1. Given two non-negative integers, num1 and num2 represented as string. 2. Return the sum of num1 and num2 as a string. 3. You must solve the problem without using any built-in library for handling large integers (such as BigInteger). 4. You must also not convert the inputs to integers directly.
1. 1 <= num1.length, num2.length <= 10^4 2. num1 and num2 consist of only digits. 3. num1 and num2 don't have any leading zeros except for the zero itself.
n1 = "1234" n2 = "897"
res = "2131"