这道题比较简单。

需要注意的一些地方：

1、impossible: 所有的标记量都是false

2、not sure：同时存在2种情况或者同时存在三种情况.

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!

直接对三种数据结构进行模拟，注意当前数据结构为空的时候的判断。

代码如下:

/*
 * UVA_11995.cpp
 *
 *  Created on: 2014年12月28日
 *      Author: Administrator
 */


#include <iostream>
#include <cstdio>
#include <queue>
#include <stack>
#include <cstring>


using namespace std;

const int maxn = 1010;

int n;
int id[maxn];
int x[maxn];


bool isStack(){
	stack<int> s;

	int i;
	for(i = 0 ; i < n ; ++i){
		if(id[i] == 1){
			s.push(x[i]);
		}else{
			if(s.empty() != true){
				int val = s.top();
				s.pop();

				if(x[i] != val){
					return false;
				}
			}else{
				return false;
			}
		}
	}

	return true;
}

bool isQueue(){
	queue<int> q;

	int i;
	for(i = 0 ; i < n ; ++i){
		if(id[i] == 1){
			q.push(x[i]);
		}else{
			if(q.empty() != true){
				int val = q.front();
				q.pop();

				if(x[i] != val){
					return false;
				}
			}else{
				return false;
			}
		}
	}

	return true;
}


bool isPriority_Queue(){
	priority_queue<int> q;

	int i;
	for(i = 0 ; i < n ; ++i){
		if(id[i] == 1){
			q.push(x[i]);
		}else{
			if(q.empty() != true){
				int val = q.top();
				q.pop();

				if(x[i] != val){
					return false;
				}
			}else{
				return false;
			}
		}
	}

	return true;

}

int main(){
	while(scanf("%d",&n) != EOF){
		memset(id,-1,sizeof(id));
		memset(x,-1,sizeof(x));

		int i;
		for(i = 0 ; i < n ; ++i){
			scanf("%d%d",&id[i],&x[i]);
		}

		bool st = isStack();
		bool q = isQueue();
		bool pq = isPriority_Queue();

		if(st == false && q == false && pq == false){
			printf("impossible\n");
		}else if((st == true && q == true && pq == true) || (st == true && q == true && pq == false) || (st == true && q == false && pq == true) || (st == false && q == true && pq == true)){
			printf("not sure\n");
		}else if(st == true && q == false && pq == false){
			printf("stack\n");
		}else if(q == true && st == false && pq == false){
			printf("queue\n");
		}else if(pq == true && st == false && q == false){
			printf("priority queue\n");
		}
	}

	return 0;
}

(DS 《算法入门经典》)UVA 11995 I Can Guess the Data Structure!(判断是哪一种数据结构)