题解 | #堆栈的使用#

#include<iostream>
#include<stack>
using namespace std;

int main(){
    
    int n;
	
	while(cin>>n){
		char op;
		stack<int> stk;
		while(n--){
			cin>>op;
			if(op == 'P'){//压栈 
				int m;
				cin>>m;
				stk.push(m);
			}else if(op == 'O'){//弹栈 
                if(!stk.empty()) stk.pop();
			}else if(op == 'A'){//输出栈顶 
				if(stk.empty()){
					cout<<'E'<<endl;
				}else{
					cout<<stk.top()<<endl;
				}
			}
			
		}
	} 
    
    return 0;
}