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Orderly Queue

orderly-queue

You are given a string s and an integer k. You can choose one of the first k letters of s and append it at the end of the string.. Return the lexicographically smallest string you could have after applying the mentioned step any number of moves.

Math,String,Sorting

Hard

RLE Iterator

rle-iterator

We can use run-length encoding (i.e., RLE) to encode a sequence of integers. In a run-length encoded array of even length encoding (0-indexed), for all even i, encoding[i] tells us the number of times that the non-negative integer value encoding[i + 1] is repeated in the sequence. Given a run-length encoded array, design an iterator that iterates through it. Implement the RLEIterator class:

Array,Design,Counting,Iterator

Medium

Online Stock Span

online-stock-span

Design an algorithm that collects daily price quotes for some stock and returns the span of that stock's price for the current day. The span of the stock's price today is defined as the maximum number of consecutive days (starting from today and going backward) for which the stock price was less than or equal to today's price. Implement the StockSpanner class:

Stack,Design,Monotonic Stack,Data Stream

Medium

739

Numbers At Most N Given Digit Set

numbers-at-most-n-given-digit-set

Given an array of digits which is sorted in non-decreasing order. You can write numbers using each digits[i] as many times as we want. For example, if digits = ['1','3','5'], we may write numbers such as '13', '551', and '1351315'. Return the number of positive integers that can be generated that are less than or equal to a given integer n.

Array,Math,Binary Search,Dynamic Programming

Hard

Valid Permutations for DI Sequence

valid-permutations-for-di-sequence

You are given a string s of length n where s[i] is either: A permutation perm of n + 1 integers of all the integers in the range [0, n] is called a valid permutation if for all valid i: Return the number of valid permutations perm. Since the answer may be large, return it modulo 109 + 7.

Dynamic Programming

Hard

Fruit Into Baskets

fruit-into-baskets

You are visiting a farm that has a single row of fruit trees arranged from left to right. The trees are represented by an integer array fruits where fruits[i] is the type of fruit the ith tree produces. You want to collect as much fruit as possible. However, the owner has some strict rules that you must follow: Given the integer array fruits, return the maximum number of fruits you can pick.

Array,Hash Table,Sliding Window

Medium

Sort Array By Parity

sort-array-by-parity

Given an integer array nums, move all the even integers at the beginning of the array followed by all the odd integers. Return any array that satisfies this condition.

Array,Two Pointers,Sorting

Easy

2283,2327

Super Palindromes

super-palindromes

Let's say a positive integer is a super-palindrome if it is a palindrome, and it is also the square of a palindrome. Given two positive integers left and right represented as strings, return the number of super-palindromes integers in the inclusive range [left, right].

Math,Enumeration

Hard

Sum of Subarray Minimums

sum-of-subarray-minimums

Given an array of integers arr, find the sum of min(b), where b ranges over every (contiguous) subarray of arr. Since the answer may be large, return the answer modulo 109 + 7.

Array,Dynamic Programming,Stack,Monotonic Stack

Medium

2227

Smallest Range I

smallest-range-i

You are given an integer array nums and an integer k. In one operation, you can choose any index i where 0 <= i < nums.length and change nums[i] to nums[i] + x where x is an integer from the range [-k, k]. You can apply this operation at most once for each index i. The score of nums is the difference between the maximum and minimum elements in nums. Return the minimum score of nums after applying the mentioned operation at most once for each index in it.

Array,Math

Easy

Snakes and Ladders

snakes-and-ladders

You are given an n x n integer matrix board where the cells are labeled from 1 to n2 in a Boustrophedon style starting from the bottom left of the board (i.e. board[n - 1][0]) and alternating direction each row. You start on square 1 of the board. In each move, starting from square curr, do the following: A board square on row r and column c has a snake or ladder if board[r][c] != -1. The destination of that snake or ladder is board[r][c]. Squares 1 and n2 do not have a snake or ladder. Note that you only take a snake or ladder at most once per move. If the destination to a snake or ladder is the start of another snake or ladder, you do not follow the subsequent snake or ladder. Return the least number of moves required to reach the square n2. If it is not possible to reach the square, return -1.

Array,Breadth-First Search,Matrix

Medium

Smallest Range II

smallest-range-ii

You are given an integer array nums and an integer k. For each index i where 0 <= i < nums.length, change nums[i] to be either nums[i] + k or nums[i] - k. The score of nums is the difference between the maximum and minimum elements in nums. Return the minimum score of nums after changing the values at each index.

Array,Math,Greedy,Sorting

Medium

Online Election

online-election

You are given two integer arrays persons and times. In an election, the ith vote was cast for persons[i] at time times[i]. For each query at a time t, find the person that was leading the election at time t. Votes cast at time t will count towards our query. In the case of a tie, the most recent vote (among tied candidates) wins. Implement the TopVotedCandidate class:

Array,Hash Table,Binary Search,Design

Medium

1483

Sort an Array

sort-an-array

Given an array of integers nums, sort the array in ascending order.

