uva 11995 I Can Guess the Data Structure!

#include<stack>
#include<queue>
#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
using namespace std;

stack<int> stk;
queue<int> que;
priority_queue<int> p_que;
int n;
bool is_stack , is_queue , is_Pqueue;

void init(){
while(!stk.empty())
stk.pop();
while(!que.empty())
que.pop();
while(!p_que.empty())
p_que.pop();
is_stack = is_queue = is_Pqueue = true;
}

void insert(int x){
stk.push(x);
que.push(x);
p_que.push(x);
}

void pop(int x){
if(is_stack){
if(!stk.empty() && x == stk.top())
stk.pop();
else
is_stack = false;
}
if(is_queue){
if(!que.empty() && x == que.front())
que.pop();
else
is_queue = false;
}
if(is_Pqueue){
if(!p_que.empty() && x == p_que.top())
p_que.pop();
else
is_Pqueue = false;
}
}

void output(){
if(is_stack && !is_queue && !is_Pqueue)
puts("stack");
else if(!is_stack && is_queue && !is_Pqueue)
puts("queue");
else if(!is_stack && !is_queue && is_Pqueue)
puts("priority queue");
else if(!is_stack && !is_queue && !is_Pqueue)
puts("impossible");
else
puts("not sure");
}

int main(){
int x , y;
while(scanf("%d" , &n) != EOF){
init();
for(int i = 0 ; i < n ; i++){
scanf("%d%d" , &x , &y);
if(x == 1)
insert(y);
else
pop(y);
}
output();
}
return 0;
}