queue2.cpp

#include <iostream>
#include <string>
using namespace std;
class Node{
    public:
        int data;
        Node *next;
    Node(int data){
        this->data = data;
    }
};

class Queue{
    public:
        Node *tail, *front;
        int _size = 0;
    Queue(){
        tail = NULL;
        front = NULL;
    }
    void push(int data){
        if(front == NULL){
            Node *node = new Node(data);
            tail = node;
            front = node;
        }
        else{
            Node *node = new Node(data);
            tail->next = node;
            tail = node;
        }
        _size++;
        cout << "ok" << endl;
    }
    void pop(){
        if(front != NULL){
            cout << front->data << endl;
            front = front->next;
        }
        _size--;
        if (front == NULL)
            tail = NULL;
    }
    void _front(){
        cout << front->data << endl;
    }
    void size(){
        cout << _size << endl;
    }
    void clear(){
        // if(front != NULL){
            front = NULL;
            tail = NULL;
            _size = 0;
            cout << "ok" << endl;
        //}
    }

};
int main(){
     string command;
    int x;
    Queue MyQueue;
    while(cin >> command){
        if(command =="push"){
            cin >>x;
            MyQueue.push(x);
        }
         else if (command == "pop"){
            if(MyQueue._size == 0){
                cout << "error" << endl;
            }
            else if (MyQueue._size != 0){ 
                MyQueue.pop();
            }
        }
        else if (command == "front"){
            if(MyQueue._size == 0){
                cout << "error" << endl;
            }
            else if (MyQueue._size != 0){ 
                MyQueue._front();
            }
        }
        else if(command == "size"){
            MyQueue.size();
        }
        else if(command == "clear"){
            MyQueue.clear();
        }
        else if(command == "exit"){
            cout << "bye" << endl;
            break;
        }
    }
}