题解 | #堆栈的使用#

#include<iostream>
#include<stack>
using namespace std;
stack<int>ss;
int main(){
    int n,b;char a;
    while(cin>>n){for(int i=0;i<n;i++){
        cin>>a;
        if(a=='A'){
            if(ss.empty())cout<<'E'<<endl;else cout<<ss.top()<<endl;
        }else if(a=='P') {cin>>b;ss.push(b);}
        else if(a=='O'){if(!ss.empty())ss.pop();}
    }

    }
}