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Bitwise AND of Numbers Range

bitwise-and-of-numbers-range

Given two integers left and right that represent the range [left, right], return the bitwise AND of all numbers in this range, inclusive.

Bit Manipulation

Medium

Happy Number

happy-number

Write an algorithm to determine if a number n is happy. A happy number is a number defined by the following process: Return true if n is a happy number, and false if not.

Hash Table,Math,Two Pointers

Easy

141,258,263,2076

Remove Linked List Elements

remove-linked-list-elements

Given the head of a linked list and an integer val, remove all the nodes of the linked list that has Node.val == val, and return the new head.

Linked List,Recursion

Easy

27,237,2216

Count Primes

count-primes

Given an integer n, return the number of prime numbers that are strictly less than n.

Array,Math,Enumeration,Number Theory

Medium

Let's start with a isPrime function. To determine if a number is prime, we need to check if it is not divisible by any number less than n. The runtime complexity of isPrime function would be O(n) and hence counting the total prime numbers up to n would be O(n2). Could we do better? As we know the number must not be divisible by any number > n / 2, we can immediately cut the total iterations half by dividing only up to n / 2. Could we still do better? Let's write down all of 12's factors: 2 × 6 = 12 3 × 4 = 12 4 × 3 = 12 6 × 2 = 12 As you can see, calculations of 4 × 3 and 6 × 2 are not necessary. Therefore, we only need to consider factors up to √n because, if n is divisible by some number p, then n = p × q and since p ≤ q, we could derive that p ≤ √n. Our total runtime has now improved to O(n1.5), which is slightly better. Is there a faster approach? public int countPrimes(int n) { int count = 0; for (int i = 1; i < n; i++) { if (isPrime(i)) count++; } return count; } private boolean isPrime(int num) { if (num <= 1) return false; // Loop's ending condition is i * i <= num instead of i <= sqrt(num) // to avoid repeatedly calling an expensive function sqrt(). for (int i = 2; i * i <= num; i++) { if (num % i == 0) return false; } return true; } The Sieve of Eratosthenes is one of the most efficient ways to find all prime numbers up to n. But don't let that name scare you, I promise that the concept is surprisingly simple. Sieve of Eratosthenes: algorithm steps for primes below 121. "Sieve of Eratosthenes Animation" by SKopp is licensed under CC BY 2.0. We start off with a table of n numbers. Let's look at the first number, 2. We know all multiples of 2 must not be primes, so we mark them off as non-primes. Then we look at the next number, 3. Similarly, all multiples of 3 such as 3 × 2 = 6, 3 × 3 = 9, ... must not be primes, so we mark them off as well. Now we look at the next number, 4, which was already marked off. What does this tell you? Should you mark off all multiples of 4 as well? 4 is not a prime because it is divisible by 2, which means all multiples of 4 must also be divisible by 2 and were already marked off. So we can skip 4 immediately and go to the next number, 5. Now, all multiples of 5 such as 5 × 2 = 10, 5 × 3 = 15, 5 × 4 = 20, 5 × 5 = 25, ... can be marked off. There is a slight optimization here, we do not need to start from 5 × 2 = 10. Where should we start marking off? In fact, we can mark off multiples of 5 starting at 5 × 5 = 25, because 5 × 2 = 10 was already marked off by multiple of 2, similarly 5 × 3 = 15 was already marked off by multiple of 3. Therefore, if the current number is p, we can always mark off multiples of p starting at p2, then in increments of p: p2 + p, p2 + 2p, ... Now what should be the terminating loop condition? It is easy to say that the terminating loop condition is p < n, which is certainly correct but not efficient. Do you still remember Hint #3? Yes, the terminating loop condition can be p < √n, as all non-primes ≥ √n must have already been marked off. When the loop terminates, all the numbers in the table that are non-marked are prime. The Sieve of Eratosthenes uses an extra O(n) memory and its runtime complexity is O(n log log n). For the more mathematically inclined readers, you can read more about its algorithm complexity on Wikipedia. public int countPrimes(int n) { boolean[] isPrime = new boolean[n]; for (int i = 2; i < n; i++) { isPrime[i] = true; } // Loop's ending condition is i * i < n instead of i < sqrt(n) // to avoid repeatedly calling an expensive function sqrt(). for (int i = 2; i * i < n; i++) { if (!isPrime[i]) continue; for (int j = i * i; j < n; j += i) { isPrime[j] = false; } } int count = 0; for (int i = 2; i < n; i++) { if (isPrime[i]) count++; } return count; }

263,264,279

Isomorphic Strings

isomorphic-strings

Given two strings s and t, determine if they are isomorphic. Two strings s and t are isomorphic if the characters in s can be replaced to get t. All occurrences of a character must be replaced with another character while preserving the order of characters. No two characters may map to the same character, but a character may map to itself.

