思路：
从末尾开始相加，进位可以最后统一处理，因为再怎么进也是最多只进一位

class Solution:def addStrings(self, num1: str, num2: str) -> str:# 确保1里是更长的字符串if len(num1) < len(num2):num1_list = list(num2)num2_list = list(num1)else:num1_list = list(num1)num2_list = list(num2)num1_list.reverse()num2_list.reverse()for pointer in range(0, len(num2_list)):num1_list[pointer] = int(num1_list[pointer]) + int(num2_list[pointer])last_remain = 0print(num1_list)for pointer in range(0, len(num1_list)):current_num = int(num1_list[pointer]) + last_remainif current_num >= 10:last_remain = current_num // 10num1_list[pointer] = current_num % 10else:num1_list[pointer] = current_numlast_remain = 0answer = ""#print(num1_list,last_remain)if last_remain > 0:answer = str(last_remain)num1_list.reverse()for each_num in num1_list:answer += str(each_num)print(answer)return answer