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Self Dividing Numbers

self-dividing-numbers

A self-dividing number is a number that is divisible by every digit it contains. A self-dividing number is not allowed to contain the digit zero. Given two integers left and right, return a list of all the self-dividing numbers in the range [left, right].

Math

Easy

For each number in the range, check whether it is self dividing by converting that number to a character array (or string in Python), then checking that each digit is nonzero and divides the original number.

507

My Calendar I

my-calendar-i

You are implementing a program to use as your calendar. We can add a new event if adding the event will not cause a double booking. A double booking happens when two events have some non-empty intersection (i.e., some moment is common to both events.). The event can be represented as a pair of integers start and end that represents a booking on the half-open interval [start, end), the range of real numbers x such that start <= x < end. Implement the MyCalendar class:

Design,Segment Tree,Ordered Set

Medium

Store the events as a sorted list of intervals. If none of the events conflict, then the new event can be added.

731,732

Count Different Palindromic Subsequences

count-different-palindromic-subsequences

Given a string s, return the number of different non-empty palindromic subsequences in s. Since the answer may be very large, return it modulo 109 + 7. A subsequence of a string is obtained by deleting zero or more characters from the string. A sequence is palindromic if it is equal to the sequence reversed. Two sequences a1, a2, ... and b1, b2, ... are different if there is some i for which ai != bi.

String,Dynamic Programming

Hard

Let dp(i, j) be the answer for the string T = S[i:j+1] including the empty sequence. The answer is the number of unique characters in T, plus palindromes of the form "a_a", "b_b", "c_c", and "d_d", where "_" represents zero or more characters.

516

My Calendar II

my-calendar-ii

You are implementing a program to use as your calendar. We can add a new event if adding the event will not cause a triple booking. A triple booking happens when three events have some non-empty intersection (i.e., some moment is common to all the three events.). The event can be represented as a pair of integers start and end that represents a booking on the half-open interval [start, end), the range of real numbers x such that start <= x < end. Implement the MyCalendarTwo class:

Design,Segment Tree,Ordered Set

Medium

Store two sorted lists of intervals: one list will be all times that are at least single booked, and another list will be all times that are definitely double booked. If none of the double bookings conflict, then the booking will succeed, and you should update your single and double bookings accordingly.

729,732

My Calendar III

my-calendar-iii

A k-booking happens when k events have some non-empty intersection (i.e., there is some time that is common to all k events.) You are given some events [start, end), after each given event, return an integer k representing the maximum k-booking between all the previous events. Implement the MyCalendarThree class:

Design,Segment Tree,Ordered Set

Hard

Treat each interval [start, end) as two events "start" and "end", and process them in sorted order.

729,731

Flood Fill

flood-fill

An image is represented by an m x n integer grid image where image[i][j] represents the pixel value of the image. You are also given three integers sr, sc, and newColor. You should perform a flood fill on the image starting from the pixel image[sr][sc]. To perform a flood fill, consider the starting pixel, plus any pixels connected 4-directionally to the starting pixel of the same color as the starting pixel, plus any pixels connected 4-directionally to those pixels (also with the same color), and so on. Replace the color of all of the aforementioned pixels with newColor. Return the modified image after performing the flood fill.

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

Easy

Write a recursive function that paints the pixel if it's the correct color, then recurses on neighboring pixels.

463

Sentence Similarity

sentence-similarity

Array,Hash Table,String

Easy

Two words w1 and w2 are similar if and only if w1 == w2, (w1, w2) was a pair, or (w2, w1) was a pair.

547,721,737

Asteroid Collision

asteroid-collision

We are given an array asteroids of integers representing asteroids in a row. For each asteroid, the absolute value represents its size, and the sign represents its direction (positive meaning right, negative meaning left). Each asteroid moves at the same speed. Find out the state of the asteroids after all collisions. If two asteroids meet, the smaller one will explode. If both are the same size, both will explode. Two asteroids moving in the same direction will never meet.

Array,Stack

Medium

Say a row of asteroids is stable. What happens when a new asteroid is added on the right?

605,2245,2317

Parse Lisp Expression

parse-lisp-expression

You are given a string expression representing a Lisp-like expression to return the integer value of. The syntax for these expressions is given as follows.

Hash Table,String,Stack,Recursion

Hard

* If the expression starts with a digit or '-', it's an integer: return it. * If the expression starts with a letter, it's a variable. Recall it by checking the current scope in reverse order. * Otherwise, group the tokens (variables or expressions) within this expression by counting the "balance" `bal` of the occurrences of `'('` minus the number of occurrences of `')'`. When the balance is zero, we have ended a token. For example, `(add 1 (add 2 3))` should have tokens `'1'` and `'(add 2 3)'`. * For add and mult expressions, evaluate each token and return the addition or multiplication of them. * For let expressions, evaluate each expression sequentially and assign it to the variable in the current scope, then return the evaluation of the final expression.

439,726,781

Sentence Similarity II

sentence-similarity-ii

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

Medium

Consider the graphs where each pair in "pairs" is an edge. Two words are similar if they are the same, or are in the same connected component of this graph.

547,721,734

Monotone Increasing Digits

monotone-increasing-digits

An integer has monotone increasing digits if and only if each pair of adjacent digits x and y satisfy x <= y. Given an integer n, return the largest number that is less than or equal to n with monotone increasing digits.

Math,Greedy

Medium

Build the answer digit by digit, adding the largest possible one that would make the number still less than or equal to N.

402

Daily Temperatures

daily-temperatures

Given an array of integers temperatures represents the daily temperatures, return an array answer such that answer[i] is the number of days you have to wait after the ith day to get a warmer temperature. If there is no future day for which this is possible, keep answer[i] == 0 instead.

