字符串乘法

import (
	"fmt"
	"strconv"
)

func main() {
	fmt.Println(multiply("999", "999"))
}

func multiply(num1 string, num2 string) string {
	if num1 == "0" || num2 == "0" {
		return "0"
	}
	m, n := len(num1), len(num2)
	ret := make([]int, m+n)
	for i := m - 1; i >= 0; i-- {
		y := int(num1[i] - '0')
		for j := n - 1; j >= 0; j-- {
			x := int(num2[j] - '0')
			temp := (x * y) + ret[i+j+1]
			ret[i+j+1] = temp % 10
			ret[i+j] += temp / 10
		}
	}

	// 去掉前缀0
	j := 0
	for j < m+n && ret[j] == 0 {
		j++
	}

	var retS string
	for i := j; i < len(ret); i++ {
		retS += strconv.Itoa(ret[i])
	}
	return retS
}