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Two Sum

two-sum

Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target. You may assume that each input would have exactly one solution, and you may not use the same element twice. You can return the answer in any order.

Array,Hash Table

Easy

A really brute force way would be to search for all possible pairs of numbers but that would be too slow. Again, it's best to try out brute force solutions for just for completeness. It is from these brute force solutions that you can come up with optimizations. So, if we fix one of the numbers, say x, we have to scan the entire array to find the next number y which is value - x where value is the input parameter. Can we change our array somehow so that this search becomes faster? The second train of thought is, without changing the array, can we use additional space somehow? Like maybe a hash map to speed up the search?

15,18,167,170,560,653,1083,1798,1830,2116,2133,2320

Add Two Numbers

add-two-numbers

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list. You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Linked List,Math,Recursion

Medium

43,67,371,415,445,1031,1774

Longest Substring Without Repeating Characters

longest-substring-without-repeating-characters

Given a string s, find the length of the longest substring without repeating characters.

Hash Table,String,Sliding Window

Medium

159,340,1034,1813,2209

Median of Two Sorted Arrays

median-of-two-sorted-arrays

Given two sorted arrays nums1 and nums2 of size m and n respectively, return the median of the two sorted arrays. The overall run time complexity should be O(log (m+n)).

Array,Binary Search,Divide and Conquer

Hard

Longest Palindromic Substring

longest-palindromic-substring

Given a string s, return the longest palindromic substring in s.

String,Dynamic Programming

Medium

How can we reuse a previously computed palindrome to compute a larger palindrome? If “aba” is a palindrome, is “xabax” a palindrome? Similarly is “xabay” a palindrome? Complexity based hint: If we use brute-force and check whether for every start and end position a substring is a palindrome we have O(n^2) start - end pairs and O(n) palindromic checks. Can we reduce the time for palindromic checks to O(1) by reusing some previous computation.

214,266,336,516,647

Zigzag Conversion

zigzag-conversion

The string "PAYPALISHIRING" is written in a zigzag pattern on a given number of rows like this: (you may want to display this pattern in a fixed font for better legibility) And then read line by line: "PAHNAPLSIIGYIR" Write the code that will take a string and make this conversion given a number of rows:

String

Medium

Reverse Integer

reverse-integer

Given a signed 32-bit integer x, return x with its digits reversed. If reversing x causes the value to go outside the signed 32-bit integer range [-231, 231 - 1], then return 0. Assume the environment does not allow you to store 64-bit integers (signed or unsigned).

Math

Medium

8,190,2238

String to Integer (atoi)

string-to-integer-atoi

Implement the myAtoi(string s) function, which converts a string to a 32-bit signed integer (similar to C/C++'s atoi function). The algorithm for myAtoi(string s) is as follows: Note:

String

Medium

7,65,2168

Palindrome Number

palindrome-number

Given an integer x, return true if x is palindrome integer. An integer is a palindrome when it reads the same backward as forward.

Math

Easy

Beware of overflow when you reverse the integer.

234,1375

Regular Expression Matching

regular-expression-matching

Given an input string s and a pattern p, implement regular expression matching with support for '.' and '*' where: The matching should cover the entire input string (not partial).

String,Dynamic Programming,Recursion

Hard

44

Container With Most Water

container-with-most-water

You are given an integer array height of length n. There are n vertical lines drawn such that the two endpoints of the ith line are (i, 0) and (i, height[i]). Find two lines that together with the x-axis form a container, such that the container contains the most water. Return the maximum amount of water a container can store. Notice that you may not slant the container.

Array,Two Pointers,Greedy

Medium

The aim is to maximize the area formed between the vertical lines. The area of any container is calculated using the shorter line as length and the distance between the lines as the width of the rectangle. Area = length of shorter vertical line * distance between lines We can definitely get the maximum width container as the outermost lines have the maximum distance between them. However, this container might not be the maximum in size as one of the vertical lines of this container could be really short. Start with the maximum width container and go to a shorter width container if there is a vertical line longer than the current containers shorter line. This way we are compromising on the width but we are looking forward to a longer length container.

42

Integer to Roman

integer-to-roman

Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M. For example, 2 is written as II in Roman numeral, just two one's added together. 12 is written as XII, which is simply X + II. The number 27 is written as XXVII, which is XX + V + II. Roman numerals are usually written largest to smallest from left to right. However, the numeral for four is not IIII. Instead, the number four is written as IV. Because the one is before the five we subtract it making four. The same principle applies to the number nine, which is written as IX. There are six instances where subtraction is used: Given an integer, convert it to a roman numeral.