Array,Stack,Monotonic Stack

Medium

If the temperature is say, 70 today, then in the future a warmer temperature must be either 71, 72, 73, ..., 99, or 100. We could remember when all of them occur next.

496,937

Delete and Earn

delete-and-earn

You are given an integer array nums. You want to maximize the number of points you get by performing the following operation any number of times: Return the maximum number of points you can earn by applying the above operation some number of times.

Array,Hash Table,Dynamic Programming

Medium

If you take a number, you might as well take them all. Keep track of what the value is of the subset of the input with maximum M when you either take or don't take M.

198

Cherry Pickup

cherry-pickup

You are given an n x n grid representing a field of cherries, each cell is one of three possible integers. Return the maximum number of cherries you can collect by following the rules below:

Array,Dynamic Programming,Matrix

Hard

64,174,2189

To Lower Case

to-lower-case

Given a string s, return the string after replacing every uppercase letter with the same lowercase letter.

String

Easy

Most languages support lowercase conversion for a string data type. However, that is certainly not the purpose of the problem. Think about how the implementation of the lowercase function call can be done easily. Think ASCII! Think about the different capital letters and their ASCII codes and how that relates to their lowercase counterparts. Does there seem to be any pattern there? Any mathematical relationship that we can use?

2235

Closest Leaf in a Binary Tree

closest-leaf-in-a-binary-tree

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

Medium

Convert the tree to a general graph, and do a breadth-first search. Alternatively, find the closest leaf for every node on the path from root to target.

Network Delay Time

network-delay-time

You are given a network of n nodes, labeled from 1 to n. You are also given times, a list of travel times as directed edges times[i] = (ui, vi, wi), where ui is the source node, vi is the target node, and wi is the time it takes for a signal to travel from source to target. We will send a signal from a given node k. Return the time it takes for all the n nodes to receive the signal. If it is impossible for all the n nodes to receive the signal, return -1.

Depth-First Search,Breadth-First Search,Graph,Heap (Priority Queue),Shortest Path

Medium

We visit each node at some time, and if that time is better than the fastest time we've reached this node, we travel along outgoing edges in sorted order. Alternatively, we could use Dijkstra's algorithm.

2151,2171

Find Smallest Letter Greater Than Target

find-smallest-letter-greater-than-target

Given a characters array letters that is sorted in non-decreasing order and a character target, return the smallest character in the array that is larger than target. Note that the letters wrap around.

Array,Binary Search

Easy

Try to find whether each of 26 next letters are in the given string array.

2269

Prefix and Suffix Search

prefix-and-suffix-search

Design a special dictionary with some words that searchs the words in it by a prefix and a suffix. Implement the WordFilter class:

String,Design,Trie

Hard

For a word like "test", consider "#test", "t#test", "st#test", "est#test", "test#test". Then if we have a query like prefix = "te", suffix = "t", we can find it by searching for something we've inserted starting with "t#te".

211

Min Cost Climbing Stairs

min-cost-climbing-stairs

You are given an integer array cost where cost[i] is the cost of ith step on a staircase. Once you pay the cost, you can either climb one or two steps. You can either start from the step with index 0, or the step with index 1. Return the minimum cost to reach the top of the floor.

Array,Dynamic Programming

Easy

Say f[i] is the final cost to climb to the top from step i. Then f[i] = cost[i] + min(f[i+1], f[i+2]).

70

Largest Number At Least Twice of Others

largest-number-at-least-twice-of-others

You are given an integer array nums where the largest integer is unique. Determine whether the largest element in the array is at least twice as much as every other number in the array. If it is, return the index of the largest element, or return -1 otherwise.

Array,Sorting

Easy

Scan through the array to find the unique largest element `m`, keeping track of it's index `maxIndex`. Scan through the array again. If we find some `x != m` with `m < 2*x`, we should return `-1`. Otherwise, we should return `maxIndex`.

2274,2327

Shortest Completing Word

shortest-completing-word

Given a string licensePlate and an array of strings words, find the shortest completing word in words. A completing word is a word that contains all the letters in licensePlate. Ignore numbers and spaces in licensePlate, and treat letters as case insensitive. If a letter appears more than once in licensePlate, then it must appear in the word the same number of times or more. For example, if licensePlate = "aBc 12c", then it contains letters 'a', 'b' (ignoring case), and 'c' twice. Possible completing words are "abccdef", "caaacab", and "cbca". Return the shortest completing word in words. It is guaranteed an answer exists. If there are multiple shortest completing words, return the first one that occurs in words.

Array,Hash Table,String

Easy

Count only the letters (possibly converted to lowercase) of each word. If a word is shorter and the count of each letter is at least the count of that letter in the licensePlate, it is the best answer we've seen yet.

Contain Virus

contain-virus

A virus is spreading rapidly, and your task is to quarantine the infected area by installing walls. The world is modeled as an m x n binary grid isInfected, where isInfected[i][j] == 0 represents uninfected cells, and isInfected[i][j] == 1 represents cells contaminated with the virus. A wall (and only one wall) can be installed between any two 4-directionally adjacent cells, on the shared boundary. Every night, the virus spreads to all neighboring cells in all four directions unless blocked by a wall. Resources are limited. Each day, you can install walls around only one region (i.e., the affected area (continuous block of infected cells) that threatens the most uninfected cells the following night). There will never be a tie. Return the number of walls used to quarantine all the infected regions. If the world will become fully infected, return the number of walls used.

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

Hard

The implementation is long - we want to perfrom the following steps: * Find all viral regions (connected components), additionally for each region keeping track of the frontier (neighboring uncontaminated cells), and the perimeter of the region. * Disinfect the most viral region, adding it's perimeter to the answer. * Spread the virus in the remaining regions outward by 1 square.

