Untitled

#include<iostream>
#include<string.h>
//#include "function.h"
using namespace std;

class ListNode
{
    friend class List_stack; //make List_stack a friend
    public:
        ListNode( const int &info ) //constructor
        : data( info ), nextPtr( NULL ), prevPtr( NULL )
        {
        } //end ListNode constructor

    private:
        int data; //data
        ListNode *nextPtr; // next node in list
        ListNode *prevPtr;
}; //end class ListNode


class List_stack {
    public:
        List_stack();
        ~List_stack();
        void push(const int &);
        void pop();
        void print();
    private:
        ListNode *head;
        ListNode *tail;
};

/**********************/
List_stack::List_stack(){
    this->head=NULL;
    this->tail=NULL;
}
List_stack::~List_stack(){
    delete(head);
    delete(tail);
}

void List_stack::pop(){
    if(this->tail==NULL) return;

    ListNode *cur=this->tail;
    this->tail=this->tail->prevPtr;
    delete(cur);
    if(this->tail==NULL) this->head=NULL;
}
void List_stack::push(const int &nu){
    if(this->head==NULL){
        this->head=new ListNode(nu);
        this->tail=this->head;
    }
    else{
        ListNode *cur=new ListNode(nu);
        this->tail->nextPtr=cur;
        cur->prevPtr=this->tail;
        this->tail=cur;
    }
}
void List_stack::print(){
    ListNode *cur=this->tail;
    if(cur==NULL) return;
    while(cur->prevPtr!=NULL){
        cout<< cur->data <<" ";
        cur=cur->prevPtr;
    }
    cout<< cur->data;
}

/**********************/

int main(){
    List_stack L_stack;
    char command[10];
    int n;
    while(cin>>command){

        if(strcmp(command,"pop")==0){
            L_stack.pop();
        }else if(strcmp(command,"push")==0){
            cin >> n;
            L_stack.push(n);
        }else if(strcmp(command, "print") == 0){
            L_stack.print();
            cout << endl;
        }
    }
    return 0;
}