X, so that it equals Y. Also two strings X and Y are similar if they are equal. For example, "tars" and "rats" are similar (swapping at positions 0 and 2), and "rats" and "arts" are similar, but "star" is not similar to "tars", "rats", or "arts". Together, these form two connected groups by similarity: {"tars", "rats", "arts"} and {"star"}.  Notice that "tars" and "arts" are in the same group even though they are not similar.  Formally, each group is such that a word is in the group if and only if it is similar to at least one other word in the group. We are given a list strs of strings where every string in strs is an anagram of every other string in strs. How many groups are there?

Array,String,Depth-First Search,Breadth-First Search,Union Find

Hard

2276

Magic Squares In Grid

magic-squares-in-grid

A 3 x 3 magic square is a 3 x 3 grid filled with distinct numbers from 1 to 9 such that each row, column, and both diagonals all have the same sum. Given a row x col grid of integers, how many 3 x 3 "magic square" subgrids are there?  (Each subgrid is contiguous).

Array,Math,Matrix

Medium

1311

Keys and Rooms

keys-and-rooms

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. When you visit a room, you may find a set of distinct keys in it. Each key has a number on it, denoting which room it unlocks, and you can take all of them with you to unlock the other rooms. Given an array rooms where rooms[i] is the set of keys that you can obtain if you visited room i, return true if you can visit all the rooms, or false otherwise.

Depth-First Search,Breadth-First Search,Graph

Medium

261

Split Array into Fibonacci Sequence

split-array-into-fibonacci-sequence

You are given a string of digits num, such as "123456579". We can split it into a Fibonacci-like sequence [123, 456, 579]. Formally, a Fibonacci-like sequence is a list f of non-negative integers such that: Note that when splitting the string into pieces, each piece must not have extra leading zeroes, except if the piece is the number 0 itself. Return any Fibonacci-like sequence split from num, or return [] if it cannot be done.

String,Backtracking

Medium

306,1013

Guess the Word

guess-the-word

This is an interactive problem. You are given an array of unique strings wordlist where wordlist[i] is 6 letters long, and one word in this list is chosen as secret. You may call Master.guess(word) to guess a word. The guessed word should have type string and must be from the original list with 6 lowercase letters. This function returns an integer type, representing the number of exact matches (value and position) of your guess to the secret word. Also, if your guess is not in the given wordlist, it will return -1 instead. For each test case, you have exactly 10 guesses to guess the word. At the end of any number of calls, if you have made 10 or fewer calls to Master.guess and at least one of these guesses was secret, then you pass the test case.

Array,Math,String,Interactive,Game Theory

Hard

Backspace String Compare

backspace-string-compare

Given two strings s and t, return true if they are equal when both are typed into empty text editors. '#' means a backspace character. Note that after backspacing an empty text, the text will continue empty.

Two Pointers,String,Stack,Simulation

Easy

1720

Longest Mountain in Array

longest-mountain-in-array

You may recall that an array arr is a mountain array if and only if: Given an integer array arr, return the length of the longest subarray, which is a mountain. Return 0 if there is no mountain subarray.

Array,Two Pointers,Dynamic Programming,Enumeration

Medium

1766,2205

Hand of Straights

hand-of-straights

Alice has some number of cards and she wants to rearrange the cards into groups so that each group is of size groupSize, and consists of groupSize consecutive cards. Given an integer array hand where hand[i] is the value written on the ith card and an integer groupSize, return true if she can rearrange the cards, or false otherwise.

Array,Hash Table,Greedy,Sorting

Medium

Shortest Path Visiting All Nodes

shortest-path-visiting-all-nodes

You have an undirected, connected graph of n nodes labeled from 0 to n - 1. You are given an array graph where graph[i] is a list of all the nodes connected with node i by an edge. Return the length of the shortest path that visits every node. You may start and stop at any node, you may revisit nodes multiple times, and you may reuse edges.

Dynamic Programming,Bit Manipulation,Breadth-First Search,Graph,Bitmask

Hard

Shifting Letters

shifting-letters

You are given a string s of lowercase English letters and an integer array shifts of the same length. Call the shift() of a letter, the next letter in the alphabet, (wrapping around so that 'z' becomes 'a'). Now for each shifts[i] = x, we want to shift the first i + 1 letters of s, x times. Return the final string after all such shifts to s are applied.

