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 (such as BigInteger). You must also not convert the inputs to integers directly.
1 <= num1.length, num2.length <= 10^4num1 and num2 consist of only digits.num1 and num2 don't have any leading zeros except for the zero itself.num1 = "11", num2 = "123""134"num1 = "456", num2 = "77""533"num1 = "0", num2 = "0""0"