Number Of Corner Rectangles

number-of-corner-rectangles

Array,Math,Dynamic Programming,Matrix

Medium

For each pair of 1s in the new row (say at `new_row[i]` and `new_row[j]`), we could create more rectangles where that pair forms the base. The number of new rectangles is the number of times some previous row had `row[i] = row[j] = 1`.

IP to CIDR

ip-to-cidr

String,Bit Manipulation

Medium

Convert the ip addresses to and from (long) integers. You want to know what is the most addresses you can put in this block starting from the "start" ip, up to n. It is the smallest between the lowest bit of start and the highest bit of n. Then, repeat this process with a new start and n.

93,468

Open the Lock

open-the-lock

You have a lock in front of you with 4 circular wheels. Each wheel has 10 slots: '0', '1', '2', '3', '4', '5', '6', '7', '8', '9'. The wheels can rotate freely and wrap around: for example we can turn '9' to be '0', or '0' to be '9'. Each move consists of turning one wheel one slot. The lock initially starts at '0000', a string representing the state of the 4 wheels. You are given a list of deadends dead ends, meaning if the lock displays any of these codes, the wheels of the lock will stop turning and you will be unable to open it. Given a target representing the value of the wheels that will unlock the lock, return the minimum total number of turns required to open the lock, or -1 if it is impossible.

Array,Hash Table,String,Breadth-First Search

Medium

We can think of this problem as a shortest path problem on a graph: there are `10000` nodes (strings `'0000'` to `'9999'`), and there is an edge between two nodes if they differ in one digit, that digit differs by 1 (wrapping around, so `'0'` and `'9'` differ by 1), and if *both* nodes are not in `deadends`.

Cracking the Safe

cracking-the-safe

There is a safe protected by a password. The password is a sequence of n digits where each digit can be in the range [0, k - 1]. The safe has a peculiar way of checking the password. When you enter in a sequence, it checks the most recent n digits that were entered each time you type a digit. Return any string of minimum length that will unlock the safe at some point of entering it.

Depth-First Search,Graph,Eulerian Circuit

Hard

We can think of this problem as the problem of finding an Euler path (a path visiting every edge exactly once) on the following graph: there are $$k^{n-1}$$ nodes with each node having $$k$$ edges. It turns out this graph always has an Eulerian circuit (path starting where it ends.) We should visit each node in "post-order" so as to not get stuck in the graph prematurely.

Reach a Number

reach-a-number

You are standing at position 0 on an infinite number line. There is a destination at position target. You can make some number of moves numMoves so that: Given the integer target, return the minimum number of moves required (i.e., the minimum numMoves) to reach the destination.

Math,Binary Search

Medium

Pour Water

pour-water

Array,Simulation

Medium

42

Pyramid Transition Matrix

pyramid-transition-matrix

You are stacking blocks to form a pyramid. Each block has a color, which is represented by a single letter. Each row of blocks contains one less block than the row beneath it and is centered on top. To make the pyramid aesthetically pleasing, there are only specific triangular patterns that are allowed. A triangular pattern consists of a single block stacked on top of two blocks. The patterns are given as a list of three-letter strings allowed, where the first two characters of a pattern represent the left and right bottom blocks respectively, and the third character is the top block. You start with a bottom row of blocks bottom, given as a single string, that you must use as the base of the pyramid. Given bottom and allowed, return true if you can build the pyramid all the way to the top such that every triangular pattern in the pyramid is in allowed, or false otherwise.

Bit Manipulation,Depth-First Search,Breadth-First Search

Medium

Convert Binary Search Tree to Sorted Doubly Linked List

convert-binary-search-tree-to-sorted-doubly-linked-list

Linked List,Stack,Tree,Depth-First Search,Binary Search Tree,Binary Tree,Doubly-Linked List

Medium

94

Set Intersection Size At Least Two

set-intersection-size-at-least-two

An integer interval [a, b] (for integers a < b) is a set of all consecutive integers from a to b, including a and b. Find the minimum size of a set S such that for every integer interval A in intervals, the intersection of S with A has a size of at least two.

Array,Greedy,Sorting

Hard

Bold Words in String

bold-words-in-string

Array,Hash Table,String,Trie,String Matching

Medium

First, determine which letters are bold and store that information in mask[i] = if i-th character is bold. Then, insert the tags at the beginning and end of groups. The start of a group is if and only if (mask[i] and (i == 0 or not mask[i-1])), and the end of a group is similar.

Employee Free Time

employee-free-time

Array,Sorting,Heap (Priority Queue)

Hard

Take all the intervals and do an "events" (or "line sweep") approach - an event of (x, OPEN) increases the number of active intervals, while (x, CLOSE) decreases it. Processing in sorted order from left to right, if the number of active intervals is zero, then you crossed a region of common free time.

56,1028

Find Anagram Mappings

find-anagram-mappings

Array,Hash Table

Easy

Create a hashmap so that D[x] = i whenever B[i] = x. Then, the answer is [D[x] for x in A].

Special Binary String

special-binary-string

Special binary strings are binary strings with the following two properties: You are given a special binary string s. A move consists of choosing two consecutive, non-empty, special substrings of s, and swapping them. Two strings are consecutive if the last character of the first string is exactly one index before the first character of the second string. Return the lexicographically largest resulting string possible after applying the mentioned operations on the string.

String,Recursion

Hard

Draw a line from (x, y) to (x+1, y+1) if we see a "1", else to (x+1, y-1). A special substring is just a line that starts and ends at the same y-coordinate, and that is the lowest y-coordinate reached. Call a mountain a special substring with no special prefixes - ie. only at the beginning and end is the lowest y-coordinate reached. If F is the answer function, and S has mountain decomposition M1,M2,M3,...,Mk, then the answer is: reverse_sorted(F(M1), F(M2), ..., F(Mk)). However, you'll also need to deal with the case that S is a mountain, such as 11011000 -> 11100100.

