Header Linked List in Data Structure

In this article we will look into an interesting topic related to Linked List: Header Linked List. We will look at its description and types and will also look at the implementation and advantages of using it.

Header Linked List is a modified version of Singly Linked List. In Header linked list, we have a special node, the Header Node present at the beginning of the linked list. The Header Node is an extra node at the front of the list storing meaningful information about the list. Such a node is not similar in structure to the other nodes in the list. It does not represent any items of the list like other nodes rather than the information present in the Header node is global for all nodes such as Count of Nodes in a List, Maximum among all Items, Minimum value among all Items etc. This gives useful information about the Linked list.

Now, Header Linked List is of two types:

  • Grounded Header Linked List
  • Circular Header Linked List.

Grounded Header Linked List

In this type of Header Linked List, the last node of the list points to NULL or holds the reference to NULL Pointer. The head pointer points to the Header node of the list. If the there is no node to the next of head pointer or head.next equals NULL then we know that the Linked List is empty. The operations performed on the Header Linked List are same as Singly Linked List such as Insertion, Deletion, and Traversal of nodes. Let us understand this with an example:

Header Linked List

The above image shows a Grounded Header Linked List. We can see the Header Node at the beginning followed by the actual nodes of the list where the last node points to NULL. The header node maintains global information about the list. Firstly, we have to create Header Node first whose data field will be NULL or 0 initially. After that we can perform any operations on that linked list and store the global information about the linked list on the header node. Here, we store the count or Total no. of nodes present in the Linked List, the Maximum and Minimum value among all nodes in the list. So, in the above Linked List Total Nodes are 4 , Maximum Value is 99 and Minimum value is 5 which are present as fields in Header Node.

Circular Header Linked List

A Linked List whose last node points back to the First node or the Head Node of the list is called a Circular Linked List. Similarly, if the last node of the Header Linked List points back to Header Node then it is a Circular Header Linked List. The last node of the list does not hold NULL reference. In this case, We have to use external pointers to maintain the last node. The end of the list is not known while traversing. It can perform same operations as its counterpart such as Insert, Delete and Search. Let us see an example.

Circular Header Linked List

We can see the last node of the list points to the header node and not the node 99. There might be problem in traversing through the nodes. It is because the structure of the last node and header node are different which might cause an Type Mismatch Exception. So while traversing we fix the Header node and keep on traversing until node.next points to header node. The rest features of the Header remains same as its Grounded counterpart.

Implementation in Java

  • For implementation, we will use a separate class for Header Node and normal Nodes which contain data respectively. We implement the Grounded Header Linked List since it is similar to the Circular Header Linked List in type of operations. We need to maintain some extra pointers in Circular variant during insertion and deletion.
  • So, We will create a Header Node then add nodes subsequently to the list. For simplicity, we add nodes to the end of the list. We maintain a pointer to last node for adding nodes. On adding nodes we check if the current node’s data is greater than maximum of header node and update the max of head accordingly. We do the same for updating min value. At the end we also increment count of nodes in the Header Node.
  • We will also perform delete operation. To keep the logic simple we delete nodes from end of the list and update the max and min accordingly before deleting the node from the list. After deletion, we also update the count of nodes. The we print the contents of the list. So let us look at the code.


Advantages of Header Linked List

  1. In the header node of the linked list we store the total nodes in the List, so we can easily get the count of nodes in O(1) time without the need to traverse the list. As the size of the list is stored in Header itself.
  2. Another upper hand we have is that while inserting a new node in the linked list we don’t need to check whether start or head node is null or not because the header node will always exist and we can insert a new node just after that.
  3. Insertion and deletion of nodes become easier as there is no overhead of checking whether the head is NULL.
  4. If we are given a list of data items and we need to search for the maximum and minimum value among them we can get them easily using the header node. If we want we can maintain any number of data items in the header node.
  5. Another application of header linked lists lies in maintaining the polynomials in memory, where the header node can represent Zero degree Polynomial.

So that’s it for the article, you can try out the code shown above with different examples and execute them in your Java IDE or Compiler to get a clear idea. Feel free to leave your suggestions/doubts in the comment section below.

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