Given a string S, find the longest palindromic substring in S. You may assume that the maximum length of S is 1000, and there exists one unique longest palindromic substring.

Given an integer, convert it to a roman numeral. Input is guaranteed to be within the range from 1 to 3999.

Given a roman numeral, convert it to an integer. Input is guaranteed to be within the range from 1 to 3999.

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, find two numbers such that they add up to a specific target number.