678

N-ary Tree Level Order Traversal

n-ary-tree-level-order-traversal

Given an n-ary tree, return the level order traversal of its nodes' values. Nary-Tree input serialization is represented in their level order traversal, each group of children is separated by the null value (See examples).

Tree,Breadth-First Search

Medium

102,775,776,2151

Serialize and Deserialize N-ary Tree

serialize-and-deserialize-n-ary-tree

String,Tree,Depth-First Search,Breadth-First Search

Hard

297,449,771

Flatten a Multilevel Doubly Linked List

flatten-a-multilevel-doubly-linked-list

You are given a doubly linked list, which contains nodes that have a next pointer, a previous pointer, and an additional child pointer. This child pointer may or may not point to a separate doubly linked list, also containing these special nodes. These child lists may have one or more children of their own, and so on, to produce a multilevel data structure as shown in the example below. Given the head of the first level of the list, flatten the list so that all the nodes appear in a single-level, doubly linked list. Let curr be a node with a child list. The nodes in the child list should appear after curr and before curr.next in the flattened list. Return the head of the flattened list. The nodes in the list must have all of their child pointers set to null.

Linked List,Depth-First Search,Doubly-Linked List

Medium

114,1796

Prime Number of Set Bits in Binary Representation

prime-number-of-set-bits-in-binary-representation

Given two integers left and right, return the count of numbers in the inclusive range [left, right] having a prime number of set bits in their binary representation. Recall that the number of set bits an integer has is the number of 1's present when written in binary.

Math,Bit Manipulation

Easy

Write a helper function to count the number of set bits in a number, then check whether the number of set bits is 2, 3, 5, 7, 11, 13, 17 or 19.

191

Partition Labels

partition-labels

You are given a string s. We want to partition the string into as many parts as possible so that each letter appears in at most one part. Note that the partition is done so that after concatenating all the parts in order, the resultant string should be s. Return a list of integers representing the size of these parts.

Hash Table,Two Pointers,String,Greedy

Medium

Try to greedily choose the smallest partition that includes the first letter. If you have something like "abaccbdeffed", then you might need to add b. You can use an map like "last['b'] = 5" to help you expand the width of your partition.

56

Largest Plus Sign

largest-plus-sign

You are given an integer n. You have an n x n binary grid grid with all values initially 1's except for some indices given in the array mines. The ith element of the array mines is defined as mines[i] = [xi, yi] where grid[xi][yi] == 0. Return the order of the largest axis-aligned plus sign of 1's contained in grid. If there is none, return 0. An axis-aligned plus sign of 1's of order k has some center grid[r][c] == 1 along with four arms of length k - 1 going up, down, left, and right, and made of 1's. Note that there could be 0's or 1's beyond the arms of the plus sign, only the relevant area of the plus sign is checked for 1's.

Array,Dynamic Programming

Medium

For each direction such as "left", find left[r][c] = the number of 1s you will see before a zero starting at r, c and walking left. You can find this in N^2 time with a dp. The largest plus sign at r, c is just the minimum of left[r][c], up[r][c] etc.

221

Couples Holding Hands

couples-holding-hands

There are n couples sitting in 2n seats arranged in a row and want to hold hands. The people and seats are represented by an integer array row where row[i] is the ID of the person sitting in the ith seat. The couples are numbered in order, the first couple being (0, 1), the second couple being (2, 3), and so on with the last couple being (2n - 2, 2n - 1). Return the minimum number of swaps so that every couple is sitting side by side. A swap consists of choosing any two people, then they stand up and switch seats.

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

Hard

Say there are N two-seat couches. For each couple, draw an edge from the couch of one partner to the couch of the other partner.

41,268,884

Encode N-ary Tree to Binary Tree

encode-n-ary-tree-to-binary-tree

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

Hard

765

Construct Quad Tree

construct-quad-tree

Given a n * n matrix grid of 0's and 1's only. We want to represent the grid with a Quad-Tree. Return the root of the Quad-Tree representing the grid. Notice that you can assign the value of a node to True or False when isLeaf is False, and both are accepted in the answer. A Quad-Tree is a tree data structure in which each internal node has exactly four children. Besides, each node has two attributes: We can construct a Quad-Tree from a two-dimensional area using the following steps: If you want to know more about the Quad-Tree, you can refer to the wiki. Quad-Tree format: The output represents the serialized format of a Quad-Tree using level order traversal, where null signifies a path terminator where no node exists below. It is very similar to the serialization of the binary tree. The only difference is that the node is represented as a list [isLeaf, val]. If the value of isLeaf or val is True we represent it as 1 in the list [isLeaf, val] and if the value of isLeaf or val is False we represent it as 0.

Array,Divide and Conquer,Tree,Matrix

Medium

Logical OR of Two Binary Grids Represented as Quad-Trees

logical-or-of-two-binary-grids-represented-as-quad-trees

A Binary Matrix is a matrix in which all the elements are either 0 or 1. Given quadTree1 and quadTree2. quadTree1 represents a n * n binary matrix and quadTree2 represents another n * n binary matrix. Return a Quad-Tree representing the n * n binary matrix which is the result of logical bitwise OR of the two binary matrixes represented by quadTree1 and quadTree2. Notice that you can assign the value of a node to True or False when isLeaf is False, and both are accepted in the answer. A Quad-Tree is a tree data structure in which each internal node has exactly four children. Besides, each node has two attributes: We can construct a Quad-Tree from a two-dimensional area using the following steps: If you want to know more about the Quad-Tree, you can refer to the wiki. Quad-Tree format: The input/output represents the serialized format of a Quad-Tree using level order traversal, where null signifies a path terminator where no node exists below. It is very similar to the serialization of the binary tree. The only difference is that the node is represented as a list [isLeaf, val]. If the value of isLeaf or val is True we represent it as 1 in the list [isLeaf, val] and if the value of isLeaf or val is False we represent it as 0.