Array,String

Medium

1954

Maximize Distance to Closest Person

maximize-distance-to-closest-person

You are given an array representing a row of seats where seats[i] = 1 represents a person sitting in the ith seat, and seats[i] = 0 represents that the ith seat is empty (0-indexed). There is at least one empty seat, and at least one person sitting. Alex wants to sit in the seat such that the distance between him and the closest person to him is maximized. Return that maximum distance to the closest person.

Array

Medium

885

Rectangle Area II

rectangle-area-ii

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 (xi2, yi2) are the coordinates of the top-right corner. Calculate the total area covered by all rectangles in the plane. Any area covered by two or more rectangles should only be counted once. Return the total area. Since the answer may be too large, return it modulo 109 + 7.

Array,Segment Tree,Line Sweep,Ordered Set

Hard

Loud and Rich

loud-and-rich

There is a group of n people labeled from 0 to n - 1 where each person has a different amount of money and a different level of quietness. You are given an array richer where richer[i] = [ai, bi] indicates that ai has more money than bi and an integer array quiet where quiet[i] is the quietness of the ith person. All the given data in richer are logically correct (i.e., the data will not lead you to a situation where x is richer than y and y is richer than x at the same time). Return an integer array answer where answer[x] = y if y is the least quiet person (that is, the person y with the smallest value of quiet[y]) among all people who definitely have equal to or more money than the person x.

Array,Depth-First Search,Graph,Topological Sort

Medium

Peak Index in a Mountain Array

peak-index-in-a-mountain-array

Let's call an array arr a mountain if the following properties hold: Given an integer array arr that is guaranteed to be a mountain, return any i such that arr[0] < arr[1] < ... arr[i - 1] < arr[i] > arr[i + 1] > ... > arr[arr.length - 1].

Array,Binary Search

Easy

162,1185,1766

Car Fleet

car-fleet

There are n cars going to the same destination along a one-lane road. The destination is target miles away. You are given two integer array position and speed, both of length n, where position[i] is the position of the ith car and speed[i] is the speed of the ith car (in miles per hour). A car can never pass another car ahead of it, but it can catch up to it and drive bumper to bumper at the same speed. The faster car will slow down to match the slower car's speed. The distance between these two cars is ignored (i.e., they are assumed to have the same position). A car fleet is some non-empty set of cars driving at the same position and same speed. Note that a single car is also a car fleet. If a car catches up to a car fleet right at the destination point, it will still be considered as one car fleet. Return the number of car fleets that will arrive at the destination.

Array,Stack,Sorting,Monotonic Stack

Medium

1902,2317

K-Similar Strings

k-similar-strings

Strings s1 and s2 are k-similar (for some non-negative integer k) if we can swap the positions of two letters in s1 exactly k times so that the resulting string equals s2. Given two anagrams s1 and s2, return the smallest k for which s1 and s2 are k-similar.

String,Breadth-First Search

Hard

770

Exam Room

exam-room

There is an exam room with n seats in a single row labeled from 0 to n - 1. When a student enters the room, they must sit in the seat that maximizes the distance to the closest person. If there are multiple such seats, they sit in the seat with the lowest number. If no one is in the room, then the student sits at seat number 0. Design a class that simulates the mentioned exam room. Implement the ExamRoom class:

Design,Ordered Set

Medium

879

Score of Parentheses

score-of-parentheses

Given a balanced parentheses string s, return the score of the string. The score of a balanced parentheses string is based on the following rule:

String,Stack

Medium

Minimum Cost to Hire K Workers

minimum-cost-to-hire-k-workers

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. We want to hire exactly k workers to form a paid group. To hire a group of k workers, we must pay them according to the following rules: Given the integer k, return the least amount of money needed to form a paid group satisfying the above conditions. Answers within 10-5 of the actual answer will be accepted.

