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Remove Invalid Parentheses

remove-invalid-parentheses

Given a string s that contains parentheses and letters, remove the minimum number of invalid parentheses to make the input string valid. Return all the possible results. You may return the answer in any order.

String,Backtracking,Breadth-First Search

Hard

Since we don't know which of the brackets can possibly be removed, we try out all the options! We can use recursion to try out all possibilities for the given expression. For each of the brackets, we have 2 options: We keep the bracket and add it to the expression that we are building on the fly during recursion. OR, we can discard the bracket and move on. The one thing all these valid expressions have in common is that they will all be of the same length i.e. as compared to the original expression, all of these expressions will have the same number of characters removed. Can we somehow find the number of misplaced parentheses and use it in our solution? For every left parenthesis, we should have a corresponding right parenthesis. We can make use of two counters which keep track of misplaced left and right parenthesis and in one iteration we can find out these two values. 0 1 2 3 4 5 6 7 ( ) ) ) ( ( ( ) i = 0, left = 1, right = 0 i = 1, left = 0, right = 0 i = 2, left = 0, right = 1 i = 3, left = 0, right = 2 i = 4, left = 1, right = 2 i = 5, left = 2, right = 2 i = 6, left = 3, right = 2 i = 7, left = 2, right = 2 We have 2 misplaced left and 2 misplaced right parentheses. We found out that the exact number of left and right parenthesis that has to be removed to get a valid expression. So, e.g. in a 1000 parentheses string, if there are 2 misplaced left and 2 misplaced right parentheses, after we are done discarding 2 left and 2 right parentheses, we will have only one option per remaining character in the expression i.e. to consider them. We can't discard them.

20,2095

Smallest Rectangle Enclosing Black Pixels

smallest-rectangle-enclosing-black-pixels

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

Hard

Range Sum Query - Immutable

range-sum-query-immutable

Given an integer array nums, handle multiple queries of the following type: Implement the NumArray class:

Array,Design,Prefix Sum

Easy

304,307,325

Range Sum Query 2D - Immutable

range-sum-query-2d-immutable

Given a 2D matrix matrix, handle multiple queries of the following type: Implement the NumMatrix class:

Array,Design,Matrix,Prefix Sum

Medium

303,308

Number of Islands II

number-of-islands-ii

Array,Union Find

Hard

200,2198

Additive Number

additive-number

An additive number is a string whose digits can form an additive sequence. A valid additive sequence should contain at least three numbers. Except for the first two numbers, each subsequent number in the sequence must be the sum of the preceding two. Given a string containing only digits, return true if it is an additive number or false otherwise. Note: Numbers in the additive sequence cannot have leading zeros, so sequence 1, 2, 03 or 1, 02, 3 is invalid.

String,Backtracking

Medium

872

Range Sum Query - Mutable

range-sum-query-mutable

Given an integer array nums, handle multiple queries of the following types: Implement the NumArray class:

Array,Design,Binary Indexed Tree,Segment Tree

Medium

303,308

Range Sum Query 2D - Mutable

range-sum-query-2d-mutable

Array,Design,Binary Indexed Tree,Segment Tree,Matrix

Hard

304,307

Best Time to Buy and Sell Stock with Cooldown

best-time-to-buy-and-sell-stock-with-cooldown

You are given an array prices where prices[i] is the price of a given stock on the ith day. Find the maximum profit you can achieve. You may complete as many transactions as you like (i.e., buy one and sell one share of the stock multiple times) with the following restrictions: Note: You may not engage in multiple transactions simultaneously (i.e., you must sell the stock before you buy again).

Array,Dynamic Programming

Medium

121,122

Minimum Height Trees

minimum-height-trees

A tree is an undirected graph in which any two vertices are connected by exactly one path. In other words, any connected graph without simple cycles is a tree. Given a tree of n nodes labelled from 0 to n - 1, and an array of n - 1 edges where edges[i] = [ai, bi] indicates that there is an undirected edge between the two nodes ai and bi in the tree, you can choose any node of the tree as the root. When you select a node x as the root, the result tree has height h. Among all possible rooted trees, those with minimum height (i.e. min(h))  are called minimum height trees (MHTs). Return a list of all MHTs' root labels. You can return the answer in any order. The height of a rooted tree is the number of edges on the longest downward path between the root and a leaf.

