Add Strings

Problem Statement:

Given two non-negative integers num1 and num2 represented as string, return the sum of num1 and num2.
Note:
The length of both num1 and num2 is < 5100.
Both num1 and num2 contains only digits 0-9.
Both num1 and num2 does not contain any leading zero.
You must not use any built-in BigInteger library or convert the inputs to integer directly


Solution:
class Solution(object):
    def addStrings(self, num1, num2):
        c=0
        stri=""
        i=len(num1)-1
        j=len(num2)-1
        while (i>=0 or j>=0 or c):
            val1=int(num1[i]) if i>=0 else 0
            val2=int(num2[j]) if j>=0 else 0
            summ=val1+val2+c
            c,r=divmod(summ,10)
            stri=str(r)+stri
            i-=1
            j-=1
        return stri