Hash Table,String

Easy

290

Reverse Linked List

reverse-linked-list

Given the head of a singly linked list, reverse the list, and return the reversed list.

Linked List,Recursion

Easy

92,156,234,2196,2236

Course Schedule

course-schedule

There are a total of numCourses courses you have to take, labeled from 0 to numCourses - 1. You are given an array prerequisites where prerequisites[i] = [ai, bi] indicates that you must take course bi first if you want to take course ai. Return true if you can finish all courses. Otherwise, return false.

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

Medium

This problem is equivalent to finding if a cycle exists in a directed graph. If a cycle exists, no topological ordering exists and therefore it will be impossible to take all courses. Topological Sort via DFS - A great video tutorial (21 minutes) on Coursera explaining the basic concepts of Topological Sort. Topological sort could also be done via BFS.

210,261,310,630

Implement Trie (Prefix Tree)

implement-trie-prefix-tree

A trie (pronounced as "try") or prefix tree is a tree data structure used to efficiently store and retrieve keys in a dataset of strings. There are various applications of this data structure, such as autocomplete and spellchecker. Implement the Trie class:

Hash Table,String,Design,Trie

Medium

211,642,648,676,1433,1949

Minimum Size Subarray Sum

minimum-size-subarray-sum

Given an array of positive integers nums and a positive integer target, return the minimal length of a contiguous subarray [numsl, numsl+1, ..., numsr-1, numsr] of which the sum is greater than or equal to target. If there is no such subarray, return 0 instead.

Array,Binary Search,Sliding Window,Prefix Sum

Medium

76,325,718,1776,2211,2329

Course Schedule II

course-schedule-ii

There are a total of numCourses courses you have to take, labeled from 0 to numCourses - 1. You are given an array prerequisites where prerequisites[i] = [ai, bi] indicates that you must take course bi first if you want to take course ai. Return the ordering of courses you should take to finish all courses. If there are many valid answers, return any of them. If it is impossible to finish all courses, return an empty array.

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

Medium

This problem is equivalent to finding the topological order in a directed graph. If a cycle exists, no topological ordering exists and therefore it will be impossible to take all courses. Topological Sort via DFS - A great video tutorial (21 minutes) on Coursera explaining the basic concepts of Topological Sort. Topological sort could also be done via BFS.

207,269,310,444,630,1101,2220

Design Add and Search Words Data Structure

design-add-and-search-words-data-structure

Design a data structure that supports adding new words and finding if a string matches any previously added string. Implement the WordDictionary class: Example: Constraints:

String,Depth-First Search,Design,Trie

Medium

You should be familiar with how a Trie works. If not, please work on this problem: Implement Trie (Prefix Tree) first.

208,746

Word Search II

word-search-ii

Given an m x n board of characters and a list of strings words, return all words on the board. Each word must be constructed from letters of sequentially adjacent cells, where adjacent cells are horizontally or vertically neighboring. The same letter cell may not be used more than once in a word.

Array,String,Backtracking,Trie,Matrix

Hard

You would need to optimize your backtracking to pass the larger test. Could you stop backtracking earlier? If the current candidate does not exist in all words' prefix, you could stop backtracking immediately. What kind of data structure could answer such query efficiently? Does a hash table work? Why or why not? How about a Trie? If you would like to learn how to implement a basic trie, please work on this problem: Implement Trie (Prefix Tree) first.

79,1022,1433

House Robber II

house-robber-ii

You are a professional robber planning to rob houses along a street. Each house has a certain amount of money stashed. All houses at this place are arranged in a circle. That means the first house is the neighbor of the last one. Meanwhile, adjacent houses have a security system connected, and it will automatically contact the police if two adjacent houses were broken into on the same night. Given an integer array nums representing the amount of money of each house, return the maximum amount of money you can rob tonight without alerting the police.

Array,Dynamic Programming

Medium

Since House[1] and House[n] are adjacent, they cannot be robbed together. Therefore, the problem becomes to rob either House[1]-House[n-1] or House[2]-House[n], depending on which choice offers more money. Now the problem has degenerated to the House Robber, which is already been solved.

