呜呜呜呜呜呜呜呜大泠蛋啊

#include <bits/stdc++.h>

using namespace std;

int main(){
    
    queue<unsigned long long> s;
    int n;
    cin >> n;
    int a;
    for (int j = 1; j <= n; j++) {
        cin >> a;
        if ( a == 1 ) {
            int num;
            cin >> num;
            s.push(num);
        } else if ( a == 2 ) {
            if ( s.empty() ) {
                cout << "ERR_CANNOT_POP" << endl;
            } else {
                s.pop();
            }
                
        } else if ( a == 3 ) {
            if ( s.empty() ) {
                cout << "ERR_CANNOT_QUERY" << endl;
            } else {
                cout << s.back() << endl;
            }
        } else if ( a == 4 ) {
            cout << s.size() << endl;
        }
            
            
        
        
        
    }


            
        
 return 0;
}

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