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

Medium

How many MHTs can a graph have at most?

207,210

Sparse Matrix Multiplication

sparse-matrix-multiplication

Array,Hash Table,Matrix

Medium

Burst Balloons

burst-balloons

You are given n balloons, indexed from 0 to n - 1. Each balloon is painted with a number on it represented by an array nums. You are asked to burst all the balloons. If you burst the ith balloon, you will get nums[i - 1] * nums[i] * nums[i + 1] coins. If i - 1 or i + 1 goes out of bounds of the array, then treat it as if there is a balloon with a 1 painted on it. Return the maximum coins you can collect by bursting the balloons wisely.

Array,Dynamic Programming

Hard

1042

Super Ugly Number

super-ugly-number

A super ugly number is a positive integer whose prime factors are in the array primes. Given an integer n and an array of integers primes, return the nth super ugly number. The nth super ugly number is guaranteed to fit in a 32-bit signed integer.

Array,Hash Table,Math,Dynamic Programming,Heap (Priority Queue)

Medium

264

Binary Tree Vertical Order Traversal

binary-tree-vertical-order-traversal

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

Medium

102

Count of Smaller Numbers After Self

count-of-smaller-numbers-after-self

You are given an integer array nums and you have to return a new counts array. The counts array has the property where counts[i] is the number of smaller elements to the right of nums[i].

Array,Binary Search,Divide and Conquer,Binary Indexed Tree,Segment Tree,Merge Sort,Ordered Set

Hard

327,406,493,1482,2280

Remove Duplicate Letters

remove-duplicate-letters

Given a string s, remove duplicate letters so that every letter appears once and only once. You must make sure your result is the smallest in lexicographical order among all possible results.

String,Stack,Greedy,Monotonic Stack

Medium

Greedily try to add one missing character. How to check if adding some character will not cause problems ? Use bit-masks to check whether you will be able to complete the sub-sequence if you add the character at some index i.

2157

Shortest Distance from All Buildings

shortest-distance-from-all-buildings

Array,Breadth-First Search,Matrix

Hard

286,296,1117

Maximum Product of Word Lengths

maximum-product-of-word-lengths

Given a string array words, return the maximum value of length(word[i]) * length(word[j]) where the two words do not share common letters. If no such two words exist, return 0.

Array,String,Bit Manipulation

Medium

Bulb Switcher

bulb-switcher

There are n bulbs that are initially off. You first turn on all the bulbs, then you turn off every second bulb. On the third round, you toggle every third bulb (turning on if it's off or turning off if it's on). For the ith round, you toggle every i bulb. For the nth round, you only toggle the last bulb. Return the number of bulbs that are on after n rounds.

Math,Brainteaser

Medium

672,1037,1491

Generalized Abbreviation

generalized-abbreviation

String,Backtracking,Bit Manipulation

Medium

78,288,411

Create Maximum Number

create-maximum-number

You are given two integer arrays nums1 and nums2 of lengths m and n respectively. nums1 and nums2 represent the digits of two numbers. You are also given an integer k. Create the maximum number of length k <= m + n from digits of the two numbers. The relative order of the digits from the same array must be preserved. Return an array of the k digits representing the answer.

Stack,Greedy,Monotonic Stack

Hard

402,670

Coin Change

coin-change

You are given an integer array coins representing coins of different denominations and an integer amount representing a total amount of money. Return the fewest number of coins that you need to make up that amount. If that amount of money cannot be made up by any combination of the coins, return -1. You may assume that you have an infinite number of each kind of coin.

Array,Dynamic Programming,Breadth-First Search

Medium

1025,1393,2345

Number of Connected Components in an Undirected Graph

number-of-connected-components-in-an-undirected-graph

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

Medium

200,261,547,2218

Wiggle Sort II

wiggle-sort-ii

Given an integer array nums, reorder it such that nums[0] < nums[1] > nums[2] < nums[3].... You may assume the input array always has a valid answer.

