```
class Solution(object):
def addStrings(self, num1, num2):
def additionStrings(num1, num2, carry):
if not num1 and not num2:
if carry: res.append(carry)
return
a, b = num1.pop() if num1 else 0, num2.pop() if num2 else 0
carry, digit = divmod(a + b + carry, 10)
res.append(digit)
additionStrings(num1, num2, carry)
res = []
additionStrings([int(c) for c in num1],
[int(c) for c in num2],
0)
return "".join([str(num) for num in res[::-1]])
```