题解 | 【模板】队列操作

#include <bits/stdc++.h>
using namespace std;
using ll = long long int;
using ld = long double;
using PII=pair<ll,ll>;
using PIII=pair<int,pair<int,int>>;
const ld ESP = 1e-10;
const ld PI = acosl(-1);
const int N=1e5+10;
const int M=2e5+10;
// const int mod = 1000000007;
const int mod = 998244353;
//随机化
random_device rd;
mt19937 gen(rd());
uniform_int_distribution<> dis(1, 1000000000);


void solve(){
    int n;
    queue<int>q;
    cin>>n;
    for(int i=1;i<=n;i++){
        int op;
        cin>>op;
        if(op==1){
            int x;
            cin>>x;
            q.push(x);
        }else if(op==2){
            if(q.empty()){
                cout<<"ERR_CANNOT_POP"<<'\n';
            }else{
                q.pop();
            }
        }else if(op==3){
            if(q.empty()){
                cout<<"ERR_CANNOT_QUERY"<<'\n';
            }else{
                cout<<q.front()<<'\n';
            }
        }else {
            cout<<q.size()<<'\n';
        }
    }
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int _=1;
    // cin>>_;
    while(_--){
        solve();
    }
    return 0;
}