题解 | 【模板】队列操作

#include <iostream>
#include<queue>
using namespace std;

int main() {
    queue<int> myq;
    int n;
    cin>>n;
    while(n--){
        int type;
        cin>>type;
        if(type==1){
            int x;
            cin>>x;
            myq.push(x);
        }else if(type==2){
            if(!myq.empty())
                myq.pop();
            else
                cout<<"ERR_CANNOT_POP"<<endl;
        }else if(type==3){
            if(!myq.empty())
                cout<<myq.front()<<endl;
            else
             cout<<"ERR_CANNOT_QUERY"<<endl;
        }else if(type==4){
            cout<<myq.size()<<endl;
        }
    }
    return 0;
}
// 64 位输出请用 printf("%lld")