10
push 6
empty
query
pop
empty
push 3
push 4
pop
query
push 6

#include <bits/stdc++.h>
using namespace std;
const int N = 100010;
int arr[N];
int main()
{
    int m,i=0,j=-1; //i:队头指针，j:队尾指针
    cin>>m;
    while(m--)
    {
        string op;
        int x;
        cin>>op;
        if(op=="push")
        {
            scanf("%d",&x);
            j++;
            arr[j]=x;
        }
        else if(op=="pop")
            i++;
        else if(op=="empty")
            if(j<i) printf("YES\n");
            else printf("NO\n");
        else
            printf("%d\n",arr[i]);
    }
    return 0;
}