Array,Divide and Conquer,Sorting,Quickselect

Medium

75,215,280,2085

Maximum Size Subarray Sum Equals k

maximum-size-subarray-sum-equals-k

Array,Hash Table

Medium

Compute the prefix sum array where psum[i] is the sum of all the elements from 0 to i. At each index i, the sum of the prefix is psum[i], so we are searching for the index x where psum[x] = psum[i] - k. The subarray [x + 1, i] will be of sum k. Use a hashmap to get the index x efficiently or to determine that it does not exist.

209,303,525,713

Power of Three

power-of-three

Given an integer n, return true if it is a power of three. Otherwise, return false. An integer n is a power of three, if there exists an integer x such that n == 3x.

Math,Recursion

Easy

231,342,1889

Count of Range Sum

count-of-range-sum

Given an integer array nums and two integers lower and upper, return the number of range sums that lie in [lower, upper] inclusive. Range sum S(i, j) is defined as the sum of the elements in nums between indices i and j inclusive, where i <= j.

Array,Binary Search,Divide and Conquer,Binary Indexed Tree,Segment Tree,Merge Sort,Ordered Set

Hard

315,493

Odd Even Linked List

odd-even-linked-list

Given the head of a singly linked list, group all the nodes with odd indices together followed by the nodes with even indices, and return the reordered list. The first node is considered odd, and the second node is even, and so on. Note that the relative order inside both the even and odd groups should remain as it was in the input. You must solve the problem in O(1) extra space complexity and O(n) time complexity.

Linked List

Medium

725

Longest Increasing Path in a Matrix

longest-increasing-path-in-a-matrix

Given an m x n integers matrix, return the length of the longest increasing path in matrix. From each cell, you can either move in four directions: left, right, up, or down. You may not move diagonally or move outside the boundary (i.e., wrap-around is not allowed).

Dynamic Programming,Depth-First Search,Breadth-First Search,Graph,Topological Sort,Memoization

Hard

Patching Array

patching-array

Given a sorted integer array nums and an integer n, add/patch elements to the array such that any number in the range [1, n] inclusive can be formed by the sum of some elements in the array. Return the minimum number of patches required.

Array,Greedy

Hard

1930

Verify Preorder Serialization of a Binary Tree

verify-preorder-serialization-of-a-binary-tree

One way to serialize a binary tree is to use preorder traversal. When we encounter a non-null node, we record the node's value. If it is a null node, we record using a sentinel value such as '#'. For example, the above binary tree can be serialized to the string "9,3,4,#,#,1,#,#,2,#,6,#,#", where '#' represents a null node. Given a string of comma-separated values preorder, return true if it is a correct preorder traversal serialization of a binary tree. It is guaranteed that each comma-separated value in the string must be either an integer or a character '#' representing null pointer. You may assume that the input format is always valid. Note: You are not allowed to reconstruct the tree.

String,Stack,Tree,Binary Tree

Medium

Reconstruct Itinerary

reconstruct-itinerary

You are given a list of airline tickets where tickets[i] = [fromi, toi] represent the departure and the arrival airports of one flight. Reconstruct the itinerary in order and return it. All of the tickets belong to a man who departs from "JFK", thus, the itinerary must begin with "JFK". If there are multiple valid itineraries, you should return the itinerary that has the smallest lexical order when read as a single string. You may assume all tickets form at least one valid itinerary. You must use all the tickets once and only once.

Depth-First Search,Graph,Eulerian Circuit

Hard

2051,2201

Largest BST Subtree

largest-bst-subtree

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

Medium

You can recursively use algorithm similar to 98. Validate Binary Search Tree at each node of the tree, which will result in O(nlogn) time complexity.

Increasing Triplet Subsequence

increasing-triplet-subsequence

Given an integer array nums, return true if there exists a triple of indices (i, j, k) such that i < j < k and nums[i] < nums[j] < nums[k]. If no such indices exists, return false.

Array,Greedy

Medium

300,2122,2280

Self Crossing

self-crossing

You are given an array of integers distance. You start at point (0,0) on an X-Y plane and you move distance[0] meters to the north, then distance[1] meters to the west, distance[2] meters to the south, distance[3] meters to the east, and so on. In other words, after each move, your direction changes counter-clockwise. Return true if your path crosses itself, and false if it does not.

Array,Math,Geometry

Hard

Palindrome Pairs

palindrome-pairs

Given a list of unique words, return all the pairs of the distinct indices (i, j) in the given list, so that the concatenation of the two words words[i] + words[j] is a palindrome.

