此題花了好多時間，開始的時候就是直接加密輸出，然後再輸出一遍源字串，結果就一直Wrong Answer.
所以，絕對不能偷懶，解密函式也要寫，因為題目並沒有說解密後和源字串一致

程式碼如下：

#include<stdio.h>
#include<string.h>
void encrypt(char *song,const int *key)
{
    int i,tmp;
    for(i = 0 ; i < strlen(song); i++)
    {
        tmp = (int)song[i]+key[i%7];
        if(tmp > 122)
            song[i] = (char)(tmp%91);
        else
            song[i] = (char)tmp;

    }
}
void decrypt(char *song,const int *key)
{
    int i,tmp;
    for(i = 0 ; i < strlen(song); i++)
    {
        tmp = (int)song[i]-key[i%7];
        if(tmp < 32)
            song[i] = (char)(tmp+91);
        else
            song[i] = (char)tmp;

    }
}
int main()
{
    int T,i,n = 1,tmp;
    char song[1024];
    int key[7]={4,9,6,2,8,7,3};
    scanf("%d",&T);
    getchar();
    while(T--)
    {
        memset(song,0,sizeof(song));
        gets(song);
        printf("Case %d:\n",n++);
        encrypt(song,key);
        printf("%s\n\n",song);
        decrypt(song,key);
        printf("%s\n",song);
    }
    return 0;
}