参考答案：

#include<iostream>

using namespace std;

const int N = 100010;
int str[N], tt;

int main()
{
    string op;
    int m;

    cin >> m;
    while (m--)
    {
        int x;
        cin >> op;
        if (op == "push")
        {
            cin >> x;
            str[++tt] = x;
        }
        else if (op == "pop") --tt;
        else if (op == "empty") cout << (tt ? "NO" : "YES") << endl;
        else cout << str[tt] << endl;
    }

    return 0;
}