Array,Hash Table,String,Trie

Hard

5,214,2237

House Robber III

house-robber-iii

The thief has found himself a new place for his thievery again. There is only one entrance to this area, called root. Besides the root, each house has one and only one parent house. After a tour, the smart thief realized that all houses in this place form a binary tree. It will automatically contact the police if two directly-linked houses were broken into on the same night. Given the root of the binary tree, return the maximum amount of money the thief can rob without alerting the police.

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

Medium

198,213

Counting Bits

counting-bits

Given an integer n, return an array ans of length n + 1 such that for each i (0 <= i <= n), ans[i] is the number of 1's in the binary representation of i.

Dynamic Programming,Bit Manipulation

Easy

You should make use of what you have produced already. Divide the numbers in ranges like [2-3], [4-7], [8-15] and so on. And try to generate new range from previous. Or does the odd/even status of the number help you in calculating the number of 1s?

191

Nested List Weight Sum

nested-list-weight-sum

Depth-First Search,Breadth-First Search

Medium

364,565,690

Longest Substring with At Most K Distinct Characters

longest-substring-with-at-most-k-distinct-characters

Hash Table,String,Sliding Window

Medium

3,159,424,1034,1046,2134

Flatten Nested List Iterator

flatten-nested-list-iterator

You are given a nested list of integers nestedList. Each element is either an integer or a list whose elements may also be integers or other lists. Implement an iterator to flatten it. Implement the NestedIterator class: Your code will be tested with the following pseudocode: If res matches the expected flattened list, then your code will be judged as correct.

Stack,Tree,Depth-First Search,Design,Queue,Iterator

Medium

251,281,385,565

Power of Four

power-of-four

Given an integer n, return true if it is a power of four. Otherwise, return false. An integer n is a power of four, if there exists an integer x such that n == 4x.

Math,Bit Manipulation,Recursion

Easy

231,326

Integer Break

integer-break

Given an integer n, break it into the sum of k positive integers, where k >= 2, and maximize the product of those integers. Return the maximum product you can get.

Math,Dynamic Programming

Medium

There is a simple O(n) solution to this problem. You may check the breaking results of n ranging from 7 to 10 to discover the regularities.

1936

Reverse String

reverse-string

Write a function that reverses a string. The input string is given as an array of characters s. You must do this by modifying the input array in-place with O(1) extra memory.

Two Pointers,String,Recursion

Easy

The entire logic for reversing a string is based on using the opposite directional two-pointer approach!

345,541

Reverse Vowels of a String

reverse-vowels-of-a-string

Given a string s, reverse only all the vowels in the string and return it. The vowels are 'a', 'e', 'i', 'o', and 'u', and they can appear in both cases.

Two Pointers,String

Easy

344,1089

Moving Average from Data Stream

moving-average-from-data-stream

Array,Design,Queue,Data Stream

Easy

2211

Top K Frequent Elements

top-k-frequent-elements

Given an integer array nums and an integer k, return the k most frequent elements. You may return the answer in any order.

Array,Hash Table,Divide and Conquer,Sorting,Heap (Priority Queue),Bucket Sort,Counting,Quickselect

Medium

192,215,451,659,692,1014,1919

Design Tic-Tac-Toe

design-tic-tac-toe

Array,Hash Table,Design,Matrix

Medium

Could you trade extra space such that move() operation can be done in O(1)? You need two arrays: int rows[n], int cols[n], plus two variables: diagonal, anti_diagonal.

810

Intersection of Two Arrays

intersection-of-two-arrays

Given two integer arrays nums1 and nums2, return an array of their intersection. Each element in the result must be unique and you may return the result in any order.