Array,Divide and Conquer,Sorting,Heap (Priority Queue),Merge Sort,Bucket Sort,Radix Sort,Counting Sort

Medium

Cat and Mouse

cat-and-mouse

A game on an undirected graph is played by two players, Mouse and Cat, who alternate turns. The graph is given as follows: graph[a] is a list of all nodes b such that ab is an edge of the graph. The mouse starts at node 1 and goes first, the cat starts at node 2 and goes second, and there is a hole at node 0. During each player's turn, they must travel along one edge of the graph that meets where they are.  For example, if the Mouse is at node 1, it must travel to any node in graph[1]. Additionally, it is not allowed for the Cat to travel to the Hole (node 0.) Then, the game can end in three ways: Given a graph, and assuming both players play optimally, return

Math,Dynamic Programming,Breadth-First Search,Graph,Memoization,Game Theory

Hard

1727

X of a Kind in a Deck of Cards

x-of-a-kind-in-a-deck-of-cards

In a deck of cards, each card has an integer written on it. Return true if and only if you can choose X >= 2 such that it is possible to split the entire deck into 1 or more groups of cards, where:

Array,Hash Table,Math,Counting,Number Theory

Easy

Partition Array into Disjoint Intervals

partition-array-into-disjoint-intervals

Given an integer array nums, partition it into two (contiguous) subarrays left and right so that: Return the length of left after such a partitioning. Test cases are generated such that partitioning exists.

Array

Medium

2138

Word Subsets

word-subsets

You are given two string arrays words1 and words2. A string b is a subset of string a if every letter in b occurs in a including multiplicity. A string a from words1 is universal if for every string b in words2, b is a subset of a. Return an array of all the universal strings in words1. You may return the answer in any order.

Array,Hash Table,String

Medium

Reverse Only Letters

reverse-only-letters

Given a string s, reverse the string according to the following rules: Return s after reversing it.

Two Pointers,String

Easy

This problem is exactly like reversing a normal string except that there are certain characters that we have to simply skip. That should be easy enough to do if you know how to reverse a string using the two-pointer approach.

Maximum Sum Circular Subarray

maximum-sum-circular-subarray

Given a circular integer array nums of length n, return the maximum possible sum of a non-empty subarray of nums. A circular array means the end of the array connects to the beginning of the array. Formally, the next element of nums[i] is nums[(i + 1) % n] and the previous element of nums[i] is nums[(i - 1 + n) % n]. A subarray may only include each element of the fixed buffer nums at most once. Formally, for a subarray nums[i], nums[i + 1], ..., nums[j], there does not exist i <= k1, k2 <= j with k1 % n == k2 % n.

Array,Divide and Conquer,Dynamic Programming,Queue,Monotonic Queue

Medium

For those of you who are familiar with the Kadane's algorithm, think in terms of that. For the newbies, Kadane's algorithm is used to finding the maximum sum subarray from a given array. This problem is a twist on that idea and it is advisable to read up on that algorithm first before starting this problem. Unless you already have a great algorithm brewing up in your mind in which case, go right ahead! What is an alternate way of representing a circular array so that it appears to be a straight array? Essentially, there are two cases of this problem that we need to take care of. Let's look at the figure below to understand those two cases: The first case can be handled by the good old Kadane's algorithm. However, is there a smarter way of going about handling the second case as well?

Complete Binary Tree Inserter

complete-binary-tree-inserter

A complete binary tree is a binary tree in which every level, except possibly the last, is completely filled, and all nodes are as far left as possible. Design an algorithm to insert a new node to a complete binary tree keeping it complete after the insertion. Implement the CBTInserter class:

Tree,Breadth-First Search,Design,Binary Tree

Medium

Number of Music Playlists

number-of-music-playlists

Your music player contains n different songs. You want to listen to goal songs (not necessarily different) during your trip. To avoid boredom, you will create a playlist so that: Given n, goal, and k, return the number of possible playlists that you can create. Since the answer can be very large, return it modulo 109 + 7.

Math,Dynamic Programming,Combinatorics

Hard

Minimum Add to Make Parentheses Valid

minimum-add-to-make-parentheses-valid

A parentheses string is valid if and only if: You are given a parentheses string s. In one move, you can insert a parenthesis at any position of the string. Return the minimum number of moves required to make s valid.

String,Stack,Greedy

Medium

2095

Sort Array By Parity II

sort-array-by-parity-ii

Given an array of integers nums, half of the integers in nums are odd, and the other half are even. Sort the array so that whenever nums[i] is odd, i is odd, and whenever nums[i] is even, i is even. Return any answer array that satisfies this condition.

Array,Two Pointers,Sorting

Easy

2271,2283,2327

3Sum With Multiplicity

3sum-with-multiplicity

Given an integer array arr, and an integer target, return the number of tuples i, j, k such that i < j < k and arr[i] + arr[j] + arr[k] == target. As the answer can be very large, return it modulo 109 + 7.

