In the previous post on singly and doubly linked lists, recall that the collections last nodes next pointers were always NULL. This is because they had nothing to point to. But what if this is not what we wanted? What if when we get to the last node, we wanted to point the next pointer back to the head node, like a photo gallery application (when we get to the last picture, the next picture should be the first). This is where circular linked lists come from (they allow us to rotate through our collection). This means that circular linked lists have no end. One must be careful if they are going to iterate through the collection, or you could end up in an infinite loop.

So in a circular linked list, there are no nodes with a NULL pointer. For eg, in a case where there is only one node (the head node), the next pointer points back to itself. Remember in previous posts that i said when a collection is empty and you add a new node, their next and previous pointers are NULL? Well in this type of linked list, they point back to itself, well until we change its pointers.

What about the previous node of the head? Remember it was NULL in previous posts? There is another type of linked list called doubly circular linked list (i know, the names are getting ridiculous). In a doubly circular linked list, the previous pointer of the head points to the tail node. Let’s look at a diagram.

In figure A we see a circular linked list, where the tail nodes next pointer, points back to the head. In figure B we see a doubly circular linked list, where head’s previous point back to the tail.