Array,Hash Table,Two Pointers,Binary Search,Sorting

Easy

350,1149,1392,2190,2282

Intersection of Two Arrays II

intersection-of-two-arrays-ii

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

Array,Hash Table,Two Pointers,Binary Search,Sorting

Easy

349,1044,1392,2282

Android Unlock Patterns

android-unlock-patterns

Dynamic Programming,Backtracking

Medium

Data Stream as Disjoint Intervals

data-stream-as-disjoint-intervals

Given a data stream input of non-negative integers a1, a2, ..., an, summarize the numbers seen so far as a list of disjoint intervals. Implement the SummaryRanges class:

Binary Search,Design,Ordered Set

Hard

228,436,715

Design Snake Game

design-snake-game

Array,Design,Queue,Matrix

Medium

Russian Doll Envelopes

russian-doll-envelopes

You are given a 2D array of integers envelopes where envelopes[i] = [wi, hi] represents the width and the height of an envelope. One envelope can fit into another if and only if both the width and height of one envelope are greater than the other envelope's width and height. Return the maximum number of envelopes you can Russian doll (i.e., put one inside the other). Note: You cannot rotate an envelope.

Array,Binary Search,Dynamic Programming,Sorting

Hard

300,2123

Design Twitter

design-twitter

Design a simplified version of Twitter where users can post tweets, follow/unfollow another user, and is able to see the 10 most recent tweets in the user's news feed. Implement the Twitter class:

Hash Table,Linked List,Design,Heap (Priority Queue)

Medium

1640

Line Reflection

line-reflection

Array,Hash Table,Math

Medium

Find the smallest and largest x-value for all points. If there is a line then it should be at y = (minX + maxX) / 2. For each point, make sure that it has a reflected point in the opposite side.

149,447

Count Numbers with Unique Digits

count-numbers-with-unique-digits

Given an integer n, return the count of all numbers with unique digits, x, where 0 <= x < 10n.

Math,Dynamic Programming,Backtracking

Medium

A direct way is to use the backtracking approach. Backtracking should contains three states which are (the current number, number of steps to get that number and a bitmask which represent which number is marked as visited so far in the current number). Start with state (0,0,0) and count all valid number till we reach number of steps equals to 10n. This problem can also be solved using a dynamic programming approach and some knowledge of combinatorics. Let f(k) = count of numbers with unique digits with length equals k. f(1) = 10, ..., f(k) = 9 * 9 * 8 * ... (9 - k + 2) [The first factor is 9 because a number cannot start with 0].

Rearrange String k Distance Apart

rearrange-string-k-distance-apart

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

Hard

621,778,2300

Logger Rate Limiter

logger-rate-limiter

Hash Table,Design

Easy

362

Sort Transformed Array

sort-transformed-array

Array,Math,Two Pointers,Sorting

Medium

x^2 + x will form a parabola. Parameter A in: A * x^2 + B * x + C dictates the shape of the parabola. Positive A means the parabola remains concave (high-low-high), but negative A inverts the parabola to be convex (low-high-low).

1019

Bomb Enemy

bomb-enemy

Array,Dynamic Programming,Matrix

Medium

2192,2200

Design Hit Counter

design-hit-counter

Array,Hash Table,Binary Search,Design,Queue

Medium

359

Max Sum of Rectangle No Larger Than K

max-sum-of-rectangle-no-larger-than-k

Given an m x n matrix matrix and an integer k, return the max sum of a rectangle in the matrix such that its sum is no larger than k. It is guaranteed that there will be a rectangle with a sum no larger than k.

Array,Binary Search,Dynamic Programming,Matrix,Ordered Set

Hard

Nested List Weight Sum II

nested-list-weight-sum-ii

Stack,Depth-First Search,Breadth-First Search

Medium

339,565

Water and Jug Problem

water-and-jug-problem

You are given two jugs with capacities jug1Capacity and jug2Capacity liters. There is an infinite amount of water supply available. Determine whether it is possible to measure exactly targetCapacity liters using these two jugs. If targetCapacity liters of water are measurable, you must have targetCapacity liters of water contained within one or both buckets by the end. Operations allowed:

Math,Depth-First Search,Breadth-First Search

Medium

Find Leaves of Binary Tree

find-leaves-of-binary-tree

Tree,Depth-First Search,Binary Tree

Medium

Valid Perfect Square

valid-perfect-square

Given a positive integer num, write a function which returns True if num is a perfect square else False. Follow up: Do not use any built-in library function such as sqrt.

Math,Binary Search

Easy

69,633

Largest Divisible Subset

largest-divisible-subset

Given a set of distinct positive integers nums, return the largest subset answer such that every pair (answer[i], answer[j]) of elements in this subset satisfies: If there are multiple solutions, return any of them.

