菜鸡写了一份，速度很慢哈哈

class Solution:
def addStrings(self, num1: str, num2: str) -> str:
res = []
carry = 0
current = (0, 0)
while current != None:
current = self.add_last(num1, num2, carry)
if num1 != '': num1 = num1[0:-1]
if num2 != '': num2 = num2[0:-1]

if current != None:
carry = current[1]
res.insert(0, str(current[0]))
if carry > 0:
res.insert(0, str(carry))
return ''.join(res)

def add_last(self, num1, num2, forth):
if len(num1) == 0 and len(num2) == 0:
return None
elif len(num1) == 0 and len(num2) > 0:
return self.add(num2[-1], forth)
elif len(num1) > 0 and len(num2) == 0:
return self.add(num1[-1], forth)
return self.add(num1[-1], num2[-1], forth)

def add(self, *nums):
total = reduce(lambda x, y: int(x) + int(y), nums)
return total%10, math.floor(total/10) if total >= 10 else 0