1 #include <iostream> 2 #include <cstring> 3 #include <queue> 4 #include <stack> 5 using namespace std; 6  7  8  9 int main (){10     int n;11     int x[1005][2];12     while (cin>>n){13         for (int i=0;i<n;i++){14             cin>>x[i][1]>>x[i][0];15         }16         queue<int> q1;17         stack<int> q2;18         priority_queue<int> q3;19         int flag[5];20         for (int i=0;i<5;i++)  flag[i]=1;21         for (int i=0;i<n;i++){22             if (x[i][1]==1){23                 q1.push (x[i][0]);24                 q2.push (x[i][0]);25                 q3.push (x[i][0]);26             }27             else {28                 if (flag[1]&&(q1.empty()||q1.front()!=x[i][0])){29                     flag[1]=0;30                 }31                 if (flag[1])32                     q1.pop ();33                 if (flag[2]&&(q2.empty()||q2.top()!=x[i][0])){34                     flag[2]=0;35                 }36                 if (flag[2])37                     q2.pop ();38                 if (flag[3]&&(q3.empty()||q3.top()!=x[i][0])){39                     flag[3]=0;40                 }41                 if (flag[3])42                     q3.pop ();43                 44                 45             }46         }47         int temp=0;48         for (int i=1;i<=3;i++)49             temp+=flag[i];50         if (temp==0)51             cout<<"impossible"<<endl;52         else if (temp>=2)53             cout<<"not sure"<<endl;54         else if (temp==1){55             if (flag[1])56                 cout<<"queue"<<endl;57             else if (flag[2])58                 cout<<"stack"<<endl;59             else 60                 cout<<"priority queue"<<endl;61         }62         63     }64     return 0;65 }