Hash Table,Math,String

Medium

13,273

Roman to Integer

roman-to-integer

Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M. For example, 2 is written as II in Roman numeral, just two one's added together. 12 is written as XII, which is simply X + II. The number 27 is written as XXVII, which is XX + V + II. Roman numerals are usually written largest to smallest from left to right. However, the numeral for four is not IIII. Instead, the number four is written as IV. Because the one is before the five we subtract it making four. The same principle applies to the number nine, which is written as IX. There are six instances where subtraction is used: Given a roman numeral, convert it to an integer.

Hash Table,Math,String

Easy

Problem is simpler to solve by working the string from back to front and using a map.

12

Longest Common Prefix

longest-common-prefix

Write a function to find the longest common prefix string amongst an array of strings. If there is no common prefix, return an empty string "".

String

Easy

3Sum

3sum

Given an integer array nums, return all the triplets [nums[i], nums[j], nums[k]] such that i != j, i != k, and j != k, and nums[i] + nums[j] + nums[k] == 0. Notice that the solution set must not contain duplicate triplets.

Array,Two Pointers,Sorting

Medium

So, we essentially need to find three numbers x, y, and z such that they add up to the given value. If we fix one of the numbers say x, we are left with the two-sum problem at hand! For the two-sum problem, if we fix one of the numbers, say x, we have to scan the entire array to find the next numbery which is value - x where value is the input parameter. Can we change our array somehow so that this search becomes faster? The second train of thought for two-sum is, without changing the array, can we use additional space somehow? Like maybe a hash map to speed up the search?

1,16,18,259

3Sum Closest

3sum-closest

Given an integer array nums of length n and an integer target, find three integers in nums such that the sum is closest to target. Return the sum of the three integers. You may assume that each input would have exactly one solution.

Array,Two Pointers,Sorting

Medium

15,259

Letter Combinations of a Phone Number

letter-combinations-of-a-phone-number

Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order. A mapping of digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.

Hash Table,String,Backtracking

Medium

22,39,401

4Sum

4sum

Given an array nums of n integers, return an array of all the unique quadruplets [nums[a], nums[b], nums[c], nums[d]] such that: You may return the answer in any order.

Array,Two Pointers,Sorting

Medium

1,15,454,2122

Remove Nth Node From End of List

remove-nth-node-from-end-of-list

Given the head of a linked list, remove the nth node from the end of the list and return its head.

Linked List,Two Pointers

Medium

Maintain two pointers and update one with a delay of n steps.

528,1618,2216

Valid Parentheses

valid-parentheses

Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid. An input string is valid if:

String,Stack

Easy

An interesting property about a valid parenthesis expression is that a sub-expression of a valid expression should also be a valid expression. (Not every sub-expression) e.g. { { } [ ] [ [ [ ] ] ] } is VALID expression [ [ [ ] ] ] is VALID sub-expression { } [ ] is VALID sub-expression Can we exploit this recursive structure somehow? What if whenever we encounter a matching pair of parenthesis in the expression, we simply remove it from the expression? This would keep on shortening the expression. e.g. { { ( { } ) } } |_| { { ( ) } } |______| { { } } |__________| { } |________________| VALID EXPRESSION! The stack data structure can come in handy here in representing this recursive structure of the problem. We can't really process this from the inside out because we don't have an idea about the overall structure. But, the stack can help us process this recursively i.e. from outside to inwards.

22,32,301,1045,2221

Merge Two Sorted Lists

merge-two-sorted-lists

You are given the heads of two sorted linked lists list1 and list2. Merge the two lists in a one sorted list. The list should be made by splicing together the nodes of the first two lists. Return the head of the merged linked list.

Linked List,Recursion

Easy

23,88,148,244,1774,2071

Generate Parentheses

generate-parentheses

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

String,Dynamic Programming,Backtracking

Medium

17,20,2221

Merge k Sorted Lists

merge-k-sorted-lists

You are given an array of k linked-lists lists, each linked-list is sorted in ascending order. Merge all the linked-lists into one sorted linked-list and return it.

Linked List,Divide and Conquer,Heap (Priority Queue),Merge Sort

Hard

21,264

Swap Nodes in Pairs

swap-nodes-in-pairs

Given a linked list, swap every two adjacent nodes and return its head. You must solve the problem without modifying the values in the list's nodes (i.e., only nodes themselves may be changed.)

Linked List,Recursion

Medium

25,528

Reverse Nodes in k-Group

reverse-nodes-in-k-group

Given the head of a linked list, reverse the nodes of the list k at a time, and return the modified list. k is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of k then left-out nodes, in the end, should remain as it is. You may not alter the values in the list's nodes, only nodes themselves may be changed.

