43-multiply-strings.py

"""
43. Multiply Strings
Medium - 42.9%

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"
"""

class Solution:
    def multiply(self, num1: str, num2: str) -> str:
        if num1 == "0" or num2 == "0":
            return "0"
        
        m, n = len(num1), len(num2)
        result = [0] * (m + n)
        
        # Multiply each digit from right to left
        for i in range(m - 1, -1, -1):
            for j in range(n - 1, -1, -1):
                mul = int(num1[i]) * int(num2[j])
                pos1, pos2 = i + j, i + j + 1
                total = mul + result[pos2]
                
                result[pos2] = total % 10
                result[pos1] += total // 10
        
        # Convert result to string
        res = ""
        for i in range(len(result)):
            if not (res == "" and result[i] == 0):
                res += str(result[i])
        
        return res if res else "0"