43. Multiply Strings

🟧 Medium

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

My Solution

func multiply(num1 string, num2 string) string {
    if num1 == "0" || num2 == "0" {
        return "0"
    }

    l1, l2 := len(num1), len(num2)

    bytes := make([]byte, l1+l2)
    for i := l1 - 1; i >= 0; i-- {
        for j := l2 - 1; j >= 0; j-- {
            v := (num1[i]-'0')*(num2[j]-'0')
            bytes[i+j+1] += v
            if bytes[i+j+1] >= 10 {
                bytes[i+j] += bytes[i+j+1]/10
                bytes[i+j+1] %= 10
            }
        }
    }

    for bytes[0] == 0 {
        bytes = bytes[1:]
    }

    for i := range bytes {
        bytes[i] += '0'
    }

    return string(bytes)
}

result

Leetcode: link