Linked List,Recursion

Hard

24,528,2196

Remove Duplicates from Sorted Array

remove-duplicates-from-sorted-array

Given an integer array nums sorted in non-decreasing order, remove the duplicates in-place such that each unique element appears only once. The relative order of the elements should be kept the same. Since it is impossible to change the length of the array in some languages, you must instead have the result be placed in the first part of the array nums. More formally, if there are k elements after removing the duplicates, then the first k elements of nums should hold the final result. It does not matter what you leave beyond the first k elements. Return k after placing the final result in the first k slots of nums. Do not allocate extra space for another array. You must do this by modifying the input array in-place with O(1) extra memory. Custom Judge: The judge will test your solution with the following code: If all assertions pass, then your solution will be accepted.

Array,Two Pointers

Easy

In this problem, the key point to focus on is the input array being sorted. As far as duplicate elements are concerned, what is their positioning in the array when the given array is sorted? Look at the image above for the answer. If we know the position of one of the elements, do we also know the positioning of all the duplicate elements? We need to modify the array in-place and the size of the final array would potentially be smaller than the size of the input array. So, we ought to use a two-pointer approach here. One, that would keep track of the current element in the original array and another one for just the unique elements. Essentially, once an element is encountered, you simply need to bypass its duplicates and move on to the next unique element.

27,80

Remove Element

remove-element

Given an integer array nums and an integer val, remove all occurrences of val in nums in-place. The relative order of the elements may be changed. Since it is impossible to change the length of the array in some languages, you must instead have the result be placed in the first part of the array nums. More formally, if there are k elements after removing the duplicates, then the first k elements of nums should hold the final result. It does not matter what you leave beyond the first k elements. Return k after placing the final result in the first k slots of nums. Do not allocate extra space for another array. You must do this by modifying the input array in-place with O(1) extra memory. Custom Judge: The judge will test your solution with the following code: If all assertions pass, then your solution will be accepted.

Array,Two Pointers

Easy

The problem statement clearly asks us to modify the array in-place and it also says that the element beyond the new length of the array can be anything. Given an element, we need to remove all the occurrences of it from the array. We don't technically need to remove that element per-say, right? We can move all the occurrences of this element to the end of the array. Use two pointers! Yet another direction of thought is to consider the elements to be removed as non-existent. In a single pass, if we keep copying the visible elements in-place, that should also solve this problem for us.

26,203,283

Implement strStr()

implement-strstr

Implement strStr(). Given two strings needle and haystack, return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack. Clarification: What should we return when needle is an empty string? This is a great question to ask during an interview. For the purpose of this problem, we will return 0 when needle is an empty string. This is consistent to C's strstr() and Java's indexOf().

Two Pointers,String,String Matching

Easy

214,459

Divide Two Integers

divide-two-integers

Given two integers dividend and divisor, divide two integers without using multiplication, division, and mod operator. The integer division should truncate toward zero, which means losing its fractional part. For example, 8.345 would be truncated to 8, and -2.7335 would be truncated to -2. Return the quotient after dividing dividend by divisor. Note: Assume we are dealing with an environment that could only store integers within the 32-bit signed integer range: [−231, 231 − 1]. For this problem, if the quotient is strictly greater than 231 - 1, then return 231 - 1, and if the quotient is strictly less than -231, then return -231.

Math,Bit Manipulation

Medium

Substring with Concatenation of All Words

substring-with-concatenation-of-all-words

You are given a string s and an array of strings words of the same length. Return all starting indices of substring(s) in s that is a concatenation of each word in words exactly once, in any order, and without any intervening characters. You can return the answer in any order.

Hash Table,String,Sliding Window

Hard

76

Next Permutation

next-permutation

A permutation of an array of integers is an arrangement of its members into a sequence or linear order. The next permutation of an array of integers is the next lexicographically greater permutation of its integer. More formally, if all the permutations of the array are sorted in one container according to their lexicographical order, then the next permutation of that array is the permutation that follows it in the sorted container. If such arrangement is not possible, the array must be rearranged as the lowest possible order (i.e., sorted in ascending order). Given an array of integers nums, find the next permutation of nums. The replacement must be in place and use only constant extra memory.

Array,Two Pointers

Medium

46,47,60,267,1978

Longest Valid Parentheses

longest-valid-parentheses

Given a string containing just the characters '(' and ')', find the length of the longest valid (well-formed) parentheses substring.

String,Dynamic Programming,Stack

Hard

20

Search in Rotated Sorted Array

search-in-rotated-sorted-array

There is an integer array nums sorted in ascending order (with distinct values). Prior to being passed to your function, nums is possibly rotated at an unknown pivot index k (1 <= k < nums.length) such that the resulting array is [nums[k], nums[k+1], ..., nums[n-1], nums[0], nums[1], ..., nums[k-1]] (0-indexed). For example, [0,1,2,4,5,6,7] might be rotated at pivot index 3 and become [4,5,6,7,0,1,2]. Given the array nums after the possible rotation and an integer target, return the index of target if it is in nums, or -1 if it is not in nums. You must write an algorithm with O(log n) runtime complexity.