198,256,276,337,600,656

Shortest Palindrome

shortest-palindrome

You are given a string s. You can convert s to a palindrome by adding characters in front of it. Return the shortest palindrome you can find by performing this transformation.

String,Rolling Hash,String Matching,Hash Function

Hard

5,28,336

Kth Largest Element in an Array

kth-largest-element-in-an-array

Given an integer array nums and an integer k, return the kth largest element in the array. Note that it is the kth largest element in the sorted order, not the kth distinct element.

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

Medium

324,347,414,789,1014,2113,2204,2250

Combination Sum III

combination-sum-iii

Find all valid combinations of k numbers that sum up to n such that the following conditions are true: Return a list of all possible valid combinations. The list must not contain the same combination twice, and the combinations may be returned in any order.

Array,Backtracking

Medium

39

Contains Duplicate

contains-duplicate

Given an integer array nums, return true if any value appears at least twice in the array, and return false if every element is distinct.

Array,Hash Table,Sorting

Easy

219,220

The Skyline Problem

the-skyline-problem

A city's skyline is the outer contour of the silhouette formed by all the buildings in that city when viewed from a distance. Given the locations and heights of all the buildings, return the skyline formed by these buildings collectively. The geometric information of each building is given in the array buildings where buildings[i] = [lefti, righti, heighti]: You may assume all buildings are perfect rectangles grounded on an absolutely flat surface at height 0. The skyline should be represented as a list of "key points" sorted by their x-coordinate in the form [[x1,y1],[x2,y2],...]. Each key point is the left endpoint of some horizontal segment in the skyline except the last point in the list, which always has a y-coordinate 0 and is used to mark the skyline's termination where the rightmost building ends. Any ground between the leftmost and rightmost buildings should be part of the skyline's contour. Note: There must be no consecutive horizontal lines of equal height in the output skyline. For instance, [...,[2 3],[4 5],[7 5],[11 5],[12 7],...] is not acceptable; the three lines of height 5 should be merged into one in the final output as such: [...,[2 3],[4 5],[12 7],...]

Array,Divide and Conquer,Binary Indexed Tree,Segment Tree,Line Sweep,Heap (Priority Queue),Ordered Set

Hard

699

Contains Duplicate II

contains-duplicate-ii

Given an integer array nums and an integer k, return true if there are two distinct indices i and j in the array such that nums[i] == nums[j] and abs(i - j) <= k.

Array,Hash Table,Sliding Window

Easy

217,220

Contains Duplicate III

contains-duplicate-iii

Given an integer array nums and two integers k and t, return true if there are two distinct indices i and j in the array such that abs(nums[i] - nums[j]) <= t and abs(i - j) <= k.

Array,Sliding Window,Sorting,Bucket Sort,Ordered Set

Medium

Time complexity O(n logk) - This will give an indication that sorting is involved for k elements. Use already existing state to evaluate next state - Like, a set of k sorted numbers are only needed to be tracked. When we are processing the next number in array, then we can utilize the existing sorted state and it is not necessary to sort next overlapping set of k numbers again.

217,219

Maximal Square

maximal-square

Given an m x n binary matrix filled with 0's and 1's, find the largest square containing only 1's and return its area.

Array,Dynamic Programming,Matrix

Medium

85,769,1312,2200

Count Complete Tree Nodes

count-complete-tree-nodes

Given the root of a complete binary tree, return the number of the nodes in the tree. According to Wikipedia, every level, except possibly the last, is completely filled in a complete binary tree, and all nodes in the last level are as far left as possible. It can have between 1 and 2h nodes inclusive at the last level h. Design an algorithm that runs in less than O(n) time complexity.

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

Medium

270

Rectangle Area

rectangle-area

Given the coordinates of two rectilinear rectangles in a 2D plane, return the total area covered by the two rectangles. The first rectangle is defined by its bottom-left corner (ax1, ay1) and its top-right corner (ax2, ay2). The second rectangle is defined by its bottom-left corner (bx1, by1) and its top-right corner (bx2, by2).

Math,Geometry

Medium

866

Basic Calculator

basic-calculator

Given a string s representing a valid expression, implement a basic calculator to evaluate it, and return the result of the evaluation. Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as eval().

