Go--实现两个大数相乘

-----

import (
    "bufio"
    "fmt"
    "os"
    "strings"
)

func multi(str1, str2 string) (result string) {

    if len(str1) == 0 && len(str2) == 0 {
        result = "0"
        return
    }

    var index1 = len(str1) - 1
    var index2 = len(str2) - 1
    var left int

    for index1 >= 0 && index2 >= 0 {
        c1 := str1[index1] - '0'
        c2 := str2[index2] - '0'

        sum := int(c1) + int(c2) + left
        if sum >= 10 {
            left = 1
        } else {
            left = 0
        }
        c3 := (sum % 10) + '0'
        result = fmt.Sprintf("%c%s", c3, result)
        index1--
        index2--
    }

    for index1 >= 0 {
        c1 := str1[index1] - '0'
        sum := int(c1) + left
        if sum >= 10 {
            left = 1
        } else {
            left = 0
        }
        c3 := (sum % 10) + '0'

        result = fmt.Sprintf("%c%s", c3, result)
        index1--
    }

    for index2 >= 0 {
        c1 := str2[index2] - '0'
        sum := int(c1) + left
        if sum >= 10 {
            left = 1
        } else {
            left = 0
        }
        c3 := (sum % 10) + '0'
        result = fmt.Sprintf("%c%s", c3, result)
        index2--
    }

    if left == 1 {
        result = fmt.Sprintf("1%s", result)
    }
    return
}

func main() {
    reader := bufio.NewReader(os.Stdin)
    result, _, err := reader.ReadLine()
    if err != nil {
        fmt.Println("read from console err:", err)
        return
    }

    strSlice := strings.Split(string(result), "+")
    if len(strSlice) != 2 {
        fmt.Println("please input a+b")
        return
    }

    strNumber1 := strings.TrimSpace(strSlice[0])
    strNumber2 := strings.TrimSpace(strSlice[1])
    fmt.Println(multi(strNumber1, strNumber2))
}