题解 | 【模板】队列操作

#include <bits/stdc++.h>
#include <iostream>
#include <queue>
using namespace std;

int main() {
   
queue<int> q;
int n;
cin>>n;
while(n--)
{
     char c;
     cin>>c;
     if(c=='1'){
        int x;
        cin>>x;
        q.push(x);
     }
    else if (c=='2') {
        if(!q.empty()){
            q.pop();
        }else cout<<"ERR_CANNOT_POP"<<endl;
    
    }else if (c=='3') {
            if(q.empty()){
                cout<< "ERR_CANNOT_QUERY"<<endl;
            }else cout<<q.front()<<endl;
    }else {
        cout<<q.size()<<endl;
    }

       
}

return 0;

}
// 64 位输出请用 printf("%lld")