Array,Math,Dynamic Programming,Sorting

Medium

Plus One Linked List

plus-one-linked-list

Linked List,Math

Medium

66

Range Addition

range-addition

Array,Prefix Sum

Medium

Thinking of using advanced data structures? You are thinking it too complicated. For each update operation, do you really need to update all elements between i and j? Update only the first and end element is sufficient. The optimal time complexity is O(k + n) and uses O(1) extra space.

598,2385

Sum of Two Integers

sum-of-two-integers

Given two integers a and b, return the sum of the two integers without using the operators + and -.

Math,Bit Manipulation

Medium

2

Super Pow

super-pow

Your task is to calculate ab mod 1337 where a is a positive integer and b is an extremely large positive integer given in the form of an array.

Math,Divide and Conquer

Medium

50

Find K Pairs with Smallest Sums

find-k-pairs-with-smallest-sums

You are given two integer arrays nums1 and nums2 sorted in ascending order and an integer k. Define a pair (u, v) which consists of one element from the first array and one element from the second array. Return the k pairs (u1, v1), (u2, v2), ..., (uk, vk) with the smallest sums.

Array,Heap (Priority Queue)

Medium

378,719,2150

Guess Number Higher or Lower

guess-number-higher-or-lower

We are playing the Guess Game. The game is as follows: I pick a number from 1 to n. You have to guess which number I picked. Every time you guess wrong, I will tell you whether the number I picked is higher or lower than your guess. You call a pre-defined API int guess(int num), which returns three possible results: Return the number that I picked.

Binary Search,Interactive

Easy

278,375,658

Guess Number Higher or Lower II

guess-number-higher-or-lower-ii

We are playing the Guessing Game. The game will work as follows: Given a particular n, return the minimum amount of money you need to guarantee a win regardless of what number I pick.

Math,Dynamic Programming,Game Theory

Medium

The best strategy to play the game is to minimize the maximum loss you could possibly face. Another strategy is to minimize the expected loss. Here, we are interested in the first scenario. Take a small example (n = 3). What do you end up paying in the worst case? Check out this article if you're still stuck. The purely recursive implementation of minimax would be worthless for even a small n. You MUST use dynamic programming. As a follow-up, how would you modify your code to solve the problem of minimizing the expected loss, instead of the worst-case loss?

294,374,464,658

Wiggle Subsequence

wiggle-subsequence

A wiggle sequence is a sequence where the differences between successive numbers strictly alternate between positive and negative. The first difference (if one exists) may be either positive or negative. A sequence with one element and a sequence with two non-equal elements are trivially wiggle sequences. A subsequence is obtained by deleting some elements (possibly zero) from the original sequence, leaving the remaining elements in their original order. Given an integer array nums, return the length of the longest wiggle subsequence of nums.

Array,Dynamic Programming,Greedy

Medium

2271

Combination Sum IV

combination-sum-iv

Given an array of distinct integers nums and a target integer target, return the number of possible combinations that add up to target. The test cases are generated so that the answer can fit in a 32-bit integer.

Array,Dynamic Programming

Medium

39

Kth Smallest Element in a Sorted Matrix

kth-smallest-element-in-a-sorted-matrix

Given an n x n matrix where each of the rows and columns is sorted in ascending order, return the kth smallest element in the matrix. Note that it is the kth smallest element in the sorted order, not the kth distinct element. You must find a solution with a memory complexity better than O(n2).

Array,Binary Search,Sorting,Heap (Priority Queue),Matrix

Medium

373,668,719,802

Design Phone Directory

design-phone-directory

Array,Hash Table,Linked List,Design,Queue

Medium

1955

Insert Delete GetRandom O(1)

insert-delete-getrandom-o1

Implement the RandomizedSet class: You must implement the functions of the class such that each function works in average O(1) time complexity.

Array,Hash Table,Math,Design,Randomized

Medium

381

Insert Delete GetRandom O(1) - Duplicates allowed

insert-delete-getrandom-o1-duplicates-allowed

RandomizedCollection is a data structure that contains a collection of numbers, possibly duplicates (i.e., a multiset). It should support inserting and removing specific elements and also removing a random element. Implement the RandomizedCollection class: You must implement the functions of the class such that each function works on average O(1) time complexity. Note: The test cases are generated such that getRandom will only be called if there is at least one item in the RandomizedCollection.

