# Instructions

Implement a doubly linked list.

Like an array, a linked list is a simple linear data structure. Several
common data types can be implemented using linked lists, like queues,
stacks, and associative arrays.

A linked list is a collection of data elements called *nodes*. In a
*singly linked list* each node holds a value and a link to the next node.
In a *doubly linked list* each node also holds a link to the previous
node.

You will write an implementation of a doubly linked list. Implement a
Node to hold a value and pointers to the next and previous nodes. Then
implement a List which holds references to the first and last node and
offers an array-like interface for adding and removing items:

* `push` (*insert value at back*);
* `pop` (*remove value at back*);
* `shift` (*remove value at front*).
* `unshift` (*insert value at front*);

To keep your implementation simple, the tests will not cover error
conditions. Specifically: `pop` or `shift` will never be called on an
empty list.

If you want to know more about linked lists, check [Wikipedia](https://en.wikipedia.org/wiki/Linked_list).
