Given the head of a singly linked list, reverse the list, and return the reversed list.
Given an input string s and a pattern p, implement regular expression matching with support for ‘.’ and ‘*’ where ‘.’ Matches any single character and ‘*’ Matches zero or more of the preceding element.
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.
Write an algorithm to determine if a number n is happy. Return true if n is a happy number, and false if not.
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.