Array,Hash Table,Two Pointers,Sorting,Counting

Medium

Minimize Malware Spread

minimize-malware-spread

You are given a network of n nodes represented as an n x n adjacency matrix graph, where the ith node is directly connected to the jth node if graph[i][j] == 1. Some nodes initial are initially infected by malware. Whenever two nodes are directly connected, and at least one of those two nodes is infected by malware, both nodes will be infected by malware. This spread of malware will continue until no more nodes can be infected in this manner. Suppose M(initial) is the final number of nodes infected with malware in the entire network after the spread of malware stops. We will remove exactly one node from initial. Return the node that, if removed, would minimize M(initial). If multiple nodes could be removed to minimize M(initial), return such a node with the smallest index. Note that if a node was removed from the initial list of infected nodes, it might still be infected later due to the malware spread.

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

Hard

Long Pressed Name

long-pressed-name

Your friend is typing his name into a keyboard. Sometimes, when typing a character c, the key might get long pressed, and the character will be typed 1 or more times. You examine the typed characters of the keyboard. Return True if it is possible that it was your friends name, with some characters (possibly none) being long pressed.

Two Pointers,String

Easy

Flip String to Monotone Increasing

flip-string-to-monotone-increasing

A binary string is monotone increasing if it consists of some number of 0's (possibly none), followed by some number of 1's (also possibly none). You are given a binary string s. You can flip s[i] changing it from 0 to 1 or from 1 to 0. Return the minimum number of flips to make s monotone increasing.

String,Dynamic Programming

Medium

Three Equal Parts

three-equal-parts

You are given an array arr which consists of only zeros and ones, divide the array into three non-empty parts such that all of these parts represent the same binary value. If it is possible, return any [i, j] with i + 1 < j, such that: If it is not possible, return [-1, -1]. Note that the entire part is used when considering what binary value it represents. For example, [1,1,0] represents 6 in decimal, not 3. Also, leading zeros are allowed, so [0,1,1] and [1,1] represent the same value.

Array,Math

Hard

Minimize Malware Spread II

minimize-malware-spread-ii

You are given a network of n nodes represented as an n x n adjacency matrix graph, where the ith node is directly connected to the jth node if graph[i][j] == 1. Some nodes initial are initially infected by malware. Whenever two nodes are directly connected, and at least one of those two nodes is infected by malware, both nodes will be infected by malware. This spread of malware will continue until no more nodes can be infected in this manner. Suppose M(initial) is the final number of nodes infected with malware in the entire network after the spread of malware stops. We will remove exactly one node from initial, completely removing it and any connections from this node to any other node. Return the node that, if removed, would minimize M(initial). If multiple nodes could be removed to minimize M(initial), return such a node with the smallest index.

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

Hard

Unique Email Addresses

unique-email-addresses

Every valid email consists of a local name and a domain name, separated by the '@' sign. Besides lowercase letters, the email may contain one or more '.' or '+'. If you add periods '.' between some characters in the local name part of an email address, mail sent there will be forwarded to the same address without dots in the local name. Note that this rule does not apply to domain names. If you add a plus '+' in the local name, everything after the first plus sign will be ignored. This allows certain emails to be filtered. Note that this rule does not apply to domain names. It is possible to use both of these rules at the same time. Given an array of strings emails where we send one email to each emails[i], return the number of different addresses that actually receive mails.

Array,Hash Table,String

Easy

Binary Subarrays With Sum

binary-subarrays-with-sum

Given a binary array nums and an integer goal, return the number of non-empty subarrays with a sum goal. A subarray is a contiguous part of the array.

Array,Hash Table,Sliding Window,Prefix Sum

Medium

Minimum Falling Path Sum

minimum-falling-path-sum

Given an n x n array of integers matrix, return the minimum sum of any falling path through matrix. A falling path starts at any element in the first row and chooses the element in the next row that is either directly below or diagonally left/right. Specifically, the next element from position (row, col) will be (row + 1, col - 1), (row + 1, col), or (row + 1, col + 1).

Array,Dynamic Programming,Matrix

Medium

1224

Beautiful Array

beautiful-array

An array nums of length n is beautiful if: Given the integer n, return any beautiful array nums of length n. There will be at least one valid answer for the given n.

Array,Math,Divide and Conquer

Medium

Number of Recent Calls

number-of-recent-calls

You have a RecentCounter class which counts the number of recent requests within a certain time frame. Implement the RecentCounter class: It is guaranteed that every call to ping uses a strictly larger value of t than the previous call.

Design,Queue,Data Stream

Easy

Shortest Bridge

shortest-bridge

You are given an n x n binary matrix grid where 1 represents land and 0 represents water. An island is a 4-directionally connected group of 1's not connected to any other 1's. There are exactly two islands in grid. You may change 0's to 1's to connect the two islands to form one island. Return the smallest number of 0's you must flip to connect the two islands.

