题解 | 【模板】队列操作

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

int main() {
    int n;
    cin >> n;
    queue<int> q;
    while(n--){
        int op;
        cin >> op;
        if(op==1){
            int x;
            cin >> x;
            q.push(x);
        }else if (op==2) {
            if(q.empty()){
                cout << "ERR_CANNOT_POP" << endl;
            }else{
                q.pop();
            }
        }else if (op==3) {
            if(q.empty()){
                cout << "ERR_CANNOT_QUERY" << endl;
            }else{
                cout << q.front() << endl;
            }
        }else if (op==4) {
            cout << q.size() << endl;
        }
    }

}
// 64 位输出请用 printf("%lld")