43. Multiply Strings

class Solution:
    def multiply(self, num1: str, num2: str) -> str:
        ZERO = '0'
        if ZERO in (num1, num2):
            return ZERO

        L1, L2 = len(num1) - 1, len(num2) - 1
        carry = 0

        result = []
        i = 0
        while (i <= L1 + L2) or carry:
            j = max(0, i - L2)
            while j <= min(i, L1):
                n1 = num1[L1 - j]
                n2 = num2[L2 - i + j]
                carry += (ord(n1) - ord(ZERO)) * (ord(n2) - ord(ZERO))
                j += 1
            r = chr(carry % 10 + ord(ZERO))
            result.append(r)
            carry //= 10
            i += 1
        return ''.join(result[::-1])