Array,Hash Table,Math,Design,Randomized

Hard

380

Linked List Random Node

linked-list-random-node

Given a singly linked list, return a random node's value from the linked list. Each node must have the same probability of being chosen. Implement the Solution class:

Linked List,Math,Reservoir Sampling,Randomized

Medium

398

Ransom Note

ransom-note

Given two strings ransomNote and magazine, return true if ransomNote can be constructed from magazine and false otherwise. Each letter in magazine can only be used once in ransomNote.

Hash Table,String,Counting

Easy

691

Shuffle an Array

shuffle-an-array

Given an integer array nums, design an algorithm to randomly shuffle the array. All permutations of the array should be equally likely as a result of the shuffling. Implement the Solution class:

Array,Math,Randomized

Medium

The solution expects that we always use the original array to shuffle() else some of the test cases fail. (Credits; @snehasingh31)

Mini Parser

mini-parser

Given a string s represents the serialization of a nested list, implement a parser to deserialize it and return the deserialized NestedInteger. Each element is either an integer or a list whose elements may also be integers or other lists.

String,Stack,Depth-First Search

Medium

341,439,722

Lexicographical Numbers

lexicographical-numbers

Given an integer n, return all the numbers in the range [1, n] sorted in lexicographical order. You must write an algorithm that runs in O(n) time and uses O(1) extra space.

Depth-First Search,Trie

Medium

First Unique Character in a String

first-unique-character-in-a-string

Given a string s, find the first non-repeating character in it and return its index. If it does not exist, return -1.

Hash Table,String,Queue,Counting

Easy

451

Longest Absolute File Path

longest-absolute-file-path

Suppose we have a file system that stores both files and directories. An example of one system is represented in the following picture: Here, we have dir as the only directory in the root. dir contains two subdirectories, subdir1 and subdir2. subdir1 contains a file file1.ext and subdirectory subsubdir1. subdir2 contains a subdirectory subsubdir2, which contains a file file2.ext. In text form, it looks like this (with ⟶ representing the tab character): If we were to write this representation in code, it will look like this: "dir\n\tsubdir1\n\t\tfile1.ext\n\t\tsubsubdir1\n\tsubdir2\n\t\tsubsubdir2\n\t\t\tfile2.ext". Note that the '\n' and '\t' are the new-line and tab characters. Every file and directory has a unique absolute path in the file system, which is the order of directories that must be opened to reach the file/directory itself, all concatenated by '/'s. Using the above example, the absolute path to file2.ext is "dir/subdir2/subsubdir2/file2.ext". Each directory name consists of letters, digits, and/or spaces. Each file name is of the form name.extension, where name and extension consist of letters, digits, and/or spaces. Given a string input representing the file system in the explained format, return the length of the longest absolute path to a file in the abstracted file system. If there is no file in the system, return 0.

String,Stack,Depth-First Search

Medium

Find the Difference

find-the-difference

You are given two strings s and t. String t is generated by random shuffling string s and then add one more letter at a random position. Return the letter that was added to t.

Hash Table,String,Bit Manipulation,Sorting

Easy

136

Elimination Game

elimination-game

You have a list arr of all integers in the range [1, n] sorted in a strictly increasing order. Apply the following algorithm on arr: Given the integer n, return the last number that remains in arr.

Math

Medium

Perfect Rectangle

perfect-rectangle

Given an array rectangles where rectangles[i] = [xi, yi, ai, bi] represents an axis-aligned rectangle. The bottom-left point of the rectangle is (xi, yi) and the top-right point of it is (ai, bi). Return true if all the rectangles together form an exact cover of a rectangular region.

Array,Line Sweep

Hard

Is Subsequence

is-subsequence

Given two strings s and t, return true if s is a subsequence of t, or false otherwise. A subsequence of a string is a new string that is formed from the original string by deleting some (can be none) of the characters without disturbing the relative positions of the remaining characters. (i.e., "ace" is a subsequence of "abcde" while "aec" is not).

