好水啊

STL的板子题，大家大概知道queue怎么用就行

不懂queue的看过来：STL之queue容器详解_stl queue 最大长度-CSDN博客

AC CODE

#include <iostream>
#include <queue>

using namespace std;

queue <int> q;
int n;

int main () {
cin >> n;
for (int i = 1; i <= n; i++) {
    int op;
    cin >> op;
    if (op == 1) {
        int x;
        cin >> x;
        q.push(x);
    }
    if (op == 2) {
        if (q.size() == 0) {
            cout << "ERR_CANNOT_POP\n";
        }
        else {
            q.pop();
        }
    }
    if (op == 3) {
        if (q.size() == 0) {
            cout << "ERR_CANNOT_QUERY\n";
        }
        else {
            cout << q.front() << "\n";
        }
    }
    if (op == 4) {
        cout << q.size() << "\n";
    }
    }
    return 0;
}