Use built-in functions ord() and chr()

For example,

ord('a') returns the integer 97

chr(97) returns the string 'a'

```
class Solution(object):
def addStrings(self, num1, num2):
i, j, carry, res = 0, 0, 0, []
while i < len(num1) or j < len(num2) or carry:
if i < len(num1): # get the value and move on
carry, i = carry+ord(num1[::-1][i])-48, i+1
if j < len(num2):
carry, j = carry+ord(num2[::-1][j])-48, j+1
res += carry%10,
carry //= 10 # carry to next digit
return "".join([chr(c+48) for c in res[::-1]])
```