Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.
Given a string, find the length of the longest substring without repeating characters.
You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.
Given an array of integers, return indices of the two numbers such that they add up to a specific target.