Divide and Conquer,Tree

Medium

Maximum Depth of N-ary Tree

maximum-depth-of-n-ary-tree

Given a n-ary tree, find its maximum depth. The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node. Nary-Tree input serialization is represented in their level order traversal, each group of children is separated by the null value (See examples).

Tree,Depth-First Search,Breadth-First Search

Easy

104,2151

N-ary Tree Preorder Traversal

n-ary-tree-preorder-traversal

Given the root of an n-ary tree, return the preorder traversal of its nodes' values. Nary-Tree input serialization is represented in their level order traversal. Each group of children is separated by the null value (See examples)

Stack,Tree,Depth-First Search

Easy

144,764,776

N-ary Tree Postorder Traversal

n-ary-tree-postorder-traversal

Given the root of an n-ary tree, return the postorder traversal of its nodes' values. Nary-Tree input serialization is represented in their level order traversal. Each group of children is separated by the null value (See examples)

Stack,Tree,Depth-First Search

Easy

145,764,775

Toeplitz Matrix

toeplitz-matrix

Given an m x n matrix, return true if the matrix is Toeplitz. Otherwise, return false. A matrix is Toeplitz if every diagonal from top-left to bottom-right has the same elements.

Array,Matrix

Easy

Check whether each value is equal to the value of it's top-left neighbor.

422

Reorganize String

reorganize-string

Given a string s, rearrange the characters of s so that any two adjacent characters are not the same. Return any possible rearrangement of s or return "" if not possible.

Hash Table,String,Greedy,Sorting,Heap (Priority Queue),Counting

Medium

Alternate placing the most common letters.

358,621,1304

Max Chunks To Make Sorted II

max-chunks-to-make-sorted-ii

You are given an integer array arr. We split arr into some number of chunks (i.e., partitions), and individually sort each chunk. After concatenating them, the result should equal the sorted array. Return the largest number of chunks we can make to sort the array.

Array,Stack,Greedy,Sorting,Monotonic Stack

Hard

Each k for which some permutation of arr[:k] is equal to sorted(arr)[:k] is where we should cut each chunk.

780

Max Chunks To Make Sorted

max-chunks-to-make-sorted

You are given an integer array arr of length n that represents a permutation of the integers in the range [0, n - 1]. We split arr into some number of chunks (i.e., partitions), and individually sort each chunk. After concatenating them, the result should equal the sorted array. Return the largest number of chunks we can make to sort the array.

Array,Stack,Greedy,Sorting,Monotonic Stack

Medium

The first chunk can be found as the smallest k for which A[:k+1] == [0, 1, 2, ...k]; then we repeat this process.

779

Basic Calculator IV

basic-calculator-iv

Given an expression such as expression = "e + 8 - a + 5" and an evaluation map such as {"e": 1} (given in terms of evalvars = ["e"] and evalints = [1]), return a list of tokens representing the simplified expression, such as ["-1*a","14"] Expressions are evaluated in the usual order: brackets first, then multiplication, then addition and subtraction. The format of the output is as follows:

Hash Table,Math,String,Stack,Recursion

Hard

One way is with a Polynomial class. For example, * `Poly:add(this, that)` returns the result of `this + that`. * `Poly:sub(this, that)` returns the result of `this - that`. * `Poly:mul(this, that)` returns the result of `this * that`. * `Poly:evaluate(this, evalmap)` returns the polynomial after replacing all free variables with constants as specified by `evalmap`. * `Poly:toList(this)` returns the polynomial in the correct output format. * `Solution::combine(left, right, symbol)` returns the result of applying the binary operator represented by `symbol` to `left` and `right`. * `Solution::make(expr)` makes a new `Poly` represented by either the constant or free variable specified by `expr`. * `Solution::parse(expr)` parses an expression into a new `Poly`.

736,785

Jewels and Stones

jewels-and-stones

You're given strings jewels representing the types of stones that are jewels, and stones representing the stones you have. Each character in stones is a type of stone you have. You want to know how many of the stones you have are also jewels. Letters are case sensitive, so "a" is considered a different type of stone from "A".

Hash Table,String

Easy

For each stone, check if it is a jewel.

Search in a Binary Search Tree

search-in-a-binary-search-tree

You are given the root of a binary search tree (BST) and an integer val. Find the node in the BST that the node's value equals val and return the subtree rooted with that node. If such a node does not exist, return null.

Tree,Binary Search Tree,Binary Tree

Easy

270,784

Insert into a Binary Search Tree

insert-into-a-binary-search-tree

You are given the root node of a binary search tree (BST) and a value to insert into the tree. Return the root node of the BST after the insertion. It is guaranteed that the new value does not exist in the original BST. Notice that there may exist multiple valid ways for the insertion, as long as the tree remains a BST after insertion. You can return any of them.

Tree,Binary Search Tree,Binary Tree

Medium

783

Basic Calculator III

basic-calculator-iii

Math,String,Stack,Recursion

Hard

224,227,781,1736

Search in a Sorted Array of Unknown Size

search-in-a-sorted-array-of-unknown-size

Array,Binary Search,Interactive

Medium

792,1672

Sliding Puzzle

sliding-puzzle

On an 2 x 3 board, there are five tiles labeled from 1 to 5, and an empty square represented by 0. A move consists of choosing 0 and a 4-directionally adjacent number and swapping it. The state of the board is solved if and only if the board is [[1,2,3],[4,5,0]]. Given the puzzle board board, return the least number of moves required so that the state of the board is solved. If it is impossible for the state of the board to be solved, return -1.

