UVA 11995 I Can Guess the Data Structure!（ADT）

I Can Guess the Data Structure!

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

题目大意：有一个类似“包包”的数据结构，支持两种操作。1x：把元素x放进包包；2：从包包中拿出一个元素。给出一系列的操作以及返回值，你的任务是猜猜这个“包包”到底是什么。它可能是一个栈，队列，优先队列或者其他什么奇怪的东西。

分析：只需要依次判断输入是否可能是栈，队列或优先队列，然后中合起来即可。注意到题目中说的“无错的返回”，因此在执行pop操作的时候要调用一下empty()，否则可能会异常退出。

代码如下：

 #include<cstdio>
 #include<queue>
 #include<stack>
 #include<cstdlib>
 using namespace std;

 const int maxn =  + ;
 int n, t[maxn], v[maxn];

 int check_stack() {
   stack<int> s;
   for(int i = ; i < n; i++) {
     if(t[i] == ) {
       if(s.empty()) return ;
       int x = s.top(); s.pop();
       if(x != v[i]) return ;
     }
     else s.push(v[i]);
   }
   return ;
 }

 int check_queue() {
   queue<int> s;
   for(int i = ; i < n; i++) {
     if(t[i] == ) {
       if(s.empty()) return ;
       int x = s.front(); s.pop();
       if(x != v[i]) return ;
     }
     else s.push(v[i]);
   }
   return ;
 }

 int check_pq() {
   priority_queue<int> s;
   for(int i = ; i < n; i++) {
     if(t[i] == ) {
       if(s.empty()) return ;
       int x = s.top(); s.pop();
       if(x != v[i]) return ;
     }
     else s.push(v[i]);
   }
   return ;
 }

 int main() {
   while(scanf("%d", &n) == ) {
     for(int i = ; i < n; i++) scanf("%d%d", &t[i], &v[i]);
     int s = check_stack();
     int q = check_queue();
     int pq = check_pq();
     if(!s && !q && !pq) printf("impossible\n");
     else if(s && !q && !pq) printf("stack\n");
     else if(!s && q && !pq) printf("queue\n");
     else if(!s && !q && pq) printf("priority queue\n");
     else printf("not sure\n");
   }
   return ;
 }