multiply_strings.py

"""
https://leetcode.com/problems/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.
"""

strToNum = {
    "0": 0,
    "1": 1,
    "2": 2,
    "3": 3,
    "4": 4,
    "5": 5,
    "6": 6,
    "7": 7,
    "8": 8,
    "9": 9,
}


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

    result = [0] * (len(num1) + len(num2))

    for i in range(len(num1) - 1, -1, -1):
        for j in range(len(num2) - 1, -1, -1):
            product = strToNum[num1[i]] * strToNum[num2[j]]

            ones = i + j + 1
            tens = i + j
            total = product + result[ones]

            result[ones] = total % 10
            result[tens] += total // 10

    return "".join(map(str, result)).lstrip("0")