题解 | 【模板】队列操作

#include <iostream>
#include <queue>
using namespace std;

int main() {
    int n;
    cin>>n;
    queue<int> queue;
    while(n--){
        int op;
        cin>>op;
        switch(op){
            case 1:
                int temp;
                cin>>temp;
                queue.push(temp);
                break;
            case 2:
                if(queue.empty()) cout<<"ERR_CANNOT_POP\n";
                else queue.pop();
                break;
            case 3:
                if(queue.empty()) cout<<"ERR_CANNOT_QUERY\n";
                else cout<<queue.front()<<"\n";
                break;
            case 4:
                cout<<queue.size()<<"\n";
        }
    }
    return 0;
}