Very similar to addBits, we need to use two pointers.

Use prev to carry 1 or 0 from prev calculations.

Question:

Given two non-negative integers, num1 and num2 represented as string, return the sum of num1 and num2 as a string.

You must solve the problem without using any built-in library for handling large integers (such as BigInteger). You must also not convert the inputs to integers directly.