Math,String,Stack,Recursion

Hard

150,227,241,282,785,2147,2328

Implement Stack using Queues

implement-stack-using-queues

Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty). Implement the MyStack class: Notes:

Stack,Design,Queue

Easy

232

Invert Binary Tree

invert-binary-tree

Given the root of a binary tree, invert the tree, and return its root.

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

Easy

Basic Calculator II

basic-calculator-ii

Given a string s which represents an expression, evaluate this expression and return its value. The integer division should truncate toward zero. You may assume that the given expression is always valid. All intermediate results will be in the range of [-231, 231 - 1]. Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as eval().

Math,String,Stack

Medium

224,282,785

Summary Ranges

summary-ranges

You are given a sorted unique integer array nums. A range [a,b] is the set of all integers from a to b (inclusive). Return the smallest sorted list of ranges that cover all the numbers in the array exactly. That is, each element of nums is covered by exactly one of the ranges, and there is no integer x such that x is in one of the ranges but not in nums. Each range [a,b] in the list should be output as:

Array

Easy

163,352

Majority Element II

majority-element-ii

Given an integer array of size n, find all elements that appear more than ⌊ n/3 ⌋ times.

Array,Hash Table,Sorting,Counting

Medium

How many majority elements could it possibly have? Do you have a better hint? Suggest it!

169,1102

Kth Smallest Element in a BST

kth-smallest-element-in-a-bst

Given the root of a binary search tree, and an integer k, return the kth smallest value (1-indexed) of all the values of the nodes in the tree.

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

Medium

Try to utilize the property of a BST. Try in-order traversal. (Credits to @chan13) What if you could modify the BST node's structure? The optimal runtime complexity is O(height of BST).

94,671

Power of Two

power-of-two

Given an integer n, return true if it is a power of two. Otherwise, return false. An integer n is a power of two, if there exists an integer x such that n == 2x.

Math,Bit Manipulation,Recursion

Easy

191,326,342

Implement Queue using Stacks

implement-queue-using-stacks

Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty). Implement the MyQueue class: Notes:

Stack,Design,Queue

Easy

225

Number of Digit One

number-of-digit-one

Given an integer n, count the total number of digit 1 appearing in all non-negative integers less than or equal to n.

Math,Dynamic Programming,Recursion

Hard

Beware of overflow.

172,1068

Palindrome Linked List

palindrome-linked-list

Given the head of a singly linked list, return true if it is a palindrome.

Linked List,Two Pointers,Stack,Recursion

Easy

9,125,206,2236

Lowest Common Ancestor of a Binary Search Tree

lowest-common-ancestor-of-a-binary-search-tree

Given a binary search tree (BST), find the lowest common ancestor (LCA) of two given nodes in the BST. According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes p and q as the lowest node in T that has both p and q as descendants (where we allow a node to be a descendant of itself).”

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

Easy

236,1190,1780,1790,1816

Lowest Common Ancestor of a Binary Tree

lowest-common-ancestor-of-a-binary-tree

Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree. According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes p and q as the lowest node in T that has both p and q as descendants (where we allow a node to be a descendant of itself).”

Tree,Depth-First Search,Binary Tree

Medium

235,1190,1354,1780,1790,1816,2217

Delete Node in a Linked List

delete-node-in-a-linked-list

Write a function to delete a node in a singly-linked list. You will not be given access to the head of the list, instead you will be given access to the node to be deleted directly. It is guaranteed that the node to be deleted is not a tail node in the list.

Linked List

Easy

203

Product of Array Except Self

product-of-array-except-self

Given an integer array nums, return an array answer such that answer[i] is equal to the product of all the elements of nums except nums[i]. The product of any prefix or suffix of nums is guaranteed to fit in a 32-bit integer. You must write an algorithm that runs in O(n) time and without using the division operation.

Array,Prefix Sum

Medium

42,152,265,2267

Sliding Window Maximum

sliding-window-maximum

You are given an array of integers nums, there is a sliding window of size k which is moving from the very left of the array to the very right. You can only see the k numbers in the window. Each time the sliding window moves right by one position. Return the max sliding window.

Array,Queue,Sliding Window,Heap (Priority Queue),Monotonic Queue

Hard

How about using a data structure such as deque (double-ended queue)? The queue size need not be the same as the window’s size. Remove redundant elements and the queue should store only elements that need to be considered.

