String Compression Question Given an array of characters, compress it in-place. The length after compression must always be smaller than or equal to the original array. Every element of the array should be a character (not int) of length 1…

Longest Palindromic Substring Question Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000. Example 1: Input: "babad" Output: "bab" Note: "aba" is also a valid answer. Example …

Design Tic-Tac-Toe Question Design a Tic-tac-toe game that is played between two players on a n x n grid. You may assume the following rules: A move is guaranteed to be valid and is placed on an empty block. Once a winning condition is rea…

Sunday I solved AtCoder questions (especially cumulative sum question) instead of LeetCode's question as LeetCode365 Day4 (It is not LeetCode365 challenge anymore?), but I believe to keep solving every day is important. Today I tackled the…

Flatten a Multilevel Doubly Linked List Question You are given a doubly linked list which in addition to the next and previous pointers, it could have a child pointer, which may or may not point to a separate doubly linked list. These chil…

Remove Nth Node From End of List Question Given a linked list, remove the n-th node from the end of list and return its head. Example: Given linked list: 1->2->3->4->5, and n = 2. After removing the second node from the end, the linked lis…

Merge two sorted lists Question Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists. Example: Input: 1->2->4, 1->3->4 Output: 1->1->2->3->4->4 Solutio…

Linked List Cycle Question Given a linked list, determine if it has a cycle in it. To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If…

Linked List Cycle II Question Given a linked list, return the node where the cycle begins. If there is no cycle, return null. To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in…

Logger Rate Limiter Question Design a logger system that receive stream of messages along with its timestamps, each message should be printed if and only if it is not printed in the last 10 seconds. Given a message and a timestamp (in seco…

Compare Strings by Frequency of the Smallest Character Question Let's define a function f(s) over a non-empty string s, which calculates the frequency of the smallest character in s. For example, if s = "dcce" then f(s) = 2 because the sma…

Backspace String Compare Given two strings S and T, return if they are equal when both are typed into empty text editors. # means a backspace character. Example 1: Input: S = "ab#c", T = "ad#c" Output: true Explanation: Both S and T become…

Lisence Key Formatting Question You are given a license key represented as a string S which consists only alphanumeric character and dashes. The string is separated into N+1 groups by N dashes. Given a number K, we would want to reformat t…

Bulls and Cows Question You are playing the following Bulls and Cows game with your friend: You write down a number and ask your friend to guess what the number is. Each time your friend makes a guess, you provide a hint that indicates how…

Find k Pairs with Smallest Sums Question 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 ar…

Maximum Subarray Question Given an integer array nums, find the contiguous subarray (containing at least one number) which has the largest sum and return its sum. Example: Input: [-2,1,-3,4,-1,2,1,-5,4], Output: 6 Explanation: [4,-1,2,1] h…

House Robber Question You are a professional robber planning to rob houses along a street. Each house has a certain amount of money stashed, the only constraint stopping you from robbing each of them is that adjacent houses have security s…

Best Time to Buy and Sell Stock Question Say you have an array for which the ith element is the price of a given stock on day i. If you were only permitted to complete at most one transaction (i.e., buy one and sell one share of the stock)…

LeetCode as routine I have started LeetCode as a daily routine from New Year's Day of 2020! My daily goal is to solve and write write-ups for 5 questions per day. Why do I do this routine? The reason is simple. I really wanna pass the codi…

Move Zeros Question Given an array nums, write a function to move all 0's to the end of it while maintaining the relative order of the non-zero elements. Example: Input: [0,1,0,3,12] Output: [1,3,12,0,0] Note: You must do this in-place wit…

Meeting Rooms Question Given an array of meeting time intervals consisting of start and end times [[s1,e1],[s2,e2],...] (si < ei), determine if a person could attend all meetings. Example 1: Input: [[0,30],[5,10],[15,20]] Output: false Exa…

Meeting Rooms II Question Given an array of meeting time intervals consisting of start and end times [[s1,e1],[s2,e2],...] (si < ei), find the minimum number of conference rooms required. Example 1: Input: [[0, 30],[5, 10],[15, 20]] Output…

Is Subsequence Question Given a string s and a string t, check if s is subsequence of t. You may assume that there is only lower case English letters in both s and t. t is potentially a very long (length ~= 500,000) string, and s is a shor…