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

Medium

Knight Dialer

knight-dialer

The chess knight has a unique movement, it may move two squares vertically and one square horizontally, or two squares horizontally and one square vertically (with both forming the shape of an L). The possible movements of chess knight are shown in this diagaram: A chess knight can move as indicated in the chess diagram below: We have a chess knight and a phone pad as shown below, the knight can only stand on a numeric cell (i.e. blue cell). Given an integer n, return how many distinct phone numbers of length n we can dial. You are allowed to place the knight on any numeric cell initially and then you should perform n - 1 jumps to dial a number of length n. All jumps should be valid knight jumps. As the answer may be very large, return the answer modulo 109 + 7.

Dynamic Programming

Medium

Stamping The Sequence

stamping-the-sequence

You are given two strings stamp and target. Initially, there is a string s of length target.length with all s[i] == '?'. In one turn, you can place stamp over s and replace every letter in the s with the corresponding letter from stamp. We want to convert s to target using at most 10 * target.length turns. Return an array of the index of the left-most letter being stamped at each turn. If we cannot obtain target from s within 10 * target.length turns, return an empty array.

String,Stack,Greedy,Queue

Hard

Reorder Data in Log Files

reorder-data-in-log-files

You are given an array of logs. Each log is a space-delimited string of words, where the first word is the identifier. There are two types of logs: Reorder these logs so that: Return the final order of the logs.

Array,String,Sorting

Easy

Range Sum of BST

range-sum-of-bst

Given the root node of a binary search tree and two integers low and high, return the sum of values of all nodes with a value in the inclusive range [low, high].

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

Easy

Minimum Area Rectangle

minimum-area-rectangle

You are given an array of points in the X-Y plane points where points[i] = [xi, yi]. Return the minimum area of a rectangle formed from these points, with sides parallel to the X and Y axes. If there is not any such rectangle, return 0.

Array,Hash Table,Math,Geometry,Sorting

Medium

Distinct Subsequences II

distinct-subsequences-ii

Given a string s, return the number of distinct non-empty subsequences of s. Since the answer may be very large, return it modulo 109 + 7.

String,Dynamic Programming

Hard

2115

Valid Mountain Array

valid-mountain-array

Given an array of integers arr, return true if and only if it is a valid mountain array. Recall that arr is a mountain array if and only if:

Array

Easy

It's very easy to keep track of a monotonically increasing or decreasing ordering of elements. You just need to be able to determine the start of the valley in the mountain and from that point onwards, it should be a valley i.e. no mini-hills after that. Use this information in regards to the values in the array and you will be able to come up with a straightforward solution.

1766

DI String Match

di-string-match

A permutation perm of n + 1 integers of all the integers in the range [0, n] can be represented as a string s of length n where: Given a string s, reconstruct the permutation perm and return it. If there are multiple valid permutations perm, return any of them.

Array,Math,Two Pointers,String,Greedy

Easy

Find the Shortest Superstring

find-the-shortest-superstring

Given an array of strings words, return the smallest string that contains each string in words as a substring. If there are multiple valid strings of the smallest length, return any of them. You may assume that no string in words is a substring of another string in words.

Array,String,Dynamic Programming,Bit Manipulation,Bitmask

Hard

Delete Columns to Make Sorted

delete-columns-to-make-sorted

You are given an array of n strings strs, all of the same length. The strings can be arranged such that there is one on each line, making a grid. For example, strs = ["abc", "bce", "cae"] can be arranged as: You want to delete the columns that are not sorted lexicographically. In the above example (0-indexed), columns 0 ('a', 'b', 'c') and 2 ('c', 'e', 'e') are sorted while column 1 ('b', 'c', 'a') is not, so you would delete column 1. Return the number of columns that you will delete.

Array,String

Easy

Minimum Increment to Make Array Unique

minimum-increment-to-make-array-unique

You are given an integer array nums. In one move, you can pick an index i where 0 <= i < nums.length and increment nums[i] by 1. Return the minimum number of moves to make every value in nums unique.

Array,Greedy,Sorting,Counting

Medium

2329

Validate Stack Sequences

validate-stack-sequences

Given two integer arrays pushed and popped each with distinct values, return true if this could have been the result of a sequence of push and pop operations on an initially empty stack, or false otherwise.

Array,Stack,Simulation

Medium

Most Stones Removed with Same Row or Column

most-stones-removed-with-same-row-or-column

On a 2D plane, we place n stones at some integer coordinate points. Each coordinate point may have at most one stone. A stone can be removed if it shares either the same row or the same column as another stone that has not been removed. Given an array stones of length n where stones[i] = [xi, yi] represents the location of the ith stone, return the largest possible number of stones that can be removed.

Depth-First Search,Union Find,Graph

Medium

Bag of Tokens

bag-of-tokens

You have an initial power of power, an initial score of 0, and a bag of tokens where tokens[i] is the value of the ith token (0-indexed). Your goal is to maximize your total score by potentially playing each token in one of two ways: Each token may be played at most once and in any order. You do not have to play all the tokens. Return the largest possible score you can achieve after playing any number of tokens.

