字符串相乘

class Solution(object):
    def multi(self, str1, s2):
        # 一位数乘以多位数
        add = 0
        res = ''
        for s1 in str1[::-1]:
            tmp = int(s1) * int(s2) + add
            res += str(tmp % 10)
            add = tmp // 10
        if add != 0:
            res += str(add)
        return res[::-1]


    def add(self, str1, str2):
        # 字符串相加
        add = 0
        if len(str1) > len(str2):
            str1, str2 = str2, str1

        i = 1
        res = ''
        while i <= len(str1):
            tmp = int(str1[-i]) + int(str2[-i]) + add
            res += str(tmp % 10)
            add = tmp // 10
            i += 1

        while i <= len(str2):
            tmp = int(int(str2[-i]) + add)
            res += str(tmp % 10)
            add = tmp // 10
            i += 1

        if add != 0:
            res += str(add)
        return res[::-1]



    def multiply(self, num1, num2):
        """
        :type num1: str
        :type num2: str
        :rtype: str
        """
        if num1 == '0' or num2 == '0':
            return '0'

        res = []
        for i, s2 in enumerate(num2[::-1]):
            res.append(self.multi(num1, s2) + '0' * i)
        res = reduce(lambda x, y: self.add(x, y), res)
        return res