76,155,159,265,1814

Search a 2D Matrix II

search-a-2d-matrix-ii

Write an efficient algorithm that searches for a value target in an m x n integer matrix matrix. This matrix has the following properties:

Array,Binary Search,Divide and Conquer,Matrix

Medium

74

Different Ways to Add Parentheses

different-ways-to-add-parentheses

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

Math,String,Dynamic Programming,Recursion,Memoization

Medium

95,224,282,2147,2328

Valid Anagram

valid-anagram

Given two strings s and t, return true if t is an anagram of s, and false otherwise. An Anagram is a word or phrase formed by rearranging the letters of a different word or phrase, typically using all the original letters exactly once.

Hash Table,String,Sorting

Easy

49,266,438

Shortest Word Distance

shortest-word-distance

Array,String

Easy

244,245,2320

Shortest Word Distance II

shortest-word-distance-ii

Array,Hash Table,Two Pointers,String,Design

Medium

21,243,245

Shortest Word Distance III

shortest-word-distance-iii

Array,String

Medium

243,244

Strobogrammatic Number

strobogrammatic-number

Hash Table,Two Pointers,String

Easy

247,248,1069

Strobogrammatic Number II

strobogrammatic-number-ii

Array,String,Recursion

Medium

Try to use recursion and notice that it should recurse with n - 2 instead of n - 1.

246,248,2202

Strobogrammatic Number III

strobogrammatic-number-iii

Array,String,Recursion

Hard

246,247

Group Shifted Strings

group-shifted-strings

Array,Hash Table,String

Medium

49

Count Univalue Subtrees

count-univalue-subtrees

Tree,Depth-First Search,Binary Tree

Medium

572,687

Flatten 2D Vector

flatten-2d-vector

Array,Two Pointers,Design,Iterator

Medium

How many variables do you need to keep track? Two variables is all you need. Try with x and y. Beware of empty rows. It could be the first few rows. To write correct code, think about the invariant to maintain. What is it? The invariant is x and y must always point to a valid point in the 2d vector. Should you maintain your invariant ahead of time or right when you need it? Not sure? Think about how you would implement hasNext(). Which is more complex? Common logic in two different places should be refactored into a common method.

173,281,284,341

Meeting Rooms

meeting-rooms

Array,Sorting

Easy

56,253

Meeting Rooms II

meeting-rooms-ii

Array,Two Pointers,Greedy,Sorting,Heap (Priority Queue)

Medium

Think about how we would approach this problem in a very simplistic way. We will allocate rooms to meetings that occur earlier in the day v/s the ones that occur later on, right? If you've figured out that we have to sort the meetings by their start time, the next thing to think about is how do we do the allocation? There are two scenarios possible here for any meeting. Either there is no meeting room available and a new one has to be allocated, or a meeting room has freed up and this meeting can take place there. An important thing to note is that we don't really care which room gets freed up while allocating a room for the current meeting. As long as a room is free, our job is done. We already know the rooms we have allocated till now and we also know when are they due to get free because of the end times of the meetings going on in those rooms. We can simply check the room which is due to get vacated the earliest amongst all the allocated rooms. Following up on the previous hint, we can make use of a min-heap to store the end times of the meetings in various rooms. So, every time we want to check if any room is free or not, simply check the topmost element of the min heap as that would be the room that would get free the earliest out of all the other rooms currently occupied. If the room we extracted from the top of the min heap isn't free, then no other room is. So, we can save time here and simply allocate a new room.

56,252,452,1184

Factor Combinations

factor-combinations

Array,Backtracking

Medium

39

Verify Preorder Sequence in Binary Search Tree

verify-preorder-sequence-in-binary-search-tree

Stack,Tree,Binary Search Tree,Recursion,Monotonic Stack,Binary Tree

Medium

144

Paint House

paint-house

Array,Dynamic Programming

Medium

198,213,265,276

Binary Tree Paths

binary-tree-paths

Given the root of a binary tree, return all root-to-leaf paths in any order. A leaf is a node with no children.

String,Backtracking,Tree,Depth-First Search,Binary Tree

Easy

113,1030,2217

Add Digits

add-digits

Given an integer num, repeatedly add all its digits until the result has only one digit, and return it.

Math,Simulation,Number Theory

Easy

A naive implementation of the above process is trivial. Could you come up with other methods? What are all the possible results? How do they occur, periodically or randomly? You may find this Wikipedia article useful.

202,1082,2076,2264

3Sum Smaller

3sum-smaller

Array,Two Pointers,Binary Search,Sorting

Medium

15,16,611,1083

Single Number III

single-number-iii

Given an integer array nums, in which exactly two elements appear only once and all the other elements appear exactly twice. Find the two elements that appear only once. You can return the answer in any order. You must write an algorithm that runs in linear runtime complexity and uses only constant extra space.

