Add Strings

'''Given two non-negative integers, num1 and num2 represented as string, 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.'''

class Solution(object):
    def addStrings(self, num1, num2):
        """
        :type num1: str
        :type num2: str
        :rtype: str
        """
        carry = 0
        res = []
        num1, num2 = list(num1), list(num2)
        while num1 or num2 or carry>0:
            if num1:
                carry += ord(num1.pop())-ord('0')
            if num2:
                carry += ord(num2.pop())-ord('0')
            res.append(carry%10)
            carry = carry//10
        return ''.join(map(str,res[::-1])