string- 43. Multiply Strings

43. 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.

Example 1:

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

Example 2:

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

Note:

1. The length of both num1 and num2 is < 110.
2. Both num1 and num2 contain only digits 0-9.
3. Both num1 and num2 do not contain any leading zero, except the number 0 itself.
4. You must not use any built-in BigInteger library or convert the inputs to integer directly.

思路：

题目要求把两个只含数字的字符串相乘，不允许使用Atoi的接口来把输入转换成整形直接相乘，所以需要模拟乘法的过程，做法就是用两个循环，依次遍历两个字符串，挨个相乘模10相加，除10进位，注意go语言里的string取每一位的时候默认是uint8，也就是byte类型。在做加法的时候，为了方便，全部用数字来表示，到最后同一转换为字符，再转换为string。

代码：

go：

func multiply(num1 string, num2 string) string {
    size1 := len(num1)
    size2 := len(num2)
    var res = make([]byte, size1 + size2)
    
    // multi
    for i := size2 - 1; i >= 0; i-- {
        for j := size1 - 1; j >= 0; j-- {
            product := (num1[j] - '0' ) * (num2[i] - '0')
            
            sum := res[i+j+1] + product
            
            res[i+j+1] = sum % 10
            res[i+j] += (sum/10)
        }
    }
    
    // remove front zero
    var start = 0
    for start < len(res) && res[start] == 0 {
        start++
    }
    if start == len(res) {
        return "0"
    }
    
    // convert to string
    for i := range res {
        res[i] += '0'
    }
    
    return string(res[start:])
}