Array,Bit Manipulation

Medium

136,137

Graph Valid Tree

graph-valid-tree

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

Medium

Given n = 5 and edges = [[0, 1], [1, 2], [3, 4]], what should your return? Is this case a valid tree? According to the definition of tree on Wikipedia: “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.”

207,323,871

Trips and Users

trips-and-users

Table: Trips Table: Users The cancellation rate is computed by dividing the number of canceled (by client or driver) requests with unbanned users by the total number of requests with unbanned users on that day. Write a SQL query to find the cancellation rate of requests with unbanned users (both client and driver must not be banned) each day between "2013-10-01" and "2013-10-03". Round Cancellation Rate to two decimal points. Return the result table in any order. The query result format is in the following example.

Database

Hard

1779,1785,1795

Ugly Number

ugly-number

An ugly number is a positive integer whose prime factors are limited to 2, 3, and 5. Given an integer n, return true if n is an ugly number.

Math

Easy

202,204,264

Ugly Number II

ugly-number-ii

An ugly number is a positive integer whose prime factors are limited to 2, 3, and 5. Given an integer n, return the nth ugly number.

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

Medium

The naive approach is to call isUgly for every number until you reach the nth one. Most numbers are not ugly. Try to focus your effort on generating only the ugly ones. An ugly number must be multiplied by either 2, 3, or 5 from a smaller ugly number. The key is how to maintain the order of the ugly numbers. Try a similar approach of merging from three sorted lists: L1, L2, and L3. Assume you have Uk, the kth ugly number. Then Uk+1 must be Min(L1 * 2, L2 * 3, L3 * 5).

23,204,263,279,313,1307

Paint House II

paint-house-ii

Array,Dynamic Programming

Hard

238,239,256,276

Palindrome Permutation

palindrome-permutation

Hash Table,String,Bit Manipulation

Easy

Consider the palindromes of odd vs even length. What difference do you notice? Count the frequency of each character. If each character occurs even number of times, then it must be a palindrome. How about character which occurs odd number of times?

5,242,267,409

Palindrome Permutation II

palindrome-permutation-ii

Hash Table,String,Backtracking

Medium

If a palindromic permutation exists, we just need to generate the first half of the string. To generate all distinct permutations of a (half of) string, use a similar approach from: Permutations II or Next Permutation.

31,47,266

Missing Number

missing-number

Given an array nums containing n distinct numbers in the range [0, n], return the only number in the range that is missing from the array.

Array,Hash Table,Math,Bit Manipulation,Sorting

Easy

41,136,287,770,2107

Alien Dictionary

alien-dictionary

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

Hard

210

Closest Binary Search Tree Value

closest-binary-search-tree-value

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

Easy

222,272,783

Encode and Decode Strings

encode-and-decode-strings

Array,String,Design

Medium

38,297,443,696

Closest Binary Search Tree Value II

closest-binary-search-tree-value-ii

Two Pointers,Stack,Tree,Depth-First Search,Binary Search Tree,Heap (Priority Queue),Binary Tree

Hard

Consider implement these two helper functions: getPredecessor(N), which returns the next smaller node to N. getSuccessor(N), which returns the next larger node to N. Try to assume that each node has a parent pointer, it makes the problem much easier. Without parent pointer we just need to keep track of the path from the root to the current node using a stack. You would need two stacks to track the path in finding predecessor and successor node separately.

94,270

Integer to English Words

integer-to-english-words

Convert a non-negative integer num to its English words representation.

Math,String,Recursion

Hard

Did you see a pattern in dividing the number into chunk of words? For example, 123 and 123000. Group the number by thousands (3 digits). You can write a helper function that takes a number less than 1000 and convert just that chunk to words. There are many edge cases. What are some good test cases? Does your code work with input such as 0? Or 1000010? (middle chunk is zero and should not be printed out)

12

H-Index

h-index

Given an array of integers citations where citations[i] is the number of citations a researcher received for their ith paper, return compute the researcher's h-index. According to the definition of h-index on Wikipedia: A scientist has an index h if h of their n papers have at least h citations each, and the other n − h papers have no more than h citations each. If there are several possible values for h, the maximum one is taken as the h-index.

Array,Sorting,Counting Sort

Medium

An easy approach is to sort the array first. What are the possible values of h-index? A faster approach is to use extra space.

