Problem Description

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.

Examples

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.

解析

給定兩個數字字串 num1, num2

要求寫一個演算來來做兩個數字相乘並且回傳相乘後的字串

題目特別要求不要直接使用字串轉數字的方式去做

也就是要採用一個 digit 一個 digit 的方式去運算

因為把兩數相乘以位元的角度是把其中一個數值固定

變動另一個數字在另一個數字的每個位元當作起始點