stack操作

#include<iostream>
#include<stack>
using namespace std;
int main()
{
    int n,c=0,x;
    char op;
    while(scanf("%d ",&n)!=EOF&&n!=0)
    {
        stack<int> a;
        if(c>0) printf("\n");
        while(n--)
        {
            scanf("%c ",&op);
            if(op=='P')
            {
                scanf("%d ",&x);
                a.push(x);
            }
            else if(op=='O')
            {
                if(!a.empty()) a.pop();
            }
            else if(op=='A')
            {
                if(!a.empty()) printf("%d\n",a.top());
                else printf("E\n");
            }
        }
        c++;
    }
    return 0;
}