Array,Binary Search

Medium

81,153,2273

Find First and Last Position of Element in Sorted Array

find-first-and-last-position-of-element-in-sorted-array

Given an array of integers nums sorted in non-decreasing order, find the starting and ending position of a given target value. If target is not found in the array, return [-1, -1]. You must write an algorithm with O(log n) runtime complexity.

Array,Binary Search

Medium

278,2165,2210

Search Insert Position

search-insert-position

Given a sorted array of distinct integers and a target value, return the index if the target is found. If not, return the index where it would be if it were inserted in order. You must write an algorithm with O(log n) runtime complexity.

Array,Binary Search

Easy

278

Valid Sudoku

valid-sudoku

Determine if a 9 x 9 Sudoku board is valid. Only the filled cells need to be validated according to the following rules: Note:

Array,Hash Table,Matrix

Medium

37,2254

Sudoku Solver

sudoku-solver

Write a program to solve a Sudoku puzzle by filling the empty cells. A sudoku solution must satisfy all of the following rules: The '.' character indicates empty cells.

Array,Backtracking,Matrix

Hard

36,1022

Count and Say

count-and-say

The count-and-say sequence is a sequence of digit strings defined by the recursive formula: To determine how you "say" a digit string, split it into the minimal number of groups so that each group is a contiguous section all of the same character. Then for each group, say the number of characters, then say the character. To convert the saying into a digit string, replace the counts with a number and concatenate every saying. For example, the saying and conversion for digit string "3322251": Given a positive integer n, return the nth term of the count-and-say sequence.

String

Medium

The following are the terms from n=1 to n=10 of the count-and-say sequence: 1. 1 2. 11 3. 21 4. 1211 5. 111221 6. 312211 7. 13112221 8. 1113213211 9. 31131211131221 10. 13211311123113112211 To generate the nth term, just count and say the n-1th term.

271,443

Combination Sum

combination-sum

Given an array of distinct integers candidates and a target integer target, return a list of all unique combinations of candidates where the chosen numbers sum to target. You may return the combinations in any order. The same number may be chosen from candidates an unlimited number of times. Two combinations are unique if the frequency of at least one of the chosen numbers is different. It is guaranteed that the number of unique combinations that sum up to target is less than 150 combinations for the given input.

Array,Backtracking

Medium

17,40,77,216,254,377

Combination Sum II

combination-sum-ii

Given a collection of candidate numbers (candidates) and a target number (target), find all unique combinations in candidates where the candidate numbers sum to target. Each number in candidates may only be used once in the combination. Note: The solution set must not contain duplicate combinations.

Array,Backtracking

Medium

39

First Missing Positive

first-missing-positive

Given an unsorted integer array nums, return the smallest missing positive integer. You must implement an algorithm that runs in O(n) time and uses constant extra space.

Array,Hash Table

Hard

Think about how you would solve the problem in non-constant space. Can you apply that logic to the existing space? We don't care about duplicates or non-positive integers Remember that O(2n) = O(n)

268,287,448,770

Trapping Rain Water

trapping-rain-water

Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it can trap after raining.

Array,Two Pointers,Dynamic Programming,Stack,Monotonic Stack

Hard

11,238,407,756

Multiply Strings

multiply-strings

Given two non-negative integers num1 and num2 represented as strings, return the product of num1 and num2, also represented as a string. Note: You must not use any built-in BigInteger library or convert the inputs to integer directly.

Math,String,Simulation

Medium

2,66,67,415

Wildcard Matching

wildcard-matching

Given an input string (s) and a pattern (p), implement wildcard pattern matching with support for '?' and '*' where: The matching should cover the entire input string (not partial).

String,Dynamic Programming,Greedy,Recursion

Hard

10

Jump Game II

jump-game-ii

Given an array of non-negative integers nums, you are initially positioned at the first index of the array. Each element in the array represents your maximum jump length at that position. Your goal is to reach the last index in the minimum number of jumps. You can assume that you can always reach the last index.

Array,Dynamic Programming,Greedy

Medium

55,1428,2001

Permutations

permutations

Given an array nums of distinct integers, return all the possible permutations. You can return the answer in any order.

Array,Backtracking

Medium

31,47,60,77

Permutations II

permutations-ii

Given a collection of numbers, nums, that might contain duplicates, return all possible unique permutations in any order.

Array,Backtracking

Medium

31,46,267,1038

Rotate Image

rotate-image

You are given an n x n 2D matrix representing an image, rotate the image by 90 degrees (clockwise). You have to rotate the image in-place, which means you have to modify the input 2D matrix directly. DO NOT allocate another 2D matrix and do the rotation.

