UVA_11995_I_Can_Guess

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <string>
#include <cctype>
#include <stack>
#include <queue>
#include <list>
#include <vector>
#include <map>
#include <set>
#include <sstream>
#include <stdlib.h>
#include <cmath>
#define FOR(i,A) for(typeof (A).begin() i = (A).begin() ; i != (A).end() ; i++)
#define debug( x ) cout << #x << " = " << x << endl
#define clr(v,x) memset( v, x , sizeof v )
#define all(x) (x).begin() , (x).end()
#define rall(x) (x).rbegin() , (x).rend()
#define TAM 110

using namespace std;

typedef pair<int,int> ii ;
typedef long long ll ;
typedef long double ld ;
typedef pair<int,ii> pii ;

int main() {
    int t,ins,x;
    while(scanf("%d",&t)!=EOF){
        bool isstck=true,isque=true,isprior=true;
        stack<int> s; queue<int> q; priority_queue<int> pq;
        for(int i=0;i<t;i++){
            scanf("%d %d",&ins,&x);
            if(ins==1){
                if(isstck){
                    s.push(x);
                }
                if(isque){
                    q.push(x);
                }
                if(isprior){
                    pq.push(x);
                }
            }else{
                if (isstck) {
                    if (s.empty() || s.top() != x)
                        isstck = false;
                    else
                        s.pop();
                }
                if (isque) {
                    if (q.empty() || q.front() != x)
                        isque = false;
                    else
                        q.pop();
                }
                if (isprior) {
                    if (pq.empty() || pq.top() != x)
                        isprior = false;
                    else
                        pq.pop();
                }
            }
        }
        if(isstck && !isque && !isprior){
            printf("stack\n");
        }else if(!isstck && isque && !isprior){
            printf("queue\n");
        }else if(!isstck && !isque && isprior){
            printf("priority queue\n");
        }else if(!isstck && !isque && !isprior){
            printf("impossible\n");
        }else{
            printf("not sure\n");
        }
    }
    return 0;
}