Array,Greedy,Sorting,Heap (Priority Queue)

Hard

Mirror Reflection

mirror-reflection

There is a special square room with mirrors on each of the four walls. Except for the southwest corner, there are receptors on each of the remaining corners, numbered 0, 1, and 2. The square room has walls of length p and a laser ray from the southwest corner first meets the east wall at a distance q from the 0th receptor. Given the two integers p and q, return the number of the receptor that the ray meets first. The test cases are guaranteed so that the ray will meet a receptor eventually.

Math,Geometry

Medium

Buddy Strings

buddy-strings

Given two strings s and goal, return true if you can swap two letters in s so the result is equal to goal, otherwise, return false. Swapping letters is defined as taking two indices i and j (0-indexed) such that i != j and swapping the characters at s[i] and s[j].

Hash Table,String

Easy

1777,1915

Lemonade Change

lemonade-change

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 pay with either a $5, $10, or $20 bill. You must provide the correct change to each customer so that the net transaction is that the customer pays $5. Note that you do not have any change in hand at first. Given an integer array bills where bills[i] is the bill the ith customer pays, return true if you can provide every customer with the correct change, or false otherwise.

Array,Greedy

Easy

Score After Flipping Matrix

score-after-flipping-matrix

You are given an m x n binary matrix grid. A move consists of choosing any row or column and toggling each value in that row or column (i.e., changing all 0's to 1's, and all 1's to 0's). Every row of the matrix is interpreted as a binary number, and the score of the matrix is the sum of these numbers. Return the highest possible score after making any number of moves (including zero moves).

Array,Greedy,Bit Manipulation,Matrix

Medium

2268

Shortest Subarray with Sum at Least K

shortest-subarray-with-sum-at-least-k

Given an integer array nums and an integer k, return the length of the shortest non-empty subarray of nums with a sum of at least k. If there is no such subarray, return -1. A subarray is a contiguous part of an array.

Array,Binary Search,Queue,Sliding Window,Heap (Priority Queue),Prefix Sum,Monotonic Queue

Hard

All Nodes Distance K in Binary Tree

all-nodes-distance-k-in-binary-tree

Given the root of a binary tree, the value of a target node target, and an integer k, return an array of the values of all nodes that have a distance k from the target node. You can return the answer in any order.

Tree,Depth-First Search,Breadth-First Search,Binary Tree

Medium

Random Pick with Blacklist

random-pick-with-blacklist

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 in the mentioned range and not in blacklist should be equally likely to be returned. Optimize your algorithm such that it minimizes the number of calls to the built-in random function of your language. Implement the Solution class:

Hash Table,Math,Binary Search,Sorting,Randomized

Hard

398,912,2107

Shortest Path to Get All Keys

shortest-path-to-get-all-keys

You are given an m x n grid grid where: You start at the starting point and one move consists of walking one space in one of the four cardinal directions. You cannot walk outside the grid, or walk into a wall. If you walk over a key, you can pick it up and you cannot walk over a lock unless you have its corresponding key. For some 1 <= k <= 6, there is exactly one lowercase and one uppercase letter of the first k letters of the English alphabet in the grid. This means that there is exactly one key for each lock, and one lock for each key; and also that the letters used to represent the keys and locks were chosen in the same order as the English alphabet. Return the lowest number of moves to acquire all keys. If it is impossible, return -1.

Bit Manipulation,Breadth-First Search

Hard

Smallest Subtree with all the Deepest Nodes

smallest-subtree-with-all-the-deepest-nodes

Given the root of a binary tree, the depth of each node is the shortest distance to the root. Return the smallest subtree such that it contains all the deepest nodes in the original tree. A node is called the deepest if it has the largest depth possible among any node in the entire tree. The subtree of a node is a tree consisting of that node, plus the set of all descendants of that node.

Hash Table,Tree,Depth-First Search,Breadth-First Search,Binary Tree

Medium

Prime Palindrome

prime-palindrome

Given an integer n, return the smallest prime palindrome greater than or equal to n. An integer is prime if it has exactly two divisors: 1 and itself. Note that 1 is not a prime number. An integer is a palindrome if it reads the same from left to right as it does from right to left. The test cases are generated so that the answer always exists and is in the range [2, 2 * 108].

