Multiply strings

We will first reverse both string and then calculate the first characters of both string, to get the digit. Then we store this digit at (i1+i2) position of our result. The (i1+i2+1) position will store the remaining value after 1st digit and (i1+i2) position we store the last digit. Then finally we join the res array as string and return.

class Solution:
    def multiply(self, num1: str, num2: str) -> str:
        if "0" in [num1, num2]:
            return "0"

        res = [0] * (len(num1) + len(num2))
        num1, num2 = num1[::-1], num2[::-1]
        for i1 in range(len(num1)):
            for i2 in range(len(num2)):
                digit = int(num1[i1]) * int(num2[i2])
                res[i1 + i2] += digit
                res[i1 + i2 + 1] += res[i1 + i2] // 10
                res[i1 + i2] = res[i1 + i2] % 10

        res, beg = res[::-1], 0
        while beg < len(res) and res[beg] == 0:
            beg += 1
        res = map(str, res[beg:])
        return "".join(res)

Time Complexity: O(n*m)
Space Complexity: O(n+m)