UVA11995: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).
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
Sample Output
queue
not sure
impossible
stack
priority queue

#include <iostream>
#include <stdio.h>
#include <string.h>
#include <stack>
#include <queue>
#include <map>
#include <set>
#include <vector>
#include <math.h>
#include <bitset>
#include <algorithm>
#include <climits>
using namespace std;

#define LS 2*i
#define RS 2*i+1
#define UP(i,x,y) for(i=x;i<=y;i++)
#define DOWN(i,x,y) for(i=x;i>=y;i--)
#define MEM(a,x) memset(a,x,sizeof(a))
#define W(a) while(a)
#define gcd(a,b) __gcd(a,b)
#define LL long long
#define N 1000005
#define MOD 1000000007
#define INF 0x3f3f3f3f
#define EXP 1e-8

struct node
{
    int x;
    bool operator<(node a)const
    {
        return x<a.x;
    }
} a;


priority_queue <node> P;
queue<node> Q;
stack<node> S;
int main()
{
    int n,k,x;
    while(~scanf("%d",&n))
    {
        while(!Q.empty())Q.pop();
        while(!S.empty())S.pop();
        while(!P.empty())P.pop();
        int kp,kq,ks;
        kp=kq=ks=1;
        while(n--)
        {
            scanf("%d%d",&k,&a.x);
            if(k==1)
            {
                P.push(a);
                Q.push(a);
                S.push(a);
            }
            else
            {
                node r;
                if(!P.empty()&&kp)
                {
                    r = P.top();
                    P.pop();
                    if(r.x!=a.x)
                        kp=0;
                }
                else
                    kp=0;

                if(!Q.empty()&&kq)
                {
                    r = Q.front();
                    Q.pop();
                    if(r.x!=a.x)
                        kq=0;
                }
                else
                    kq = 0;

                if(!S.empty()&&ks)
                {
                    r = S.top();
                    S.pop();
                    if(r.x!=a.x)
                        ks=0;
                }
                else
                    ks = 0;
            }
        }
        if(ks&&!kq&&!kp)
            puts("stack");
        else if(!ks&&kq&&!kp)
            puts("queue");
        else if(!ks&&!kq&&kp)
            puts("priority queue");
        else if(!ks&&!kq&&!kp)
            puts("impossible");
        else
            puts("not sure");
    }

    return 0;
}