golang 解法

package main

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

func main() {
    bs := bufio.NewScanner(os.Stdin)
    isAdd := true
    for bs.Scan() {
        input := bs.Text()
        if isAdd {
            ret := add(input)
            fmt.Println(ret)
            isAdd = false
            continue
        }
        ret := down(input)
        fmt.Println(ret)
        isAdd = true
    }
}

func down(str string) string {
    s := ""
    for _, i := range str {
        switch{
        case i > 'a' && i <= 'z':
            s = s + string(i-33)
        case i == 'a':
            s = s + "Z"
        case i > 'A' && i <='Z':
            s = s + string(i+31)
        case i == 'A':
            s = s + "z"

        case i > '0' && i <= '9':
            s = s + string(i-1)
        case i == '0':
            s = s + "9"
        }
    }
    return s
}

func add(str string) string {
    s := ""
    for _, i := range str {
        switch {
        case i >= 'a' && i < 'z':
            s = s + string(i-31)
        case i == 'z':
            s = s + "A"
        case i >= 'A' && i < 'Z':
            s = s + string(i+33)
        case i == 'Z':
            s = s + "a"
        case i >= '0' && i < '9':
            s = s + string(i+1)
        case i == '9':
            s = s + "0"
        }
    }
    return s
}