Array,Breadth-First Search,Matrix

Hard

Perform a breadth-first-search, where the nodes are the puzzle boards and edges are if two puzzle boards can be transformed into one another with one move.

Minimize Max Distance to Gas Station

minimize-max-distance-to-gas-station

Array,Binary Search

Hard

Use a binary search. We'll binary search the monotone function "possible(D) = can we use K or less gas stations to ensure each adjacent distance between gas stations is at most D?"

907

Kth Largest Element in a Stream

kth-largest-element-in-a-stream

Design a class to find the kth largest element in a stream. Note that it is the kth largest element in the sorted order, not the kth distinct element. Implement KthLargest class:

Tree,Design,Binary Search Tree,Heap (Priority Queue),Binary Tree,Data Stream

Easy

215,1953,2207

Global and Local Inversions

global-and-local-inversions

You are given an integer array nums of length n which represents a permutation of all the integers in the range [0, n - 1]. The number of global inversions is the number of the different pairs (i, j) where: The number of local inversions is the number of indices i where: Return true if the number of global inversions is equal to the number of local inversions.

Array,Math

Medium

Where can the 0 be placed in an ideal permutation? What about the 1?

Split BST

split-bst

Tree,Binary Search Tree,Recursion,Binary Tree

Medium

Use recursion. If root.val <= V, you split root.right into two halves, then join it's left half back on root.right.

450

Binary Search

binary-search

Given an array of integers nums which is sorted in ascending order, and an integer target, write a function to search target in nums. If target exists, then return its index. Otherwise, return -1. You must write an algorithm with O(log n) runtime complexity.

Array,Binary Search

Easy

786

Swap Adjacent in LR String

swap-adjacent-in-lr-string

In a string composed of 'L', 'R', and 'X' characters, like "RXXLRXRXL", a move consists of either replacing one occurrence of "XL" with "LX", or replacing one occurrence of "RX" with "XR". Given the starting string start and the ending string end, return True if and only if there exists a sequence of moves to transform one string to the other.

Two Pointers,String

Medium

Think of the L and R's as people on a horizontal line, where X is a space. The people can't cross each other, and also you can't go from XRX to RXX.

Swim in Rising Water

swim-in-rising-water

You are given an n x n integer matrix grid where each value grid[i][j] represents the elevation at that point (i, j). The rain starts to fall. At time t, the depth of the water everywhere is t. You can swim from a square to another 4-directionally adjacent square if and only if the elevation of both squares individually are at most t. You can swim infinite distances in zero time. Of course, you must stay within the boundaries of the grid during your swim. Return the least time until you can reach the bottom right square (n - 1, n - 1) if you start at the top left square (0, 0).

Array,Binary Search,Depth-First Search,Breadth-First Search,Union Find,Heap (Priority Queue),Matrix

Hard

Use either Dijkstra's, or binary search for the best time T for which you can reach the end if you only step on squares at most T.

1753

K-th Symbol in Grammar

k-th-symbol-in-grammar

We build a table of n rows (1-indexed). We start by writing 0 in the 1st row. Now in every subsequent row, we look at the previous row and replace each occurrence of 0 with 01, and each occurrence of 1 with 10. Given two integer n and k, return the kth (1-indexed) symbol in the nth row of a table of n rows.

Math,Bit Manipulation,Recursion

Medium

Try to represent the current (N, K) in terms of some (N-1, prevK). What is prevK ?

Reaching Points

reaching-points

Given four integers sx, sy, tx, and ty, return true if it is possible to convert the point (sx, sy) to the point (tx, ty) through some operations, or false otherwise. The allowed operation on some point (x, y) is to convert it to either (x, x + y) or (x + y, y).

Math

Hard

Rabbits in Forest

rabbits-in-forest

There is a forest with an unknown number of rabbits. We asked n rabbits "How many rabbits have the same color as you?" and collected the answers in an integer array answers where answers[i] is the answer of the ith rabbit. Given the array answers, return the minimum number of rabbits that could be in the forest.

Array,Hash Table,Math,Greedy

Medium

Transform to Chessboard

transform-to-chessboard

You are given an n x n binary grid board. In each move, you can swap any two rows with each other, or any two columns with each other. Return the minimum number of moves to transform the board into a chessboard board. If the task is impossible, return -1. A chessboard board is a board where no 0's and no 1's are 4-directionally adjacent.

Array,Math,Bit Manipulation,Matrix

Hard

Minimum Distance Between BST Nodes

minimum-distance-between-bst-nodes

Given the root of a Binary Search Tree (BST), return the minimum difference between the values of any two different nodes in the tree.

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

Easy

94

Letter Case Permutation

letter-case-permutation

Given a string s, you can transform every letter individually to be lowercase or uppercase to create another string. Return a list of all possible strings we could create. Return the output in any order.

String,Backtracking,Bit Manipulation

Medium

78,1076

Is Graph Bipartite?

is-graph-bipartite

There is an undirected graph with n nodes, where each node is numbered between 0 and n - 1. You are given a 2D array graph, where graph[u] is an array of nodes that node u is adjacent to. More formally, for each v in graph[u], there is an undirected edge between node u and node v. The graph has the following properties: A graph is bipartite if the nodes can be partitioned into two independent sets A and B such that every edge in the graph connects a node in set A and a node in set B. Return true if and only if it is bipartite.

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

Medium

K-th Smallest Prime Fraction

k-th-smallest-prime-fraction

You are given a sorted integer array arr containing 1 and prime numbers, where all the integers of arr are unique. You are also given an integer k. For every i and j where 0 <= i < j < arr.length, we consider the fraction arr[i] / arr[j]. Return the kth smallest fraction considered. Return your answer as an array of integers of size 2, where answer[0] == arr[i] and answer[1] == arr[j].

