题解

#include<bits/stdc++.h>
#define ll long long                                                                                       //放抄袭标记
using namespace std;
int T;
int a[1000005],l = 0,r = 0;
int main(){
	cin>>T;
	while(T--){
		int c;
		cin>>c;
		if(c==1){
			int b;
			cin>>b;
			
			a[r] = b;
			r++;
		}
		else if(c == 2){
			if(l==r) cout<<"impossible!"<<endl;
			else l++;
		}
		else{
			if(l == r) cout<<"impossible!\n";
			else cout<<a[l]<<endl;
		}
	}
	return 0;
}