Math

Medium

2202

Transpose Matrix

transpose-matrix

Given a 2D integer array matrix, return the transpose of matrix. The transpose of a matrix is the matrix flipped over its main diagonal, switching the matrix's row and column indices.

Array,Matrix,Simulation

Easy

We don't need any special algorithms to do this. You just need to know what the transpose of a matrix looks like. Rows become columns and vice versa!

Binary Gap

binary-gap

Given a positive integer n, find and return the longest distance between any two adjacent 1's in the binary representation of n. If there are no two adjacent 1's, return 0. Two 1's are adjacent if there are only 0's separating them (possibly no 0's). The distance between two 1's is the absolute difference between their bit positions. For example, the two 1's in "1001" have a distance of 3.

Math,Bit Manipulation

Easy

Reordered Power of 2

reordered-power-of-2

You are given an integer n. We reorder the digits in any order (including the original order) such that the leading digit is not zero. Return true if and only if we can do this so that the resulting number is a power of two.

Math,Sorting,Counting,Enumeration

Medium

Advantage Shuffle

advantage-shuffle

You are given two integer arrays nums1 and nums2 both of the same length. The advantage of nums1 with respect to nums2 is the number of indices i for which nums1[i] > nums2[i]. Return any permutation of nums1 that maximizes its advantage with respect to nums2.

Array,Greedy,Sorting

Medium

Minimum Number of Refueling Stops

minimum-number-of-refueling-stops

A car travels from a starting position to a destination which is target miles east of the starting position. There are gas stations along the way. The gas stations are represented as an array stations where stations[i] = [positioni, fueli] indicates that the ith gas station is positioni miles east of the starting position and has fueli liters of gas. The car starts with an infinite tank of gas, which initially has startFuel liters of fuel in it. It uses one liter of gas per one mile that it drives. When the car reaches a gas station, it may stop and refuel, transferring all the gas from the station into the car. Return the minimum number of refueling stops the car must make in order to reach its destination. If it cannot reach the destination, return -1. Note that if the car reaches a gas station with 0 fuel left, the car can still refuel there. If the car reaches the destination with 0 fuel left, it is still considered to have arrived.

Array,Dynamic Programming,Greedy,Heap (Priority Queue)

Hard

Implement Rand10() Using Rand7()

implement-rand10-using-rand7

Given the API rand7() that generates a uniform random integer in the range [1, 7], write a function rand10() that generates a uniform random integer in the range [1, 10]. You can only call the API rand7(), and you shouldn't call any other API. Please do not use a language's built-in random API. Each test case will have one internal argument n, the number of times that your implemented function rand10() will be called while testing. Note that this is not an argument passed to rand10().

Math,Rejection Sampling,Randomized,Probability and Statistics

Medium

Leaf-Similar Trees

leaf-similar-trees

Consider all the leaves of a binary tree, from left to right order, the values of those leaves form a leaf value sequence. For example, in the given tree above, the leaf value sequence is (6, 7, 4, 9, 8). Two binary trees are considered leaf-similar if their leaf value sequence is the same. Return true if and only if the two given trees with head nodes root1 and root2 are leaf-similar.

Tree,Depth-First Search,Binary Tree

Easy

Length of Longest Fibonacci Subsequence

length-of-longest-fibonacci-subsequence

A sequence x1, x2, ..., xn is Fibonacci-like if: Given a strictly increasing array arr of positive integers forming a sequence, return the length of the longest Fibonacci-like subsequence of arr. If one does not exist, return 0. A subsequence is derived from another sequence arr by deleting any number of elements (including none) from arr, without changing the order of the remaining elements. For example, [3, 5, 8] is a subsequence of [3, 4, 5, 6, 7, 8].