Array,Math,Matrix

Medium

2015

Group Anagrams

group-anagrams

Given an array of strings strs, group the anagrams together. You can return the answer in any order. 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

Medium

242,249

Pow(x, n)

powx-n

Implement pow(x, n), which calculates x raised to the power n (i.e., xn).

Math,Recursion

Medium

69,372

N-Queens

n-queens

The n-queens puzzle is the problem of placing n queens on an n x n chessboard such that no two queens attack each other. Given an integer n, return all distinct solutions to the n-queens puzzle. You may return the answer in any order. Each solution contains a distinct board configuration of the n-queens' placement, where 'Q' and '.' both indicate a queen and an empty space, respectively.

Array,Backtracking

Hard

52,1043

N-Queens II

n-queens-ii

The n-queens puzzle is the problem of placing n queens on an n x n chessboard such that no two queens attack each other. Given an integer n, return the number of distinct solutions to the n-queens puzzle.

Backtracking

Hard

51

Maximum Subarray

maximum-subarray

Given an integer array nums, find the contiguous subarray (containing at least one number) which has the largest sum and return its sum. A subarray is a contiguous part of an array.

Array,Divide and Conquer,Dynamic Programming

Easy

121,152,697,1020,1849,1893

Spiral Matrix

spiral-matrix

Given an m x n matrix, return all elements of the matrix in spiral order.

Array,Matrix,Simulation

Medium

Well for some problems, the best way really is to come up with some algorithms for simulation. Basically, you need to simulate what the problem asks us to do. We go boundary by boundary and move inwards. That is the essential operation. First row, last column, last row, first column and then we move inwards by 1 and then repeat. That's all, that is all the simulation that we need. Think about when you want to switch the progress on one of the indexes. If you progress on i out of [i, j], you'd be shifting in the same column. Similarly, by changing values for j, you'd be shifting in the same row. Also, keep track of the end of a boundary so that you can move inwards and then keep repeating. It's always best to run the simulation on edge cases like a single column or a single row to see if anything breaks or not.

59,921

Jump Game

jump-game

You are given an integer array nums. You are initially positioned at the array's first index, and each element in the array represents your maximum jump length at that position. Return true if you can reach the last index, or false otherwise.

Array,Dynamic Programming,Greedy

Medium

45,1428,2001

Merge Intervals

merge-intervals

Given an array of intervals where intervals[i] = [starti, endi], merge all overlapping intervals, and return an array of the non-overlapping intervals that cover all the intervals in the input.

Array,Sorting

Medium

57,252,253,495,616,715,761,768,1028,2297,2319

Insert Interval

insert-interval

You are given an array of non-overlapping intervals intervals where intervals[i] = [starti, endi] represent the start and the end of the ith interval and intervals is sorted in ascending order by starti. You are also given an interval newInterval = [start, end] that represents the start and end of another interval. Insert newInterval into intervals such that intervals is still sorted in ascending order by starti and intervals still does not have any overlapping intervals (merge overlapping intervals if necessary). Return intervals after the insertion.

Array

Medium

56,715

Length of Last Word

length-of-last-word

Given a string s consisting of some words separated by some number of spaces, return the length of the last word in the string. A word is a maximal substring consisting of non-space characters only.

String

Easy

Spiral Matrix II

spiral-matrix-ii

Given a positive integer n, generate an n x n matrix filled with elements from 1 to n2 in spiral order.

Array,Matrix,Simulation

Medium

54,921

Permutation Sequence

permutation-sequence

The set [1, 2, 3, ..., n] contains a total of n! unique permutations. By listing and labeling all of the permutations in order, we get the following sequence for n = 3: Given n and k, return the kth permutation sequence.

Math,Recursion

Hard

31,46

Rotate List

rotate-list

Given the head of a linked list, rotate the list to the right by k places.

Linked List,Two Pointers

Medium

189,725

Unique Paths

unique-paths

There is a robot on an m x n grid. The robot is initially located at the top-left corner (i.e., grid[0][0]). The robot tries to move to the bottom-right corner (i.e., grid[m - 1][n - 1]). The robot can only move either down or right at any point in time. Given the two integers m and n, return the number of possible unique paths that the robot can take to reach the bottom-right corner. The test cases are generated so that the answer will be less than or equal to 2 * 109.

Math,Dynamic Programming,Combinatorics

Medium

63,64,174,2192

Unique Paths II

unique-paths-ii

You are given an m x n integer array grid. There is a robot initially located at the top-left corner (i.e., grid[0][0]). The robot tries to move to the bottom-right corner (i.e., grid[m-1][n-1]). The robot can only move either down or right at any point in time. An obstacle and space are marked as 1 or 0 respectively in grid. A path that the robot takes cannot include any square that is an obstacle. Return the number of possible unique paths that the robot can take to reach the bottom-right corner. The testcases are generated so that the answer will be less than or equal to 2 * 109.

