题解 | 【模板】队列操作

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

int main() {
    int n, code, x;
    cin >> n;
    queue<int> q;

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

    return 0;
}