43. Multiply Strings - LeetCode

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

        def ord_(x):
            return ord(x) - ord(ZERO)

        def chr_(x):
            return chr(x + ord(ZERO))

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