Array,Two Pointers,Greedy,Sorting

Medium

Largest Time for Given Digits

largest-time-for-given-digits

Given an array arr of 4 digits, find the latest 24-hour time that can be made using each digit exactly once. 24-hour times are formatted as "HH:MM", where HH is between 00 and 23, and MM is between 00 and 59. The earliest 24-hour time is 00:00, and the latest is 23:59. Return the latest 24-hour time in "HH:MM" format. If no valid time can be made, return an empty string.

String,Enumeration

Medium

Reveal Cards In Increasing Order

reveal-cards-in-increasing-order

You are given an integer array deck. There is a deck of cards where every card has a unique integer. The integer on the ith card is deck[i]. You can order the deck in any order you want. Initially, all the cards start face down (unrevealed) in one deck. You will do the following steps repeatedly until all cards are revealed: Return an ordering of the deck that would reveal the cards in increasing order. Note that the first entry in the answer is considered to be the top of the deck.

Array,Queue,Sorting,Simulation

Medium

Flip Equivalent Binary Trees

flip-equivalent-binary-trees

For a binary tree T, we can define a flip operation as follows: choose any node, and swap the left and right child subtrees. A binary tree X is flip equivalent to a binary tree Y if and only if we can make X equal to Y after some number of flip operations. Given the roots of two binary trees root1 and root2, return true if the two trees are flip equivalent or false otherwise.

Tree,Depth-First Search,Binary Tree

Medium

Largest Component Size by Common Factor

largest-component-size-by-common-factor

You are given an integer array of unique positive integers nums. Consider the following graph: Return the size of the largest connected component in the graph.

Array,Math,Union Find

Hard

2276

Verifying an Alien Dictionary

verifying-an-alien-dictionary

In an alien language, surprisingly, they also use English lowercase letters, but possibly in a different order. The order of the alphabet is some permutation of lowercase letters. Given a sequence of words written in the alien language, and the order of the alphabet, return true if and only if the given words are sorted lexicographically in this alien language.

Array,Hash Table,String

Easy

Array of Doubled Pairs

array-of-doubled-pairs

Given an integer array of even length arr, return true if it is possible to reorder arr such that arr[2 * i + 1] = 2 * arr[2 * i] for every 0 <= i < len(arr) / 2, or false otherwise.

Array,Hash Table,Greedy,Sorting

Medium

2117

Delete Columns to Make Sorted II

delete-columns-to-make-sorted-ii

You are given an array of n strings strs, all of the same length. We may choose any deletion indices, and we delete all the characters in those indices for each string. For example, if we have strs = ["abcdef","uvwxyz"] and deletion indices {0, 2, 3}, then the final array after deletions is ["bef", "vyz"]. Suppose we chose a set of deletion indices answer such that after deletions, the final array has its elements in lexicographic order (i.e., strs[0] <= strs[1] <= strs[2] <= ... <= strs[n - 1]). Return the minimum possible value of answer.length.

Array,String,Greedy

Medium

Tallest Billboard

tallest-billboard

You are installing a billboard and want it to have the largest height. The billboard will have two steel supports, one on each side. Each steel support must be an equal height. You are given a collection of rods that can be welded together. For example, if you have rods of lengths 1, 2, and 3, you can weld them together to make a support of length 6. Return the largest possible height of your billboard installation. If you cannot support the billboard, return 0.

Array,Dynamic Programming

Hard

2162

Prison Cells After N Days

prison-cells-after-n-days

There are 8 prison cells in a row and each cell is either occupied or vacant. Each day, whether the cell is occupied or vacant changes according to the following rules: Note that because the prison is a row, the first and the last cells in the row can't have two adjacent neighbors. You are given an integer array cells where cells[i] == 1 if the ith cell is occupied and cells[i] == 0 if the ith cell is vacant, and you are given an integer n. Return the state of the prison after n days (i.e., n such changes described above).

Array,Hash Table,Math,Bit Manipulation

Medium

Check Completeness of a Binary Tree

check-completeness-of-a-binary-tree

Given the root of a binary tree, determine if it is a complete binary tree. In a complete binary tree, every level, except possibly the last, is completely filled, and all nodes in the last level are as far left as possible. It can have between 1 and 2h nodes inclusive at the last level h.

Tree,Breadth-First Search,Binary Tree

Medium

Regions Cut By Slashes

regions-cut-by-slashes

An n x n grid is composed of 1 x 1 squares where each 1 x 1 square consists of a '/', '\', or blank space ' '. These characters divide the square into contiguous regions. Given the grid grid represented as a string array, return the number of regions. Note that backslash characters are escaped, so a '\' is represented as '\\'.

