UVA - 11995 I Can Guess the Data Structure!

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

int main() {
int n;
while (scanf("%d", &n) != EOF) {
stack<int> st;
queue<int> qu;
priority_queue<int> pq;
int S = 1, Q = 1, P = 1;
for (int i = 0; i < n; i++) {
int c, x;
scanf("%d%d", &c, &x);
if (c == 1) {
st.push(x);
qu.push(x);
pq.push(x);
} else {
if (st.empty()) {
S = Q = P = 0;
continue;
}
if (S)
S = (st.top() == x);
if (Q)
Q = (qu.front() == x);
if (P)
P = (pq.top() == x);
st.pop();
qu.pop();
pq.pop();
}
}

if (S + Q + P > 1)
puts("not sure");
else if (S)
puts("stack");
else if (Q)
puts("queue");
else if (P)
puts("priority queue");
else
puts("impossible");
}

return 0;
}