Summing Numeric Strings

Given two strings, num_str1 and num_str2, write a function to sum the two strings together without directly converting them to integers.

Note: Return the output in string format.

Example:

Input:

num_ str1 = '122' #assume that in practice the two strings are huge
num_str2 = '222'

Output:

sum_strings(num_str1,num_str2) -> '344'