Array,Hash Table,Dynamic Programming

Medium

1013

Walking Robot Simulation

walking-robot-simulation

A robot on an infinite XY-plane starts at point (0, 0) facing north. The robot can receive a sequence of these three possible types of commands: Some of the grid squares are obstacles. The ith obstacle is at grid point obstacles[i] = (xi, yi). If the robot runs into an obstacle, then it will instead stay in its current location and move on to the next command. Return the maximum Euclidean distance that the robot ever gets from the origin squared (i.e. if the distance is 5, return 25). Note:

Array,Simulation

Medium

2178

Koko Eating Bananas

koko-eating-bananas

Koko loves to eat bananas. There are n piles of bananas, the ith pile has piles[i] bananas. The guards have gone and will come back in h hours. Koko can decide her bananas-per-hour eating speed of k. Each hour, she chooses some pile of bananas and eats k bananas from that pile. If the pile has less than k bananas, she eats all of them instead and will not eat any more bananas during this hour. Koko likes to eat slowly but still wants to finish eating all the bananas before the guards return. Return the minimum integer k such that she can eat all the bananas within h hours.

Array,Binary Search

Medium

788,1335,2188

Middle of the Linked List

middle-of-the-linked-list

Given the head of a singly linked list, return the middle node of the linked list. If there are two middle nodes, return the second middle node.

Linked List,Two Pointers

Easy

2216,2236

Stone Game

stone-game

Alice and Bob play a game with piles of stones. There are an even number of piles arranged in a row, and each pile has a positive integer number of stones piles[i]. The objective of the game is to end with the most stones. The total number of stones across all the piles is odd, so there are no ties. Alice and Bob take turns, with Alice starting first. Each turn, a player takes the entire pile of stones either from the beginning or from the end of the row. This continues until there are no more piles left, at which point the person with the most stones wins. Assuming Alice and Bob play optimally, return true if Alice wins the game, or false if Bob wins.

Array,Math,Dynamic Programming,Game Theory

Medium

1685,1788,1808,2002,2156

Nth Magical Number

nth-magical-number

A positive integer is magical if it is divisible by either a or b. Given the three integers n, a, and b, return the nth magical number. Since the answer may be very large, return it modulo 109 + 7.

Math,Binary Search

Hard

Profitable Schemes

profitable-schemes

There is a group of n members, and a list of various crimes they could commit. The ith crime generates a profit[i] and requires group[i] members to participate in it. If a member participates in one crime, that member can't participate in another crime. Let's call a profitable scheme any subset of these crimes that generates at least minProfit profit, and the total number of members participating in that subset of crimes is at most n. Return the number of schemes that can be chosen. Since the answer may be very large, return it modulo 109 + 7.

Array,Dynamic Programming

Hard

Random Pick with Weight

random-pick-with-weight

You are given a 0-indexed array of positive integers w where w[i] describes the weight of the ith index. You need to implement the function pickIndex(), which randomly picks an index in the range [0, w.length - 1] (inclusive) and returns it. The probability of picking an index i is w[i] / sum(w).

Math,Binary Search,Prefix Sum,Randomized

Medium

398,894,914

Random Flip Matrix

random-flip-matrix

There is an m x n binary grid matrix with all the values set 0 initially. Design an algorithm to randomly pick an index (i, j) where matrix[i][j] == 0 and flips it to 1. All the indices (i, j) where matrix[i][j] == 0 should be equally likely to be returned. Optimize your algorithm to minimize the number of calls made to the built-in random function of your language and optimize the time and space complexity. Implement the Solution class:

Hash Table,Math,Reservoir Sampling,Randomized

Medium

Random Point in Non-overlapping Rectangles

random-point-in-non-overlapping-rectangles

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, yi) is the top-right corner point of the ith rectangle. Design an algorithm to pick a random integer point inside the space covered by one of the given rectangles. A point on the perimeter of a rectangle is included in the space covered by the rectangle. Any integer point inside the space covered by one of the given rectangles should be equally likely to be returned. Note that an integer point is a point that has integer coordinates. Implement the Solution class:

Math,Binary Search,Reservoir Sampling,Prefix Sum,Ordered Set,Randomized

Medium

912,915

Generate Random Point in a Circle

generate-random-point-in-a-circle

Given the radius and the position of the center of a circle, implement the function randPoint which generates a uniform random point inside the circle. Implement the Solution class:

Math,Geometry,Rejection Sampling,Randomized

Medium

914

Decoded String at Index

decoded-string-at-index

You are given an encoded string s. To decode the string to a tape, the encoded string is read one character at a time and the following steps are taken: Given an integer k, return the kth letter (1-indexed) in the decoded string.

