Lab Week 4: Linked List

Lab Week 4:  Linked List 
Implement an ordered list using a doublylinked list

Before doing this lab make sure to read over section 3.22, 3.23 carefully. You will be extending the implementation as described in the text. 
http://www.openbookproject.net/books/pythonds/BasicDS/TheOrderedListAbstractDataType.html
http://www.openbookproject.net/books/pythonds/BasicDS/ImplementinganOrderedList.html

The structure of an ordered list is a collection of items where each item holds a relative position that is based upon some underlying characteristic of the item. The ordering is typically either ascending or descending and we assume that list items have a meaningful comparison operation that is already defined. Many of the ordered list operations are the same as those of the unordered list.

Implement the following operations for an ordered list of integers ordered in ascending order using a doubly linked list.  The “head” of the list be where the “smallest items are and let “tail” be where the largest items are.  You may use whatever mechanism you like to keep track of the head and tail of the list.  E.g. references or sentinel nodes.

•    OrderedList() creates a new ordered list that is empty. It needs no parameters and returns an empty list.
•    add(item) adds a new item to the list making sure that the order is preserved. It needs the item and returns nothing. Assume the item is not already in the list.
•    remove(item) removes the item from the list. It needs the item and modifies the list.  Assume the item is present in the list.
•    search_forward(item) searches for the item in the list. It needs the item and returns the boolean value True if the item is in the list and False if the item is not in the list.
•    search_backward(item) searches for the item in the list starting from the tail of the list.. It needs the item and returns the boolean value True if the item is in the list and False if the item is not in the list.
•    is_empty() tests to see whether the list is empty. It needs no parameters and returns a boolean value.  True if the list is empty and False if any items are in the list.
•    size() returns the number of items in the list. It needs no parameters and returns an integer.
•    index(item) returns the position of item in the list. It needs the item and returns the index. Assume the item is in the list.
•    pop() removes and returns the last item in the list. It needs nothing and returns an item. Assume the list has at least one item.
•    pop(pos) removes and returns the item at position pos. It needs the position and returns the item. Assume the item is in the list.  pop(pos) should compare pos to the size of the list and search from the head if pos <= size/2 and from the rear if pos > size/2.
Submit two files to PolyLearn
1.    ordered_list.py
2.    ordered_list_tests.py