Array,Dynamic Programming,Matrix

Medium

The robot can only move either down or right. Hence any cell in the first row can only be reached from the cell left to it. However, if any cell has an obstacle, you don't let that cell contribute to any path. So, for the first row, the number of ways will simply be if obstacleGrid[i][j] is not an obstacle obstacleGrid[i,j] = obstacleGrid[i,j - 1] else obstacleGrid[i,j] = 0 You can do a similar processing for finding out the number of ways of reaching the cells in the first column. For any other cell, we can find out the number of ways of reaching it, by making use of the number of ways of reaching the cell directly above it and the cell to the left of it in the grid. This is because these are the only two directions from which the robot can come to the current cell. Since we are making use of pre-computed values along the iteration, this becomes a dynamic programming problem. if obstacleGrid[i][j] is not an obstacle obstacleGrid[i,j] = obstacleGrid[i,j - 1] + obstacleGrid[i - 1][j] else obstacleGrid[i,j] = 0

62,1022

Minimum Path Sum

minimum-path-sum

Given a m x n grid filled with non-negative numbers, find a path from top left to bottom right, which minimizes the sum of all numbers along its path. Note: You can only move either down or right at any point in time.

Array,Dynamic Programming,Matrix

Medium

62,174,741,2067,2192

Valid Number

valid-number

A valid number can be split up into these components (in order): A decimal number can be split up into these components (in order): An integer can be split up into these components (in order): For example, all the following are valid numbers: ["2", "0089", "-0.1", "+3.14", "4.", "-.9", "2e10", "-90E3", "3e+7", "+6e-1", "53.5e93", "-123.456e789"], while the following are not valid numbers: ["abc", "1a", "1e", "e3", "99e2.5", "--6", "-+3", "95a54e53"]. Given a string s, return true if s is a valid number.

String

Hard

8

Plus One

plus-one

You are given a large integer represented as an integer array digits, where each digits[i] is the ith digit of the integer. The digits are ordered from most significant to least significant in left-to-right order. The large integer does not contain any leading 0's. Increment the large integer by one and return the resulting array of digits.

Array,Math

Easy

43,67,369,1031

Add Binary

add-binary

Given two binary strings a and b, return their sum as a binary string.

Math,String,Bit Manipulation,Simulation

Easy

2,43,66,1031

Text Justification

text-justification

Given an array of strings words and a width maxWidth, format the text such that each line has exactly maxWidth characters and is fully (left and right) justified. You should pack your words in a greedy approach; that is, pack as many words as you can in each line. Pad extra spaces ' ' when necessary so that each line has exactly maxWidth characters. Extra spaces between words should be distributed as evenly as possible. If the number of spaces on a line does not divide evenly between words, the empty slots on the left will be assigned more spaces than the slots on the right. For the last line of text, it should be left-justified and no extra space is inserted between words. Note:

Array,String,Simulation

Hard

1714,2260

Sqrt(x)

sqrtx

Given a non-negative integer x, compute and return the square root of x. Since the return type is an integer, the decimal digits are truncated, and only the integer part of the result is returned. Note: You are not allowed to use any built-in exponent function or operator, such as pow(x, 0.5) or x ** 0.5.

Math,Binary Search

Easy

Try exploring all integers. (Credits: @annujoshi) Use the sorted property of integers to reduced the search space. (Credits: @annujoshi)

50,367

Climbing Stairs

climbing-stairs

You are climbing a staircase. It takes n steps to reach the top. Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?

Math,Dynamic Programming,Memoization

Easy

To reach nth step, what could have been your previous steps? (Think about the step sizes)

747,1013,1236

Simplify Path

simplify-path

Given a string path, which is an absolute path (starting with a slash '/') to a file or directory in a Unix-style file system, convert it to the simplified canonical path. In a Unix-style file system, a period '.' refers to the current directory, a double period '..' refers to the directory up a level, and any multiple consecutive slashes (i.e. '//') are treated as a single slash '/'. For this problem, any other format of periods such as '...' are treated as file/directory names. The canonical path should have the following format: Return the simplified canonical path.

String,Stack

Medium

Edit Distance

edit-distance

Given two strings word1 and word2, return the minimum number of operations required to convert word1 to word2. You have the following three operations permitted on a word:

String,Dynamic Programming

Hard

161,583,712,1105,2311

Set Matrix Zeroes

set-matrix-zeroes

Given an m x n integer matrix matrix, if an element is 0, set its entire row and column to 0's. You must do it in place.