String,Stack

Medium

Boats to Save People

boats-to-save-people

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 most two people at the same time, provided the sum of the weight of those people is at most limit. Return the minimum number of boats to carry every given person.

Array,Two Pointers,Greedy,Sorting

Medium

Reachable Nodes In Subdivided Graph

reachable-nodes-in-subdivided-graph

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 new nodes varying between each edge. The graph is given as a 2D array of edges where edges[i] = [ui, vi, cnti] indicates that there is an edge between nodes ui and vi in the original graph, and cnti is the total number of new nodes that you will subdivide the edge into. Note that cnti == 0 means you will not subdivide the edge. To subdivide the edge [ui, vi], replace it with (cnti + 1) new edges and cnti new nodes. The new nodes are x1, x2, ..., xcnti, and the new edges are [ui, x1], [x1, x2], [x2, x3], ..., [xcnti-1, xcnti], [xcnti, vi]. In this new graph, you want to know how many nodes are reachable from the node 0, where a node is reachable if the distance is maxMoves or less. Given the original graph and maxMoves, return the number of nodes that are reachable from node 0 in the new graph.

Graph,Heap (Priority Queue),Shortest Path

Hard

2213,2218

Projection Area of 3D Shapes

projection-area-of-3d-shapes

You are given an n x n grid where we place some 1 x 1 x 1 cubes that are axis-aligned with the x, y, and z axes. Each value v = grid[i][j] represents a tower of v cubes placed on top of the cell (i, j). We view the projection of these cubes onto the xy, yz, and zx planes. A projection is like a shadow, that maps our 3-dimensional figure to a 2-dimensional plane. We are viewing the "shadow" when looking at the cubes from the top, the front, and the side. Return the total area of all three projections.

Array,Math,Geometry,Matrix

Easy

Uncommon Words from Two Sentences

uncommon-words-from-two-sentences

A sentence is a string of single-space separated words where each word consists only of lowercase letters. A word is uncommon if it appears exactly once in one of the sentences, and does not appear in the other sentence. Given two sentences s1 and s2, return a list of all the uncommon words. You may return the answer in any order.

Hash Table,String

Easy

2190

Spiral Matrix III

spiral-matrix-iii

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. You will walk in a clockwise spiral shape to visit every position in this grid. Whenever you move outside the grid's boundary, we continue our walk outside the grid (but may return to the grid boundary later.). Eventually, we reach all rows * cols spaces of the grid. Return an array of coordinates representing the positions of the grid in the order you visited them.

Array,Matrix,Simulation

Medium

54,59

Possible Bipartition

possible-bipartition

We want to split a group of n people (labeled from 1 to n) into two groups of any size. Each person may dislike some other people, and they should not go into the same group. Given the integer n and the array dislikes where dislikes[i] = [ai, bi] indicates that the person labeled ai does not like the person labeled bi, return true if it is possible to split everyone into two groups in this way.

Depth-First Search,Breadth-First Search,Union Find,Graph

Medium

Super Egg Drop

super-egg-drop

You are given k identical eggs and you have access to a building with n floors labeled from 1 to n. You know that there exists a floor f where 0 <= f <= n such that any egg dropped at a floor higher than f will break, and any egg dropped at or below floor f will not break. Each move, you may take an unbroken egg and drop it from any floor x (where 1 <= x <= n). If the egg breaks, you can no longer use it. However, if the egg does not break, you may reuse it in future moves. Return the minimum number of moves that you need to determine with certainty what the value of f is.

Math,Binary Search,Dynamic Programming

Hard

2031

Fair Candy Swap

fair-candy-swap

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 has and bobSizes[j] is the number of candies of the jth box of candy that Bob has. Since they are friends, they would like to exchange one candy box each so that after the exchange, they both have the same total amount of candy. The total amount of candy a person has is the sum of the number of candies in each box they have. Return an integer array answer where answer[0] is the number of candies in the box that Alice must exchange, and answer[1] is the number of candies in the box that Bob must exchange. If there are multiple answers, you may return any one of them. It is guaranteed that at least one answer exists.

