leetcode: 43. Multiply Strings

# Given two numbers represented as strings, 
# return multiplication of the numbers as a string.
# 
# Note: The numbers can be arbitrarily large and are non-negative.

class Solution():
    def multiply(self, x1, x2):
        x1, x2, res = x1[::-1], x2[::-1], [0]*(len(x1) + len(x2))
        for i in range(len(x1)):
            for j in range(len(x2)):
                res[i + j] += int(x1[i]) * int(x2[j])
                res[i + j + 1] += res[i + j] // 10
                res[i + j] %= 10
        i = len(res) - 1
        while i > 0 and res[i] == 0:
            i -= 1
        return ''.join(map(str, res[i::-1]))


if __name__ == "__main__":
    assert Solution().multiply("123", "1000") == '123000'