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

Medium

Delete Columns to Make Sorted III

delete-columns-to-make-sorted-iii

You are given an array of n strings strs, all of the same length. We may choose any deletion indices, and we delete all the characters in those indices for each string. For example, if we have strs = ["abcdef","uvwxyz"] and deletion indices {0, 2, 3}, then the final array after deletions is ["bef", "vyz"]. Suppose we chose a set of deletion indices answer such that after deletions, the final array has every string (row) in lexicographic order. (i.e., (strs[0][0] <= strs[0][1] <= ... <= strs[0][strs[0].length - 1]), and (strs[1][0] <= strs[1][1] <= ... <= strs[1][strs[1].length - 1]), and so on). Return the minimum possible value of answer.length.

Array,String,Dynamic Programming

Hard

N-Repeated Element in Size 2N Array

n-repeated-element-in-size-2n-array

You are given an integer array nums with the following properties: Return the element that is repeated n times.

Array,Hash Table

Easy

Maximum Width Ramp

maximum-width-ramp

A ramp in an integer array nums is a pair (i, j) for which i < j and nums[i] <= nums[j]. The width of such a ramp is j - i. Given an integer array nums, return the maximum width of a ramp in nums. If there is no ramp in nums, return 0.

Array,Stack,Monotonic Stack

Medium

Minimum Area Rectangle II

minimum-area-rectangle-ii

You are given an array of points in the X-Y plane points where points[i] = [xi, yi]. Return the minimum area of any rectangle formed from these points, with sides not necessarily parallel to the X and Y axes. If there is not any such rectangle, return 0. Answers within 10-5 of the actual answer will be accepted.

Array,Math,Geometry

Medium

Least Operators to Express Number

least-operators-to-express-number

Given a single positive integer x, we will write an expression of the form x (op1) x (op2) x (op3) x ... where each operator op1, op2, etc. is either addition, subtraction, multiplication, or division (+, -, *, or /). For example, with x = 3, we might write 3 * 3 / 3 + 3 - 3 which is a value of 3. When writing such an expression, we adhere to the following conventions: We would like to write an expression with the least number of operators such that the expression equals the given target. Return the least number of operators used.

Math,Dynamic Programming

Hard

Univalued Binary Tree

univalued-binary-tree

A binary tree is uni-valued if every node in the tree has the same value. Given the root of a binary tree, return true if the given tree is uni-valued, or false otherwise.

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

Easy

1609

Vowel Spellchecker

vowel-spellchecker

Given a wordlist, we want to implement a spellchecker that converts a query word into a correct word. For a given query word, the spell checker handles two categories of spelling mistakes: In addition, the spell checker operates under the following precedence rules: Given some queries, return a list of words answer, where answer[i] is the correct word for query = queries[i].

Array,Hash Table,String

Medium

Numbers With Same Consecutive Differences

numbers-with-same-consecutive-differences

Return all non-negative integers of length n such that the absolute difference between every two consecutive digits is k. Note that every number in the answer must not have leading zeros. For example, 01 has one leading zero and is invalid. You may return the answer in any order.

Backtracking,Breadth-First Search

Medium

Binary Tree Cameras

binary-tree-cameras

You are given the root of a binary tree. We install cameras on the tree nodes where each camera at a node can monitor its parent, itself, and its immediate children. Return the minimum number of cameras needed to monitor all nodes of the tree.

Dynamic Programming,Tree,Depth-First Search,Binary Tree

Hard

1021

Pancake Sorting

pancake-sorting

Given an array of integers arr, sort the array by performing a series of pancake flips. In one pancake flip we do the following steps: For example, if arr = [3,2,1,4] and we performed a pancake flip choosing k = 3, we reverse the sub-array [3,2,1], so arr = [1,2,3,4] after the pancake flip at k = 3. Return an array of the k-values corresponding to a sequence of pancake flips that sort arr. Any valid answer that sorts the array within 10 * arr.length flips will be judged as correct.

Array,Two Pointers,Greedy,Sorting

Medium

Powerful Integers

powerful-integers

Given three integers x, y, and bound, return a list of all the powerful integers that have a value less than or equal to bound. An integer is powerful if it can be represented as xi + yj for some integers i >= 0 and j >= 0. You may return the answer in any order. In your answer, each value should occur at most once.

Hash Table,Math

Medium

Flip Binary Tree To Match Preorder Traversal

flip-binary-tree-to-match-preorder-traversal

You are given the root of a binary tree with n nodes, where each node is uniquely assigned a value from 1 to n. You are also given a sequence of n values voyage, which is the desired pre-order traversal of the binary tree. Any node in the binary tree can be flipped by swapping its left and right subtrees. For example, flipping node 1 will have the following effect: Flip the smallest number of nodes so that the pre-order traversal of the tree matches voyage. Return a list of the values of all flipped nodes. You may return the answer in any order. If it is impossible to flip the nodes in the tree to make the pre-order traversal match voyage, return the list [-1].