Array,Binary Search,Heap (Priority Queue)

Hard

378,668,719

Cheapest Flights Within K Stops

cheapest-flights-within-k-stops

There are n cities connected by some number of flights. You are given an array flights where flights[i] = [fromi, toi, pricei] indicates that there is a flight from city fromi to city toi with cost pricei. You are also given three integers src, dst, and k, return the cheapest price from src to dst with at most k stops. If there is no such route, return -1.

Dynamic Programming,Depth-First Search,Breadth-First Search,Graph,Heap (Priority Queue),Shortest Path

Medium

568,2230

Rotated Digits

rotated-digits

An integer x is a good if after rotating each digit individually by 180 degrees, we get a valid number that is different from x. Each digit must be rotated - we cannot choose to leave it alone. A number is valid if each digit remains a digit after rotation. For example: Given an integer n, return the number of good integers in the range [1, n].

Math,Dynamic Programming

Medium

Escape The Ghosts

escape-the-ghosts

You are playing a simplified PAC-MAN game on an infinite 2-D grid. You start at the point [0, 0], and you are given a destination point target = [xtarget, ytarget] that you are trying to get to. There are several ghosts on the map with their starting positions given as a 2D array ghosts, where ghosts[i] = [xi, yi] represents the starting position of the ith ghost. All inputs are integral coordinates. Each turn, you and all the ghosts may independently choose to either move 1 unit in any of the four cardinal directions: north, east, south, or west, or stay still. All actions happen simultaneously. You escape if and only if you can reach the target before any ghost reaches you. If you reach any square (including the target) at the same time as a ghost, it does not count as an escape. Return true if it is possible to escape regardless of how the ghosts move, otherwise return false.

Array,Math

Medium

1727

Domino and Tromino Tiling

domino-and-tromino-tiling

You have two types of tiles: a 2 x 1 domino shape and a tromino shape. You may rotate these shapes. Given an integer n, return the number of ways to tile an 2 x n board. Since the answer may be very large, return it modulo 109 + 7. In a tiling, every square must be covered by a tile. Two tilings are different if and only if there are two 4-directionally adjacent cells on the board such that exactly one of the tilings has both squares occupied by a tile.

Dynamic Programming

Medium

Custom Sort String

custom-sort-string

You are given two strings order and s. All the words of order are unique and were sorted in some custom order previously. Permute the characters of s so that they match the order that order was sorted. More specifically, if a character x occurs before a character y in order, then x should occur before y in the permuted string. Return any permutation of s that satisfies this property.

Hash Table,String,Sorting

Medium

Number of Matching Subsequences

number-of-matching-subsequences

Given a string s and an array of strings words, return the number of words[i] that is a subsequence of s. A subsequence of a string is a new string generated from the original string with some characters (can be none) deleted without changing the relative order of the remaining characters.

Hash Table,String,Trie,Sorting

Medium

392,1051,2186

Preimage Size of Factorial Zeroes Function

preimage-size-of-factorial-zeroes-function

Let f(x) be the number of zeroes at the end of x!. Recall that x! = 1 * 2 * 3 * ... * x and by convention, 0! = 1. Given an integer k, return the number of non-negative integers x have the property that f(x) = k.

Math,Binary Search

Hard

172

Valid Tic-Tac-Toe State

valid-tic-tac-toe-state

Given a Tic-Tac-Toe board as a string array board, return true if and only if it is possible to reach this board position during the course of a valid tic-tac-toe game. The board is a 3 x 3 array that consists of characters ' ', 'X', and 'O'. The ' ' character represents an empty square. Here are the rules of Tic-Tac-Toe:

Array,String

Medium

348

Number of Subarrays with Bounded Maximum

number-of-subarrays-with-bounded-maximum

Given an integer array nums and two integers left and right, return the number of contiguous non-empty subarrays such that the value of the maximum array element in that subarray is in the range [left, right]. The test cases are generated so that the answer will fit in a 32-bit integer.

Array,Two Pointers

Medium

Rotate String

rotate-string

Given two strings s and goal, return true if and only if s can become goal after some number of shifts on s. A shift on s consists of moving the leftmost character of s to the rightmost position.

String,String Matching

Easy

All Paths From Source to Target

all-paths-from-source-to-target

Given a directed acyclic graph (DAG) of n nodes labeled from 0 to n - 1, find all possible paths from node 0 to node n - 1 and return them in any order. The graph is given as follows: graph[i] is a list of all nodes you can visit from node i (i.e., there is a directed edge from node i to node graph[i][j]).

Backtracking,Depth-First Search,Breadth-First Search,Graph

Medium

2090

Smallest Rotation with Highest Score

smallest-rotation-with-highest-score

You are given an array nums. You can rotate it by a non-negative integer k so that the array becomes [nums[k], nums[k + 1], ... nums[nums.length - 1], nums[0], nums[1], ..., nums[k-1]]. Afterward, any entries that are less than or equal to their index are worth one point. Return the rotation index k that corresponds to the highest score we can achieve if we rotated nums by it. If there are multiple answers, return the smallest such index k.

Array,Prefix Sum

Hard

Champagne Tower

champagne-tower

We stack glasses in a pyramid, where the first row has 1 glass, the second row has 2 glasses, and so on until the 100th row.  Each glass holds one cup of champagne. Then, some champagne is poured into the first glass at the top.  When the topmost glass is full, any excess liquid poured will fall equally to the glass immediately to the left and right of it.  When those glasses become full, any excess champagne will fall equally to the left and right of those glasses, and so on.  (A glass at the bottom row has its excess champagne fall on the floor.) For example, after one cup of champagne is poured, the top most glass is full.  After two cups of champagne are poured, the two glasses on the second row are half full.  After three cups of champagne are poured, those two cups become full - there are 3 full glasses total now.  After four cups of champagne are poured, the third row has the middle glass half full, and the two outside glasses are a quarter full, as pictured below. Now after pouring some non-negative integer cups of champagne, return how full the jth glass in the ith row is (both i and j are 0-indexed.)

