Question

we say that a header linked list is that which consists of a special node called header node that marks the beginning of the list
but i dont understand what is really the importance of this header node.
please help me?

Answer 1

Having sentinel nodes prevents you from having to handle certain edge cases.

The biggest is the null check: You always know that there will be a node at the top of the list that you can insert nodes after, so you don’t have to deal with checking if head is null. (also it helps having a tail node for similar reasons)

Consider the two cases:

With a head and a tail node:

addNewDataAtHead( data ):
    newNode = new Node(data);
    newNode.next = head.next;
    newNode.prev = head;
    head.next.prev = newNode;
    head.next = newNode;

Without:

addNewDataAtHead( data ):
    newNode = new Node(data);
    if (head == null):
        head = newNode;
    newNode.next = head;
    head.prev = newNode;
    head =  newNode;

the intent of the first one is a lot clearer because it is like inserting anywhere else. The second case requires you to check for a special circumstance.