已知下列程序，请将display改为普通函数，分别作为Time类和Date的友元函数。在主函数中调用display函数，实现输出和原来程序一样的结果。

#include<iostream>
using namespace std;
class Time;
class Date{
    int year,month,day;
public:
    Date(int y=0,int m=0,int d=0):year(y),month(m),day(d){}
    friend void display(Date &d,Time &t);
};
class Time{
    int hour,minute,second;
public:
    Time(int h=0,int m=0,int s=0):hour(h),minute(m),second(s){}
    friend void display(Date &d,Time &t);
};
void display(Date &d,Time &t){
    cout<<d.year<<"-"<<d.month<<"-"<<d.day<<" ";
    cout<<t.hour<<":"<<t.minute<<":"<<t.second<<endl;
}
int main(){
    Date d(2022,10,1);
    Time t(12,30,0);
    display(d,t);
    return 0;
}

修改后的程序如下：

#include<iostream>
using namespace std;
class Time;
class Date{
    int year,month,day;
public:
    Date(int y=0,int m=0,int d=0):year(y),month(m),day(d){}
    friend void display(Date &d, Time &t);
};
class Time{
    int hour,minute,second;
public:
    Time(int h=0,int m=0,int s=0):hour(h),minute(m),second(s){}
    friend void display(Date &d, Time &t);
};
void display(Date &d, Time &t){
    cout<<d.year<<"-"<<d.month<<"-"<<d.day<<" ";
    cout<<t.hour<<":"<<t.minute<<":"<<t.second<<endl;
}
int main(){
    Date d(2022,10,1);
    Time t(12,30,0);
    display(d, t);
    return 0;
}