Array,Hash Table,Matrix

Medium

If any cell of the matrix has a zero we can record its row and column number using additional memory. But if you don't want to use extra memory then you can manipulate the array instead. i.e. simulating exactly what the question says. Setting cell values to zero on the fly while iterating might lead to discrepancies. What if you use some other integer value as your marker? There is still a better approach for this problem with 0(1) space. We could have used 2 sets to keep a record of rows/columns which need to be set to zero. But for an O(1) space solution, you can use one of the rows and and one of the columns to keep track of this information. We can use the first cell of every row and column as a flag. This flag would determine whether a row or column has been set to zero.

289,2244,2259,2314

Search a 2D Matrix

search-a-2d-matrix

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,Matrix

Medium

240

Sort Colors

sort-colors

Given an array nums with n objects colored red, white, or blue, sort them in-place so that objects of the same color are adjacent, with the colors in the order red, white, and blue. We will use the integers 0, 1, and 2 to represent the color red, white, and blue, respectively. You must solve this problem without using the library's sort function.

Array,Two Pointers,Sorting

Medium

A rather straight forward solution is a two-pass algorithm using counting sort. Iterate the array counting number of 0's, 1's, and 2's. Overwrite array with the total number of 0's, then 1's and followed by 2's.

148,280,324

Minimum Window Substring

minimum-window-substring

Given two strings s and t of lengths m and n respectively, return the minimum window substring of s such that every character in t (including duplicates) is included in the window. If there is no such substring, return the empty string "". The testcases will be generated such that the answer is unique. A substring is a contiguous sequence of characters within the string.

Hash Table,String,Sliding Window

Hard

Use two pointers to create a window of letters in S, which would have all the characters from T. Since you have to find the minimum window in S which has all the characters from T, you need to expand and contract the window using the two pointers and keep checking the window for all the characters. This approach is also called Sliding Window Approach. L ------------------------ R , Suppose this is the window that contains all characters of T         L----------------- R , this is the contracted window. We found a smaller window that still contains all the characters in T When the window is no longer valid, start expanding again using the right pointer.

30,209,239,567,632,727

Combinations

combinations

Given two integers n and k, return all possible combinations of k numbers out of the range [1, n]. You may return the answer in any order.

Backtracking

Medium

39,46

Subsets

subsets

Given an integer array nums of unique elements, return all possible subsets (the power set). The solution set must not contain duplicate subsets. Return the solution in any order.

Array,Backtracking,Bit Manipulation

Medium

90,320,800,2109,2170

Word Search

word-search

Given an m x n grid of characters board and a string word, return true if word exists in the grid. The word can 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.

Array,Backtracking,Matrix

Medium

212

Remove Duplicates from Sorted Array II

remove-duplicates-from-sorted-array-ii

Given an integer array nums sorted in non-decreasing order, remove some duplicates in-place such that each unique element appears at most twice. The relative order of the elements should be kept the same. Since it is impossible to change the length of the array in some languages, you must instead have the result be placed in the first part of the array nums. More formally, if there are k elements after removing the duplicates, then the first k elements of nums should hold the final result. It does not matter what you leave beyond the first k elements. Return k after placing the final result in the first k slots of nums. Do not allocate extra space for another array. You must do this by modifying the input array in-place with O(1) extra memory. Custom Judge: The judge will test your solution with the following code: If all assertions pass, then your solution will be accepted.

Array,Two Pointers

Medium

26

Search in Rotated Sorted Array II

search-in-rotated-sorted-array-ii

There is an integer array nums sorted in non-decreasing order (not necessarily with distinct values). Before being passed to your function, nums is rotated at an unknown pivot index k (0 <= k < nums.length) such that the resulting array is [nums[k], nums[k+1], ..., nums[n-1], nums[0], nums[1], ..., nums[k-1]] (0-indexed). For example, [0,1,2,4,4,4,5,6,6,7] might be rotated at pivot index 5 and become [4,5,6,6,7,0,1,2,4,4]. Given the array nums after the rotation and an integer target, return true if target is in nums, or false if it is not in nums. You must decrease the overall operation steps as much as possible.

Array,Binary Search

Medium

33

Remove Duplicates from Sorted List II

remove-duplicates-from-sorted-list-ii

Given the head of a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list. Return the linked list sorted as well.

Linked List,Two Pointers

Medium

83,1982

Remove Duplicates from Sorted List

remove-duplicates-from-sorted-list

Given the head of a sorted linked list, delete all duplicates such that each element appears only once. Return the linked list sorted as well.

Linked List

Easy

82,1982

Largest Rectangle in Histogram

largest-rectangle-in-histogram

Given an array of integers heights representing the histogram's bar height where the width of each bar is 1, return the area of the largest rectangle in the histogram.

Array,Stack,Monotonic Stack

Hard

85,1918

Maximal Rectangle

maximal-rectangle

Given a rows x cols binary matrix filled with 0's and 1's, find the largest rectangle containing only 1's and return its area.