Dynamic Programming

Medium

1385

Design HashSet

design-hashset

Design a HashSet without using any built-in hash table libraries. Implement MyHashSet class:

Array,Hash Table,Linked List,Design,Hash Function

Easy

817,1337

Design HashMap

design-hashmap

Design a HashMap without using any built-in hash table libraries. Implement the MyHashMap class:

Array,Hash Table,Linked List,Design,Hash Function

Easy

816,1337

Similar RGB Color

similar-rgb-color

Math,String,Enumeration

Easy

Minimum Swaps To Make Sequences Increasing

minimum-swaps-to-make-sequences-increasing

You are given two integer arrays of the same length nums1 and nums2. In one operation, you are allowed to swap nums1[i] with nums2[i]. Return the minimum number of needed operations to make nums1 and nums2 strictly increasing. The test cases are generated so that the given input always makes it possible. An array arr is strictly increasing if and only if arr[0] < arr[1] < arr[2] < ... < arr[arr.length - 1].

Array,Dynamic Programming

Hard

2234

Find Eventual Safe States

find-eventual-safe-states

There is a directed graph of n nodes with each node labeled from 0 to n - 1. The graph is represented by a 0-indexed 2D integer array graph where graph[i] is an integer array of nodes adjacent to node i, meaning there is an edge from node i to each node in graph[i]. A node is a terminal node if there are no outgoing edges. A node is a safe node if every possible path starting from that node leads to a terminal node. Return an array containing all the safe nodes of the graph. The answer should be sorted in ascending order.

Depth-First Search,Breadth-First Search,Graph,Topological Sort

Medium

Bricks Falling When Hit

bricks-falling-when-hit

You are given an m x n binary grid, where each 1 represents a brick and 0 represents an empty space. A brick is stable if: You are also given an array hits, which is a sequence of erasures we want to apply. Each time we want to erase the brick at the location hits[i] = (rowi, coli). The brick on that location (if it exists) will disappear. Some other bricks may no longer be stable because of that erasure and will fall. Once a brick falls, it is immediately erased from the grid (i.e., it does not land on other stable bricks). Return an array result, where each result[i] is the number of bricks that will fall after the ith erasure is applied. Note that an erasure may refer to a location with no brick, and if it does, no bricks drop.

Array,Union Find,Matrix

Hard

2101,2322

Unique Morse Code Words

unique-morse-code-words

International Morse Code defines a standard encoding where each letter is mapped to a series of dots and dashes, as follows: For convenience, the full table for the 26 letters of the English alphabet is given below: Given an array of strings words where each word can be written as a concatenation of the Morse code of each letter. Return the number of different transformations among all words we have.

Array,Hash Table,String

Easy

Split Array With Same Average

split-array-with-same-average

You are given an integer array nums. You should move each element of nums into one of the two arrays A and B such that A and B are non-empty, and average(A) == average(B). Return true if it is possible to achieve that and false otherwise. Note that for an array arr, average(arr) is the sum of all the elements of arr over the length of arr.

Array,Math,Dynamic Programming,Bit Manipulation,Bitmask

Hard

2162

Number of Lines To Write String

number-of-lines-to-write-string

You are given a string s of lowercase English letters and an array widths denoting how many pixels wide each lowercase English letter is. Specifically, widths[0] is the width of 'a', widths[1] is the width of 'b', and so on. You are trying to write s across several lines, where each line is no longer than 100 pixels. Starting at the beginning of s, write as many letters on the first line such that the total width does not exceed 100 pixels. Then, from where you stopped in s, continue writing as many letters as you can on the second line. Continue this process until you have written all of s. Return an array result of length 2 where:

Array,String

Easy

Max Increase to Keep City Skyline

max-increase-to-keep-city-skyline

There is a city composed of n x n blocks, where each block contains a single building shaped like a vertical square prism. You are given a 0-indexed n x n integer matrix grid where grid[r][c] represents the height of the building located in the block at row r and column c. A city's skyline is the the outer contour formed by all the building when viewing the side of the city from a distance. The skyline from each cardinal direction north, east, south, and west may be different. We are allowed to increase the height of any number of buildings by any amount (the amount can be different per building). The height of a 0-height building can also be increased. However, increasing the height of a building should not affect the city's skyline from any cardinal direction. Return the maximum total sum that the height of the buildings can be increased by without changing the city's skyline from any cardinal direction.

Array,Greedy,Matrix

Medium

Soup Servings

soup-servings

There are two types of soup: type A and type B. Initially, we have n ml of each type of soup. There are four kinds of operations: When we serve some soup, we give it to someone, and we no longer have it. Each turn, we will choose from the four operations with an equal probability 0.25. If the remaining volume of soup is not enough to complete the operation, we will serve as much as possible. We stop once we no longer have some quantity of both types of soup. Note that we do not have an operation where all 100 ml's of soup B are used first. Return the probability that soup A will be empty first, plus half the probability that A and B become empty at the same time. Answers within 10-5 of the actual answer will be accepted.

Math,Dynamic Programming,Probability and Statistics

Medium

Expressive Words

expressive-words

Sometimes people repeat letters to represent extra feeling. For example: In these strings like "heeellooo", we have groups of adjacent letters that are all the same: "h", "eee", "ll", "ooo". You are given a string s and an array of query strings words. A query word is stretchy if it can be made to be equal to s by any number of applications of the following extension operation: choose a group consisting of characters c, and add some number of characters c to the group so that the size of the group is three or more. Return the number of query strings that are stretchy.

Array,Two Pointers,String

Medium