栈和队列

题目：

代码： 

#include <iostream>
using namespace std;
const int N = 100010;
int st[N];
int top = -1;
int n;

int main()
{
    cin >> n;
    while(n--)
    {
        string s;
        cin >> s;

        //插入
        if(s == "push")
        {
            int a;
            cin >> a;
            st[++top] = a;
        }

        //弹出
        if(s == "pop")
        {
            top --;
        }
       
        //栈顶
        if(s == "query")
        {
            cout << st[top] << endl;
        }
       
        //判断栈是否为空
        if(s == "empty")
        {
            cout << (top == -1 ? "YES" : "NO") << endl;
        }
    }
}