275

H-Index II

h-index-ii

Given an array of integers citations where citations[i] is the number of citations a researcher received for their ith paper and citations is sorted in an ascending order, return compute the researcher's h-index. According to the definition of h-index on Wikipedia: A scientist has an index h if h of their n papers have at least h citations each, and the other n − h papers have no more than h citations each. If there are several possible values for h, the maximum one is taken as the h-index. You must write an algorithm that runs in logarithmic time.

Array,Binary Search

Medium

Expected runtime complexity is in O(log n) and the input is sorted.

274

Paint Fence

paint-fence

Dynamic Programming

Medium

198,213,256,265

Find the Celebrity

find-the-celebrity

Two Pointers,Greedy,Graph,Interactive

Medium

The best hint for this problem can be provided by the following figure: Well, if you understood the gist of the above idea, you can extend it to find a candidate that can possibly be a celebrity. Why do we say a "candidate"? That is for you to think. This is clearly a greedy approach to find the answer. However, there is some information that would still remain to be verified without which we can't obtain an answer with certainty. To get that stake in the ground, we would need some more calls to the knows API.

1039

First Bad Version

first-bad-version

You are a product manager and currently leading a team to develop a new product. Unfortunately, the latest version of your product fails the quality check. Since each version is developed based on the previous version, all the versions after a bad version are also bad. Suppose you have n versions [1, 2, ..., n] and you want to find out the first bad one, which causes all the following ones to be bad. You are given an API bool isBadVersion(version) which returns whether version is bad. Implement a function to find the first bad version. You should minimize the number of calls to the API.

Binary Search,Interactive

Easy

34,35,374

Perfect Squares

perfect-squares

Given an integer n, return the least number of perfect square numbers that sum to n. A perfect square is an integer that is the square of an integer; in other words, it is the product of some integer with itself. For example, 1, 4, 9, and 16 are perfect squares while 3 and 11 are not.

Math,Dynamic Programming,Breadth-First Search

Medium

204,264

Wiggle Sort

wiggle-sort

Array,Greedy,Sorting

Medium

75,324,2085

Zigzag Iterator

zigzag-iterator

Array,Design,Queue,Iterator

Medium

173,251,284,341,1894

Expression Add Operators

expression-add-operators

Given a string num that contains only digits and an integer target, return all possibilities to insert the binary operators '+', '-', and/or '*' between the digits of num so that the resultant expression evaluates to the target value. Note that operands in the returned expressions should not contain leading zeros.

Math,String,Backtracking

Hard

Note that a number can contain multiple digits. Since the question asks us to find all of the valid expressions, we need a way to iterate over all of them. (Hint: Recursion!) We can keep track of the expression string and evaluate it at the very end. But that would take a lot of time. Can we keep track of the expression's value as well so as to avoid the evaluation at the very end of recursion? Think carefully about the multiply operator. It has a higher precedence than the addition and subtraction operators. 1 + 2 = 3 1 + 2 - 4 --> 3 - 4 --> -1 1 + 2 - 4 * 12 --> -1 * 12 --> -12 (WRONG!) 1 + 2 - 4 * 12 --> -1 - (-4) + (-4 * 12) --> 3 + (-48) --> -45 (CORRECT!) We simply need to keep track of the last operand in our expression and reverse it's effect on the expression's value while considering the multiply operator.

150,224,227,241,494

Move Zeroes

move-zeroes

Given an integer array nums, move all 0's to the end of it while maintaining the relative order of the non-zero elements. Note that you must do this in-place without making a copy of the array.

Array,Two Pointers

Easy

In-place means we should not be allocating any space for extra array. But we are allowed to modify the existing array. However, as a first step, try coming up with a solution that makes use of additional space. For this problem as well, first apply the idea discussed using an additional array and the in-place solution will pop up eventually. A two-pointer approach could be helpful here. The idea would be to have one pointer for iterating the array and another pointer that just works on the non-zero elements of the array.

27

Peeking Iterator

peeking-iterator

Design an iterator that supports the peek operation on an existing iterator in addition to the hasNext and the next operations. Implement the PeekingIterator class: Note: Each language may have a different implementation of the constructor and Iterator, but they all support the int next() and boolean hasNext() functions.

Array,Design,Iterator

Medium

Think of "looking ahead". You want to cache the next element. Is one variable sufficient? Why or why not? Test your design with call order of peek() before next() vs next() before peek(). For a clean implementation, check out Google's guava library source code.

