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.
shift will never be called on an empty list.
Read more about linked lists on Wikipedia.