#include <iostream>
#include <cstdlib>// to define NULL

using namespace std;

class D{ //a dumb double linked list.
public: D(char c, D* p){chr=c; prev=p; next=NULL; }

	char chr; // data for list
	D* prev;  // previous node in list
	D* next;  // next node in list
};// class D
main()
{
	D* t; D* h; // t for tail and h for head of list.
    // construct a list with one element
	t = h = new D('a', NULL);
    // add next element to h and update t to point at it.
	t = h->next = new D('b', t);
    // add thrid element to h and update t to point at it.
	t = h->next->next = new D('c', t);
    // add fourth element to h and update t to point at it.
	t = h->next->next->next = new D('d', t);

    //test out what we've got -- from head to tail
	cout << h->chr
	     <<	h->next->chr
	     <<	h->next->next->chr
	     <<	h->next->next->next->chr << endl;

    //test out what we've got -- from back to front
	cout << t->chr
	     <<	t->prev->chr
	     <<	t->prev->prev->chr
	     <<	t->prev->prev->prev->chr << endl;
   return 0;
}