Multiply Strings

Given two non-negative integers num1 and num2 represented as strings, return the product of num1 and num2, also represented as a string.

Note: You must not use any built-in BigInteger library or convert the inputs to integer directly.

Example 1:

Input: num1 = "2", num2 = "3"
Output: "6"

Example 2:

Input: num1 = "123", num2 = "456"
Output: "56088"

Constraints:

• 1 <= num1.length, num2.length <= 200
• num1 and num2 consist of digits only.
• Both num1 and num2 do not contain any leading zero, except the number 0 itself.

SOLUTION:

class Solution:
    def addStrings(self, num1: str, num2: str) -> str:
        m = len(num1)
        n = len(num2)
        if m < n:
            num1 = "0" * (n - m) + num1
        else:
            num2 = "0" * (m - n) + num2
        carry = 0
        i = max(m, n) - 1
        op = ""
        while i >= 0:
            val = carry + int(num1[i]) + int(num2[i])
            op = str(val % 10) + op
            carry = val // 10
            i -= 1
        if carry > 0:
            op = "1" + op
        return op

    def mulNtimes(self, num, n):
        if n == 0:
            return "0"
        if n == 1:
            return num
        op = "0"
        if n & 1:
            a = num
            b = self.mulNtimes(num, (n - 1) // 2)
            b = self.addStrings(b, b)
            return self.addStrings(a, b)
        else:
            a = self.mulNtimes(num, n // 2)
            return self.addStrings(a, a)

    def multiply(self, num1: str, num2: str) -> str:
        op = "0"
        n = len(num2)
        for i in range(n):
            curr = self.mulNtimes(num1, int(num2[n - i - 1]))
            if curr != "0":
                curr += "0" * i
            op = self.addStrings(op, curr)
        return op