30分求助

#include <iostream>
#include <cstring>
#include <iomanip>
#include <cmath>
#include <map>
#include <stack>
#include <algorithm>
#include <string>
#include <queue>
using namespace std;
queue <int> que;

int main(){
	int n,x,qu;
	cin>>n;
	for(int i=0;i<n;i++){
		cin>>x;
		if(x==1){
			cin>>qu;
			que.push(qu);
		}
		else if(x==2){
			if(que.size()!=0){
				que.pop();
			}
			else cout<<"ERR_CANNOT_POP"<<endl;
		}
		else if(x==3){
			if(que.front()!=0){
				cout<<que.front()<<endl;	
			}
			else {
				cout<<"ERR_CANNOT_QUERY"<<endl;
			}
		}
		else if(x==4){
			cout<<que.size()<<endl;
		}
	}
}