题解 | 【模板】队列操作

#include<bits/stdc++.h>
#include <iostream>
#include <queue>
using namespace std;
queue<int>q;int n,temp,sig;
int main(){
    cin>>n;
    for (int i=0; i<n;++i) {
        cin>>sig;
        if (sig==1) {
            cin>>temp;
            q.push(temp);
        }else if (sig==2) {
            if (!(q.empty())) {
                q.pop();
            }else {
                cout<<"ERR_CANNOT_POP"<<endl;
            }
        }else if (sig==3) {
            if (!(q.empty())) {
                temp=q.front();cout<<temp<<endl;
            }else {
                cout<<"ERR_CANNOT_QUERY"<<endl;
            }
        }else {
        cout<<q.size()<<endl;
        }
    }
    return 0;
}