173,251,281

Inorder Successor in BST

inorder-successor-in-bst

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

Medium

94,173,509

Walls and Gates

walls-and-gates

Array,Breadth-First Search,Matrix

Medium

130,200,317,865,1036

Find the Duplicate Number

find-the-duplicate-number

Given an array of integers nums containing n + 1 integers where each integer is in the range [1, n] inclusive. There is only one repeated number in nums, return this repeated number. You must solve the problem without modifying the array nums and uses only constant extra space.

Array,Two Pointers,Binary Search,Bit Manipulation

Medium

41,136,142,268,645

Unique Word Abbreviation

unique-word-abbreviation

Array,Hash Table,String,Design

Medium

170,320

Game of Life

game-of-life

According to Wikipedia's article: "The Game of Life, also known simply as Life, is a cellular automaton devised by the British mathematician John Horton Conway in 1970." The board is made up of an m x n grid of cells, where each cell has an initial state: live (represented by a 1) or dead (represented by a 0). Each cell interacts with its eight neighbors (horizontal, vertical, diagonal) using the following four rules (taken from the above Wikipedia article): The next state is created by applying the above rules simultaneously to every cell in the current state, where births and deaths occur simultaneously. Given the current state of the m x n grid board, return the next state.

Array,Matrix,Simulation

Medium

73

Word Pattern

word-pattern

Given a pattern and a string s, find if s follows the same pattern. Here follow means a full match, such that there is a bijection between a letter in pattern and a non-empty word in s.

Hash Table,String

Easy

205,291

Word Pattern II

word-pattern-ii

Hash Table,String,Backtracking

Medium

290

Nim Game

nim-game

You are playing the following Nim Game with your friend: Given n, the number of stones in the heap, return true if you can win the game assuming both you and your friend play optimally, otherwise return false.

Math,Brainteaser,Game Theory

Easy

If there are 5 stones in the heap, could you figure out a way to remove the stones such that you will always be the winner?

294

Flip Game

flip-game

String

Easy

294

Flip Game II

flip-game-ii

Math,Dynamic Programming,Backtracking,Memoization,Game Theory

Medium

292,293,375,464

Find Median from Data Stream

find-median-from-data-stream

The median is the middle value in an ordered integer list. If the size of the list is even, there is no middle value and the median is the mean of the two middle values. Implement the MedianFinder class:

Two Pointers,Design,Sorting,Heap (Priority Queue),Data Stream

Hard

480,1953,2207

Best Meeting Point

best-meeting-point

Array,Math,Sorting,Matrix

Hard

Try to solve it in one dimension first. How can this solution apply to the two dimension case?

317,462

Serialize and Deserialize Binary Tree

serialize-and-deserialize-binary-tree

Serialization is the process of converting a data structure or object into a sequence of bits so that it can be stored in a file or memory buffer, or transmitted across a network connection link to be reconstructed later in the same or another computer environment. Design an algorithm to serialize and deserialize a binary tree. There is no restriction on how your serialization/deserialization algorithm should work. You just need to ensure that a binary tree can be serialized to a string and this string can be deserialized to the original tree structure. Clarification: The input/output format is the same as how LeetCode serializes a binary tree. You do not necessarily need to follow this format, so please be creative and come up with different approaches yourself.

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

Hard

271,449,652,765

Binary Tree Longest Consecutive Sequence

binary-tree-longest-consecutive-sequence

Tree,Depth-First Search,Binary Tree

Medium

128,549,1416

Bulls and Cows

bulls-and-cows

You are playing the Bulls and Cows game with your friend. You write down a secret number and ask your friend to guess what the number is. When your friend makes a guess, you provide a hint with the following info: Given the secret number secret and your friend's guess guess, return the hint for your friend's guess. The hint should be formatted as "xAyB", where x is the number of bulls and y is the number of cows. Note that both secret and guess may contain duplicate digits.

Hash Table,String,Counting

Medium

Longest Increasing Subsequence

longest-increasing-subsequence

Given an integer array nums, return the length of the longest strictly increasing subsequence. A subsequence is a sequence that can be derived from an array by deleting some or no elements without changing the order of the remaining elements. For example, [3,6,2,7] is a subsequence of the array [0,3,1,6,2,2,7].

Array,Binary Search,Dynamic Programming

Medium

334,354,646,673,712,1766,2096,2234