题解 | 【模板】队列操作

#include <iostream>
using namespace std;
#include <queue>
int main() {
    queue<int> duilie;

    int n;
    cin >> n;

    while (n--) {
        int a;
        cin >> a;

        switch (a) {
            case 1:
                int b;
                cin >> b;
                duilie.push(b);
                break;
            case 2:
                if (duilie.size() != 0){
                    duilie.pop();
                    break;
                }
                else{
                    cout << "ERR_CANNOT_POP" << endl;
                    break;
                }
            case 3:
                if (duilie.size() != 0){
                    cout << duilie.front() << endl;
                    break;
                }
                else{
                    cout << "ERR_CANNOT_QUERY" << endl;
                    break;
                }
            case 4:
                int shuliang = duilie.size();
                cout << shuliang << endl;
                break;
        }
    }
}
// 64 位输出请用 printf("%lld")