Tree,Depth-First Search,Binary Tree

Medium

Equal Rational Numbers

equal-rational-numbers

Given two strings s and t, each of which represents a non-negative rational number, return true if and only if they represent the same number. The strings may use parentheses to denote the repeating part of the rational number. A rational number can be represented using up to three parts: <IntegerPart>, <NonRepeatingPart>, and a <RepeatingPart>. The number will be represented in one of the following three ways: The repeating portion of a decimal expansion is conventionally denoted within a pair of round brackets. For example:

Math,String

Hard

Fibonacci Number

fibonacci-number

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, Given n, calculate F(n).

Math,Dynamic Programming,Recursion,Memoization

Easy

70,872,905,1236

K Closest Points to Origin

k-closest-points-to-origin

Given an array of points where points[i] = [xi, yi] represents a point on the X-Y plane and an integer k, return the k closest points to the origin (0, 0). The distance between two points on the X-Y plane is the Euclidean distance (i.e., √(x1 - x2)2 + (y1 - y2)2). You may return the answer in any order. The answer is guaranteed to be unique (except for the order that it is in).

Array,Math,Divide and Conquer,Geometry,Sorting,Heap (Priority Queue),Quickselect

Medium

215,347,692,1888

Subarray Sums Divisible by K

subarray-sums-divisible-by-k

Given an integer array nums and an integer k, return the number of non-empty subarrays that have a sum divisible by k. A subarray is a contiguous part of an array.

Array,Hash Table,Prefix Sum

Medium

560,1694

Odd Even Jump

odd-even-jump

You are given an integer array arr. From some starting index, you can make a series of jumps. The (1st, 3rd, 5th, ...) jumps in the series are called odd-numbered jumps, and the (2nd, 4th, 6th, ...) jumps in the series are called even-numbered jumps. Note that the jumps are numbered, not the indices. You may jump forward from index i to index j (with i < j) in the following way: A starting index is good if, starting from that index, you can reach the end of the array (index arr.length - 1) by jumping some number of times (possibly 0 or more than once). Return the number of good starting indices.

Array,Dynamic Programming,Stack,Monotonic Stack,Ordered Set

Hard

Largest Perimeter Triangle

largest-perimeter-triangle

Given an integer array nums, return the largest perimeter of a triangle with a non-zero area, formed from three of these lengths. If it is impossible to form any triangle of a non-zero area, return 0.

Array,Math,Greedy,Sorting

Easy

830

Squares of a Sorted Array

squares-of-a-sorted-array

Given an integer array nums sorted in non-decreasing order, return an array of the squares of each number sorted in non-decreasing order.

Array,Two Pointers,Sorting

Easy

88,360

Longest Turbulent Subarray

longest-turbulent-subarray

Given an integer array arr, return the length of a maximum size turbulent subarray of arr. A subarray is turbulent if the comparison sign flips between each adjacent pair of elements in the subarray. More formally, a subarray [arr[i], arr[i + 1], ..., arr[j]] of arr is said to be turbulent if and only if:

Array,Dynamic Programming,Sliding Window

Medium

53

Distribute Coins in Binary Tree

distribute-coins-in-binary-tree

You are given the root of a binary tree with n nodes where each node in the tree has node.val coins. There are n coins in total throughout the whole tree. In one move, we may choose two adjacent nodes and move one coin from one node to another. A move may be from parent to child, or from child to parent. Return the minimum number of moves required to make every node have exactly one coin.

Tree,Depth-First Search,Binary Tree

Medium

863,1008

Unique Paths III

unique-paths-iii

You are given an m x n integer array grid where grid[i][j] could be: Return the number of 4-directional walks from the starting square to the ending square, that walk over every non-obstacle square exactly once.

Array,Backtracking,Bit Manipulation,Matrix

Hard

37,63,212

Time Based Key-Value Store

time-based-key-value-store

Design a time-based key-value data structure that can store multiple values for the same key at different time stamps and retrieve the key's value at a certain timestamp. Implement the TimeMap class:

Hash Table,String,Binary Search,Design

Medium

2161

Triples with Bitwise AND Equal To Zero

triples-with-bitwise-and-equal-to-zero

Given an integer array nums, return the number of AND triples. An AND triple is a triple of indices (i, j, k) such that:

Array,Hash Table,Bit Manipulation

Hard

Minimum Cost For Tickets

minimum-cost-for-tickets

You have planned some train traveling one year in advance. The days of the year in which you will travel are given as an integer array days. Each day is an integer from 1 to 365. Train tickets are sold in three different ways: The passes allow that many days of consecutive travel. Return the minimum number of dollars you need to travel every day in the given list of days.

Array,Dynamic Programming

Medium

322

String Without AAA or BBB

string-without-aaa-or-bbb

Given two integers a and b, return any string s such that:

String,Greedy

Medium

Sum of Even Numbers After Queries

sum-of-even-numbers-after-queries

You are given an integer array nums and an array queries where queries[i] = [vali, indexi]. For each query i, first, apply nums[indexi] = nums[indexi] + vali, then print the sum of the even values of nums. Return an integer array answer where answer[i] is the answer to the ith query.

