String Addition

Problem

Given two non-negative integers num1 and num2 represented as string, return the sum of num1 and num2 as a string. You must not use any built-in library for handling large integers (such as BigInteger).

You must not convert the inputs to integers directly.

Examples

addStrings("2", "3")
// returns "5"

addStrings("40", "60")
// returns "100"

addStrings("25", "75")
// returns "100"