uva 11995 - I Can Guess the Data Structure!

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

using namespace std;

int main()
{
int i,n,t,x,f1,f2,f3,tmp;
while(scanf("%d",&n)!=EOF)
{
queue<int>q;
stack<int>s;
priority_queue<int>r;
f1=1,f2=1,f3=1;
for(i=0;i<n;i++)
{
scanf("%d%d",&t,&x);
if(t==1)
{
q.push(x);
s.push(x);
r.push(x);
}
else if(!q.empty())
{
if(f1 && s.top()!=x) f1=0;s.pop();
if(f2 && q.front()!=x) f2=0;q.pop();
if(f3 && r.top()!=x) f3=0;r.pop();
}
else f1=f2=f3=0;
}
tmp=f1+f2+f3;
if(tmp==0) printf("impossible\n");
else if(tmp==1)
{
if(f1) printf("stack\n");
else if(f2) printf("queue\n");
else printf("priority queue\n");
}
else printf("not sure\n");
}
return 0;
}