题目

#include <iostream>
#include <string>
using namespace std;
int main(){
	char b;
	string a[1001]={};
	int n,hd=0;
	cin>>n;
	for (int i=1;i<=n;i++){
		a[i]="";
		cin>>b;
		if (b=='V'){
			hd++;
			cin>>a[hd];
			cout<<a[hd];
			for (int j=hd+1;j<=n;j++){
				a[j]="";
			}
		}
		if (b=='B'){
			if (a[hd-1]!=""){
				hd--;
				cout<<a[hd];
			}else{
				cout<<"?";
			}
		}
		if (b=='F'){
			if (a[hd+1]!=""){
				hd++;
				cout<<a[hd];
			}else{
				cout<<"?";
			}
		}
        if (i!=n){
            cout<<endl;
        }
	}
	return 0;
}