题解

#include<bits/stdc++.h>
using namespace std;
stack<int> a;
int main(){
    int n;
    cin >> n;
    for ( int i=0; i<n; i++){
        string s;
        cin >> s;
        if ( s[0] == 'e'){
            cout << (a.empty()?"yes":"no") << endl;
        }else if ( s[0] == 'p'){
            if ( s[1] == 'u'){
                int x;
                cin >> x;
                a.push(x);
            }else{
                if ( a.empty()){
                    cout << "pop fail" << endl;
                }else{
                    cout << "pop " << a.top() << endl;
                    a.pop();
                }
            }
        }else if ( s[0] == 't'){
            if ( a.empty()){
                cout << "top fail" << endl;
            }else{
                cout << "top = " << a.top() << endl;
            }
        }else if ( s[0] == 's'){
            cout << "size = " << a.size() << endl;
        }
    }
}