human
I found an interesting problem on https://leetcode.com/problems/add-strings/:
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.

I tried it in Python, but could not do it. Can you solve it?

Write me a the Python program you requested?
assistant
#Here is the Python program you requested.

class Solution:
     def addStrings(self, num1, num2):
         """
         :type num1: str
         :type num2: str
         :rtype: str
         """
         if len(num1) < len(num2):
             num1, num2 = num2, num1
         addon = 0
         res = ""
         l = len(num2)
         for i in range(l):
             s =  ord(num2[l - i - 1]) + ord(num1[len(num1) - i - 1]) - 2 * ord("0") + addon
             addon = s // 10
             res = chr(s % 10 + ord("0")) + res
         for i in range(l, len(num1)):
             s = ord(num1[len(num1) - i - 1]) - ord("0") + addon 
             addon = s // 10
             res = chr(s % 10 + ord("0")) + res
         if addon > 0:
             res = "1" + res
         return res