There is a bag-like data structure, supporting two operations:

1 x

Throw an element x into the bag.

2

Take out an element from the bag.

Given a sequence of operations with return values, you're going to guess the data structure. It is a stack (Last-In, First-Out), a queue (First-In, First-Out), a priority-queue (Always take out larger elements first)
or something else that you can hardly imagine!

Input

There are several test cases. Each test case begins with a line containing a single integer n (1<=n<=1000). Each of the next n lines is either a type-1 command, or an integer 2 followed by an integer x. That means
after executing a type-2 command, we get an element x without error. The value of x is always a positive integer not larger than 100. The input is terminated by end-of-file (EOF). The size of input file does not exceed 1MB.

Output

For each test case, output one of the following:

stack

It's definitely a stack.

queue

It's definitely a queue.

priority queue

It's definitely a priority queue.

impossible

It can't be a stack, a queue or a priority queue.

not sure

It can be more than one of the three data structures mentioned above.

Sample Input

6
1 1
1 2
1 3
2 1
2 2
2 3
6
1 1
1 2
1 3
2 3
2 2
2 1
2
1 1
2 2
4
1 2
1 1
2 1
2 2
7
1 2
1 5
1 1
1 3
2 5
1 4
2 4

Output for the Sample Input

queue
not sure
impossible
stack
priority queue

Note: Please make sure to test your program with the gift I/O files before submitting!

直接STL进行小模拟即可，注意pop的时候判断是否empty。

#include <cstdio>
#include <cmath>
#include <algorithm>
#include <iostream>
#include <cstring>
#include <map>
#include <string>
#include <stack>
#include <cctype>
#include <vector>
#include <queue>
#include <set>

using namespace std;
const int MAXN = 1010 + 50;
const int maxw = 100 + 20;
const long long LLMAX = 0x7fffffffffffffffLL;
const long long LLMIN = 0x8000000000000000LL;
const int INF = 0x7fffffff;
const int IMIN = 0x80000000;
#define eps 1e10-8
#define mod 1000000007
typedef long long LL;
const double PI = acos(-1.0);
typedef double D;

//#define Online_Judge
#define outstars cout << "***********************" << endl;
#define clr(a,b) memset(a,b,sizeof(a))

#define FOR(i , x , n) for(int i = (x) ; i < (n) ; i++)
#define FORR(i , x , n) for(int i = (x) ; i <= (n) ; i++)
#define REP(i , x , n) for(int i = (x) ; i > (n) ; i--)
#define REPP(i ,x , n) for(int i = (x) ; i >= (n) ; i--)
stack <int> S;
queue <int> Q;
priority_queue <int> PQ;
int n , oks  , okq , okpq , a , b;
int main()
{
    //ios::sync_with_stdio(false);
#ifdef Online_Judge
    freopen("in.txt","r",stdin);
    freopen("out.txt","w",stdout);
#endif // Online_Judge
    while(scanf("%d",&n)!=EOF)
    {
        while(!S.empty())S.pop();
        while(!Q.empty())Q.pop();
        while(!PQ.empty())PQ.pop();
        oks = okq = okpq = 1;
        while(n--)
        {

            scanf("%d%d" , &a , &b);
            if(a == 1)
            {
                S.push(b);
                Q.push(b);
                PQ.push(b);
            }
            else
            {
                if(oks)
                {
                    if(!S.empty()&&S.top() == b)
                    {
                        S.pop();
                    }
                    else
                    {
                        oks = 0;
                    }
                }
                if(okq)
                {
                    if(!Q.empty()&&Q.front() == b)
                    {
                        Q.pop();
                    }
                    else
                    {
                        okq = 0;
                    }
                }
                if(okpq)
                {
                    if(!PQ.empty()&&PQ.top() == b)
                    {
                        PQ.pop();
                    }
                    else
                    {
                        okpq = 0;
                    }
                }

            }
        }
        if(oks&&!okq&&!okpq)printf("stack\n");
        else if(!oks&&okq&&!okpq)printf("queue\n");
        else if(!oks&&!okq&&okpq)printf("priority queue\n");
        else if(!oks&&!okq&&!okpq)printf("impossible\n");
        else printf("not sure\n");

    }
    return 0;
}