Given two non-negative integers num1 and num2 represented as string, return the sum of num1 and num2.
- The length of both num1 and num2 is < 5100.
- Both num1 and num2 contains only digits 0-9.
- Both num1 and num2 does not contain any leading zero.
- You must not use any built-in BigInteger library or convert the inputs to integer directly.
Input: '0', '0'
Output: 0