Array,Dynamic Programming,Stack,Matrix,Monotonic Stack

Hard

84,221

Partition List

partition-list

Given the head of a linked list and a value x, partition it such that all nodes less than x come before nodes greater than or equal to x. You should preserve the original relative order of the nodes in each of the two partitions.

Linked List,Two Pointers

Medium

2265

Scramble String

scramble-string

We can scramble a string s to get a string t using the following algorithm: Given two strings s1 and s2 of the same length, return true if s2 is a scrambled string of s1, otherwise, return false.

String,Dynamic Programming

Hard

Merge Sorted Array

merge-sorted-array

You are given two integer arrays nums1 and nums2, sorted in non-decreasing order, and two integers m and n, representing the number of elements in nums1 and nums2 respectively. Merge nums1 and nums2 into a single array sorted in non-decreasing order. The final sorted array should not be returned by the function, but instead be stored inside the array nums1. To accommodate this, nums1 has a length of m + n, where the first m elements denote the elements that should be merged, and the last n elements are set to 0 and should be ignored. nums2 has a length of n.

Array,Two Pointers,Sorting

Easy

You can easily solve this problem if you simply think about two elements at a time rather than two arrays. We know that each of the individual arrays is sorted. What we don't know is how they will intertwine. Can we take a local decision and arrive at an optimal solution? If you simply consider one element each at a time from the two arrays and make a decision and proceed accordingly, you will arrive at the optimal solution.

21,1019,1028

Gray Code

gray-code

An n-bit gray code sequence is a sequence of 2n integers where: Given an integer n, return any valid n-bit gray code sequence.

Math,Backtracking,Bit Manipulation

Medium

717

Subsets II

subsets-ii

Given an integer array nums that may contain duplicates, return all possible subsets (the power set). The solution set must not contain duplicate subsets. Return the solution in any order.

Array,Backtracking,Bit Manipulation

Medium

78,2109

Decode Ways

decode-ways

A message containing letters from A-Z can be encoded into numbers using the following mapping: To decode an encoded message, all the digits must be grouped then mapped back into letters using the reverse of the mapping above (there may be multiple ways). For example, "11106" can be mapped into: Note that the grouping (1 11 06) is invalid because "06" cannot be mapped into 'F' since "6" is different from "06". Given a string s containing only digits, return the number of ways to decode it. The test cases are generated so that the answer fits in a 32-bit integer.

String,Dynamic Programming

Medium

639,2091

Reverse Linked List II

reverse-linked-list-ii

Given the head of a singly linked list and two integers left and right where left <= right, reverse the nodes of the list from position left to position right, and return the reversed list.

Linked List

Medium

206

Restore IP Addresses

restore-ip-addresses

A valid IP address consists of exactly four integers separated by single dots. Each integer is between 0 and 255 (inclusive) and cannot have leading zeros. Given a string s containing only digits, return all possible valid IP addresses that can be formed by inserting dots into s. You are not allowed to reorder or remove any digits in s. You may return the valid IP addresses in any order.

String,Backtracking

Medium

752

Binary Tree Inorder Traversal

binary-tree-inorder-traversal

Given the root of a binary tree, return the inorder traversal of its nodes' values.

Stack,Tree,Depth-First Search,Binary Tree

Easy

98,144,145,173,230,272,285,758,799

Unique Binary Search Trees II

unique-binary-search-trees-ii

Given an integer n, return all the structurally unique BST's (binary search trees), which has exactly n nodes of unique values from 1 to n. Return the answer in any order.

Dynamic Programming,Backtracking,Tree,Binary Search Tree,Binary Tree

Medium

96,241

Unique Binary Search Trees

unique-binary-search-trees

Given an integer n, return the number of structurally unique BST's (binary search trees) which has exactly n nodes of unique values from 1 to n.

Math,Dynamic Programming,Tree,Binary Search Tree,Binary Tree

Medium

95

Interleaving String

interleaving-string

Given strings s1, s2, and s3, find whether s3 is formed by an interleaving of s1 and s2. An interleaving of two strings s and t is a configuration where they are divided into non-empty substrings such that: Note: a + b is the concatenation of strings a and b.

String,Dynamic Programming

Medium

Validate Binary Search Tree

validate-binary-search-tree

Given the root of a binary tree, determine if it is a valid binary search tree (BST). A valid BST is defined as follows:

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

Medium

94,501

Recover Binary Search Tree

recover-binary-search-tree

You are given the root of a binary search tree (BST), where the values of exactly two nodes of the tree were swapped by mistake. Recover the tree without changing its structure.

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

Medium

Same Tree

same-tree

Given the roots of two binary trees p and q, write a function to check if they are the same or not. Two binary trees are considered the same if they are structurally identical, and the nodes have the same value.

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

Easy