Array,Simulation

Medium

Interval List Intersections

interval-list-intersections

You are given two lists of closed intervals, firstList and secondList, where firstList[i] = [starti, endi] and secondList[j] = [startj, endj]. Each list of intervals is pairwise disjoint and in sorted order. Return the intersection of these two interval lists. A closed interval [a, b] (with a <= b) denotes the set of real numbers x with a <= x <= b. The intersection of two closed intervals is a set of real numbers that are either empty or represented as a closed interval. For example, the intersection of [1, 3] and [2, 4] is [2, 3].

Array,Two Pointers

Medium

56,88,761

Vertical Order Traversal of a Binary Tree

vertical-order-traversal-of-a-binary-tree

Given the root of a binary tree, calculate the vertical order traversal of the binary tree. For each node at position (row, col), its left and right children will be at positions (row + 1, col - 1) and (row + 1, col + 1) respectively. The root of the tree is at (0, 0). The vertical order traversal of a binary tree is a list of top-to-bottom orderings for each column index starting from the leftmost column and ending on the rightmost column. There may be multiple nodes in the same row and same column. In such a case, sort these nodes by their values. Return the vertical order traversal of the binary tree.

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

Hard

Smallest String Starting From Leaf

smallest-string-starting-from-leaf

You are given the root of a binary tree where each node has a value in the range [0, 25] representing the letters 'a' to 'z'. Return the lexicographically smallest string that starts at a leaf of this tree and ends at the root. As a reminder, any shorter prefix of a string is lexicographically smaller. A leaf of a node is a node that has no children.

String,Tree,Depth-First Search,Binary Tree

Medium

129,257

Add to Array-Form of Integer

add-to-array-form-of-integer

The array-form of an integer num is an array representing its digits in left to right order. Given num, the array-form of an integer, and an integer k, return the array-form of the integer num + k.

Array,Math

Easy

2,66,67,415

Satisfiability of Equality Equations

satisfiability-of-equality-equations

You are given an array of strings equations that represent relationships between variables where each string equations[i] is of length 4 and takes one of two different forms: "xi==yi" or "xi!=yi".Here, xi and yi are lowercase letters (not necessarily different) that represent one-letter variable names. Return true if it is possible to assign integers to variable names so as to satisfy all the given equations, or false otherwise.

Array,String,Union Find,Graph

Medium

Broken Calculator

broken-calculator

There is a broken calculator that has the integer startValue on its display initially. In one operation, you can: Given two integers startValue and target, return the minimum number of operations needed to display target on the calculator.

Math,Greedy

Medium

650

Subarrays with K Different Integers

subarrays-with-k-different-integers

Given an integer array nums and an integer k, return the number of good subarrays of nums. A good array is an array where the number of different integers in that array is exactly k. A subarray is a contiguous part of an array.

Array,Hash Table,Sliding Window,Counting

Hard

3,159,340,2186,2247

Cousins in Binary Tree

cousins-in-binary-tree

Given the root of a binary tree with unique values and the values of two different nodes of the tree x and y, return true if the nodes corresponding to the values x and y in the tree are cousins, or false otherwise. Two nodes of a binary tree are cousins if they have the same depth with different parents. Note that in a binary tree, the root node is at the depth 0, and children of each depth k node are at the depth k + 1.

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

Easy

102

Rotting Oranges

rotting-oranges

You are given an m x n grid where each cell can have one of three values: Every minute, any fresh orange that is 4-directionally adjacent to a rotten orange becomes rotten. Return the minimum number of minutes that must elapse until no cell has a fresh orange. If this is impossible, return -1.

Array,Breadth-First Search,Matrix

Medium

286,2206

Minimum Number of K Consecutive Bit Flips

minimum-number-of-k-consecutive-bit-flips

You are given a binary array nums and an integer k. A k-bit flip is choosing a subarray of length k from nums and simultaneously changing every 0 in the subarray to 1, and every 1 in the subarray to 0. Return the minimum number of k-bit flips required so that there is no 0 in the array. If it is not possible, return -1. A subarray is a contiguous part of an array.

Array,Bit Manipulation,Sliding Window,Prefix Sum

Hard

319,2286

Number of Squareful Arrays

number-of-squareful-arrays

An array is squareful if the sum of every pair of adjacent elements is a perfect square. Given an integer array nums, return the number of permutations of nums that are squareful. Two permutations perm1 and perm2 are different if there is some index i such that perm1[i] != perm2[i].

Array,Math,Dynamic Programming,Backtracking,Bit Manipulation,Bitmask

Hard

47

Find the Town Judge

find-the-town-judge

In a town, there are n people labeled from 1 to n. There is a rumor that one of these people is secretly the town judge. If the town judge exists, then: You are given an array trust where trust[i] = [ai, bi] representing that the person labeled ai trusts the person labeled bi. Return the label of the town judge if the town judge exists and can be identified, or return -1 otherwise.

Array,Hash Table,Graph

Easy

277