2.3. Linked Lists¶
2.3.1. Linked Lists¶
In this module we present one of the two traditional implementations for lists, usually called a linked list. The linked list uses dynamic memory allocation, that is, it allocates memory for new list elements as needed. The following diagram illustrates the linked list concept. Here there are three nodes that are “linked” together. Each node has two boxes. The box on the right holds a link to the next node in the list. Notice that the rightmost node has a diagonal slash through its link box, signifying that there is no link coming out of this box.
Because a list node is a distinct object (as opposed to simply a cell
in an array), it is good practice to make a separate list node class.
(We can also re-use the list node class to implement linked
implementations for the stack and
queue data structures.
Here is an implementation for list nodes, called the Link class.
Objects in the Link class contain an element field to
store the element value, and a next field to store a pointer to
the next node on the list.
The list built from such nodes is called a singly linked list,
or a one-way list, because each list node
has a single pointer to the next node on the list.
The Link class is quite simple.
There are two forms for its constructor, one with
an initial element value and one without.
Member functions allow the link user to get or set the element
and link fields.
2.3.2. Why This Has Problems¶
There are a number of problems with the representation just
described.
First, there are lots of special cases to code for.
For example, when the list is empty we have
no element for head, tail, and curr to point to.
Implementing special cases for insert and remove
increases code complexity, making it harder to understand,
and thus increases the chance of introducing bugs.
2.3.3. A Better Solution¶
Fortunately, there is a fairly easy way to deal with all of the special cases, as well as the problem with deleting the last node. Many special cases can be eliminated by implementing linked lists with an additional header node as the first node of the list. This header node is a link node like any other, but its value is ignored and it is not considered to be an actual element of the list. The header node saves coding effort because we no longer need to consider special cases for empty lists or when the current position is at one end of the list. The cost of this simplification is the space for the header node. However, there are space savings due to smaller code size, because statements to handle the special cases are omitted. We get rid of the remaining special cases related to being at the end of the list by adding a “trailer” node that also never stores a value.
The following diagram shows initial conditions for a linked list with header and trailer nodes.
Here is what a list with some elements looks like with the header and trailer nodes added.
Adding the trailer node also solves our problem with deleting the last node on the list, as we will see when we take a closer look at the remove method’s implementation.
2.3.4. Linked List insertion¶
- Here are some special cases for linked list insertion: Inserting at
the end, and inserting to an empty list.
