MultiplyStrings.py

def multiply(self, num1: str, num2: str) -> str:
    if num1 == "0" or num2 == "0":
        return "0"

    ans = "0"
    m, n = len(num1), len(num2)
    for i in range(n - 1, -1, -1):
        add = 0
        y = int(num2[i])
        curr = ["0"] * (n - i - 1)
        for j in range(m - 1, -1, -1):
            product = int(num1[j]) * y + add
            curr.append(str(product % 10))
            add = product // 10
        if add > 0:
            curr.append(str(add))
        curr = "".join(curr[::-1])
        ans = self.addStrings(ans, curr)

    return ans

def addStrings(self, num1: str, num2: str) -> str:
    i, j = len(num1) - 1, len(num2) - 1
    add = 0
    ans = list()
    while i >= 0 or j >= 0 or add != 0:
        x = int(num1[i]) if i >= 0 else 0
        y = int(num2[j]) if j >= 0 else 0
        result = x + y + add
        ans.append(str(result % 10))
        add = result // 10
        i -= 1
        j -= 1
    return "".join(ans[::-1])