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.

  Example 1:

Input: num1 = "11", num2 = "123" Output: "134"

Example 2:

Input: num1 = "456", num2 = "77" Output: "533"

Example 3:

Input: num1 = "0", num2 = "0" Output: "0"

  Constraints:

1 <= num1.length, num2.length <= 10⁴
num1 and num2 consist of only digits.
num1 and num2 don't have any leading zeros except for the zero itself.