Two Pointers,String,Dynamic Programming

Easy

808,1051

UTF-8 Validation

utf-8-validation

Given an integer array data representing the data, return whether it is a valid UTF-8 encoding (i.e. it translates to a sequence of valid UTF-8 encoded characters). A character in UTF8 can be from 1 to 4 bytes long, subjected to the following rules: This is how the UTF-8 encoding would work: x denotes a bit in the binary form of a byte that may be either 0 or 1. Note: The input is an array of integers. Only the least significant 8 bits of each integer is used to store the data. This means each integer represents only 1 byte of data.

Array,Bit Manipulation

Medium

All you have to do is follow the rules. For a given integer, obtain its binary representation in the string form and work with the rules given in the problem. An integer can either represent the start of a UTF-8 character, or a part of an existing UTF-8 character. There are two separate rules for these two scenarios in the problem. If an integer is a part of an existing UTF-8 character, simply check the 2 most significant bits of in the binary representation string. They should be 10. If the integer represents the start of a UTF-8 character, then the first few bits would be 1 followed by a 0. The number of initial bits (most significant) bits determines the length of the UTF-8 character. Note: The array can contain multiple valid UTF-8 characters. String manipulation will work fine here. But, it is too slow. Can we instead use bit manipulation to do the validations instead of string manipulations? We can use bit masking to check how many initial bits are set for a given number. We only need to work with the 8 least significant bits as mentioned in the problem. mask = 1 << 7 while mask & num: n_bytes += 1 mask = mask >> 1 Can you use bit-masking to perform the second validation as well i.e. checking if the most significant bit is 1 and the second most significant bit a 0? To check if the most significant bit is a 1 and the second most significant bit is a 0, we can make use of the following two masks. mask1 = 1 << 7 mask2 = 1 << 6 if not (num & mask1 and not (num & mask2)): return False

Decode String

decode-string

Given an encoded string, return its decoded string. The encoding rule is: k[encoded_string], where the encoded_string inside the square brackets is being repeated exactly k times. Note that k is guaranteed to be a positive integer. You may assume that the input string is always valid; there are no extra white spaces, square brackets are well-formed, etc. Furthermore, you may assume that the original data does not contain any digits and that digits are only for those repeat numbers, k. For example, there will not be input like 3a or 2[4].

String,Stack,Recursion

Medium

471,726,1076

Longest Substring with At Least K Repeating Characters

longest-substring-with-at-least-k-repeating-characters

Given a string s and an integer k, return the length of the longest substring of s such that the frequency of each character in this substring is greater than or equal to k.

Hash Table,String,Divide and Conquer,Sliding Window

Medium

2140,2209

Rotate Function

rotate-function

You are given an integer array nums of length n. Assume arrk to be an array obtained by rotating nums by k positions clock-wise. We define the rotation function F on nums as follow: Return the maximum value of F(0), F(1), ..., F(n-1). The test cases are generated so that the answer fits in a 32-bit integer.

Array,Math,Dynamic Programming

Medium

Integer Replacement

integer-replacement

Given a positive integer n, you can apply one of the following operations: Return the minimum number of operations needed for n to become 1.

Dynamic Programming,Greedy,Bit Manipulation,Memoization

Medium

Random Pick Index

random-pick-index

Given an integer array nums with possible duplicates, randomly output the index of a given target number. You can assume that the given target number must exist in the array. Implement the Solution class:

Hash Table,Math,Reservoir Sampling,Randomized

Medium

382,894,912

Evaluate Division

evaluate-division

You are given an array of variable pairs equations and an array of real numbers values, where equations[i] = [Ai, Bi] and values[i] represent the equation Ai / Bi = values[i]. Each Ai or Bi is a string that represents a single variable. You are also given some queries, where queries[j] = [Cj, Dj] represents the jth query where you must find the answer for Cj / Dj = ?. Return the answers to all queries. If a single answer cannot be determined, return -1.0. Note: The input is always valid. You may assume that evaluating the queries will not result in division by zero and that there is no contradiction.

Array,Depth-First Search,Breadth-First Search,Union Find,Graph,Shortest Path

Medium

Do you recognize this as a graph problem?

Nth Digit

nth-digit

Given an integer n, return the nth digit of the infinite integer sequence [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, ...].

Math,Binary Search

Medium