Given two non-negative integers, num1 and num2 represented as strings, 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 and without converting the inputs to integers directly.

Example 1:

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

Example 2:

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

Constraints:

• 1 <= num1.length, num2.length <= 10^4
• num1 and num2 consist of only digits