Array,Hash Table,Binary Search,Sorting

Easy

Construct Binary Tree from Preorder and Postorder Traversal

construct-binary-tree-from-preorder-and-postorder-traversal

Given two integer arrays, preorder and postorder where preorder is the preorder traversal of a binary tree of distinct values and postorder is the postorder traversal of the same tree, reconstruct and return the binary tree. If there exist multiple answers, you can return any of them.

Array,Hash Table,Divide and Conquer,Tree,Binary Tree

Medium

Find and Replace Pattern

find-and-replace-pattern

Given a list of strings words and a string pattern, return a list of words[i] that match pattern. You may return the answer in any order. A word matches the pattern if there exists a permutation of letters p so that after replacing every letter x in the pattern with p(x), we get the desired word. Recall that a permutation of letters is a bijection from letters to letters: every letter maps to another letter, and no two letters map to the same letter.

Array,Hash Table,String

Medium

Sum of Subsequence Widths

sum-of-subsequence-widths

The width of a sequence is the difference between the maximum and minimum elements in the sequence. Given an array of integers nums, return the sum of the widths of all the non-empty subsequences of nums. Since the answer may be very large, return it modulo 109 + 7. A subsequence is a sequence that can be derived from an array by deleting some or no elements without changing the order of the remaining elements. For example, [3,6,2,7] is a subsequence of the array [0,3,1,6,2,2,7].

Array,Math,Sorting

Hard

Surface Area of 3D Shapes

surface-area-of-3d-shapes

You are given an n x n grid where you have placed some 1 x 1 x 1 cubes. Each value v = grid[i][j] represents a tower of v cubes placed on top of cell (i, j). After placing these cubes, you have decided to glue any directly adjacent cubes to each other, forming several irregular 3D shapes. Return the total surface area of the resulting shapes. Note: The bottom face of each shape counts toward its surface area.

Array,Math,Geometry,Matrix

Easy

Groups of Special-Equivalent Strings

groups-of-special-equivalent-strings

You are given an array of strings of the same length words. In one move, you can swap any two even indexed characters or any two odd indexed characters of a string words[i]. Two strings words[i] and words[j] are special-equivalent if after any number of moves, words[i] == words[j]. A group of special-equivalent strings from words is a non-empty subset of words such that: Return the number of groups of special-equivalent strings from words.

Array,Hash Table,String

Medium

All Possible Full Binary Trees

all-possible-full-binary-trees

Given an integer n, return a list of all possible full binary trees with n nodes. Each node of each tree in the answer must have Node.val == 0. Each element of the answer is the root node of one possible tree. You may return the final list of trees in any order. A full binary tree is a binary tree where each node has exactly 0 or 2 children.

Dynamic Programming,Tree,Recursion,Memoization,Binary Tree

Medium

Maximum Frequency Stack

maximum-frequency-stack

Design a stack-like data structure to push elements to the stack and pop the most frequent element from the stack. Implement the FreqStack class:

Hash Table,Stack,Design,Ordered Set

Hard

Monotonic Array

monotonic-array

An array is monotonic if it is either monotone increasing or monotone decreasing. An array nums is monotone increasing if for all i <= j, nums[i] <= nums[j]. An array nums is monotone decreasing if for all i <= j, nums[i] >= nums[j]. Given an integer array nums, return true if the given array is monotonic, or false otherwise.

Array

Easy

2316

Increasing Order Search Tree

increasing-order-search-tree

Given the root of a binary search tree, rearrange the tree in in-order so that the leftmost node in the tree is now the root of the tree, and every node has no left child and only one right child.

Stack,Tree,Depth-First Search,Binary Search Tree,Binary Tree

Easy

Bitwise ORs of Subarrays

bitwise-ors-of-subarrays

We have an array arr of non-negative integers. For every (contiguous) subarray sub = [arr[i], arr[i + 1], ..., arr[j]] (with i <= j), we take the bitwise OR of all the elements in sub, obtaining a result arr[i] | arr[i + 1] | ... | arr[j]. Return the number of possible results. Results that occur more than once are only counted once in the final answer

Array,Dynamic Programming,Bit Manipulation

Medium