# 10998 - Stack

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

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

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

/* main.c */
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;
}

/* function.c */
/* List_stack */

/* constructor */
List_stack::List_stack(){
    tail = NULL; head = NULL;
}
/* destructor */
// since there are movable memories
// we need to delete it manually
List_stack::~List_stack(){
    while(tail != head){
        tail = tail->prevPtr;
        delete tail->nextPtr;
    } delete tail;
}
// Special Case: head = NULL
void List_stack::push(const int &k){
    if(head == NULL){
        head = new ListNode(k);
        tail = head;
    }
    else{
        tail->nextPtr = new ListNode(k);
        tail->nextPtr->prevPtr = tail;
        tail = tail->nextPtr;
    }
}
// Special Case: Only one node (tail = head)
void List_stack::pop(){
    if(tail != head){
        tail = tail->prevPtr;
        delete tail->nextPtr;
    }
    else if(head != NULL){
        delete head;
        tail = head = NULL;
    }
}
// Print from tail to head
void List_stack::print(){
    ListNode *Lp = tail;
    if(Lp != NULL){
        while(Lp != head){
            cout << Lp->data << ' ';
            Lp = Lp->prevPtr;
        } cout << Lp->data;
    }
}