#include <iostream>
#include <vector>
using namespace std;
const int N = 100010;

vector<int> stack;

int main()
{
    int m;
    cin>>m;
    while(m--)
    {
        string s;
        cin>>s;
        if(s == "push")
        {
            int x;
            cin>>x;
            stack.push_back(x);
        }
        if(s == "pop")
        {
            stack.pop_back();
        }
        if(s == "empty")
        {
            if(stack.empty()) cout<<"YES"<<endl;
            else cout<<"NO"<<endl;
        }
        if(s == "query")
        {
            cout<<stack.back()<<endl;
        }
    }
    return 0;
}

判断empty的时候，还可以通过判断stack.size()是否为0来判断栈是否为空。