分别用三个函数：输入（time_input）、输出（time_output）、转换(time_change)函数

    int hour,minute;
    char p;
    char noon;
   do{
    time_input(hour,minute);
    time_change(hour,minute,noon);
    time_output(hour,minute,noon);
    cout<<\"Enter \'r\' to again. \";
    cin>>p;

   }while(p==\'R\'||p==\'r\');

 system(\"pause\");//某些编译器此处可以不要
    return 0;
}

void time_input(int& hour,int& minute)
{
    cout<<\"Enter hour:\";
    cin>>hour;
    cout<<\"Enter minte:\";
    cin>>minute;
}
void time_output(int& hour,int& minute,char& noon)
{
    cout<<\"12-hours: \"
         <<hour
         <<\":\"
         <<minute
         <<\" \"
         <<noon
         <<endl;
}
void time_change(int& hour,int& minute,char& noon)
{
    if((hour>12&&hour<=24)&&(minute>=0&&minute<=60))
    {
    noon=\'P\';
    hour-=12;
    }
    else if((hour>=0&&hour<12)&&(minute>=0&&minute<=60)) {noon=\'A\';}
    else {cout<<\"Time is error!\\n\";}
}