What is a sentinel node in the context of linked lists?

A sentinel node in linked lists is a dummy node used as a marker or placeholder to simplify certain operations.

In the context of linked lists, a sentinel node is a special type of node that does not hold any meaningful data itself but serves a specific purpose. It is often used as a marker or placeholder to simplify certain operations and to avoid special cases in linked list manipulations.

For instance, in a singly linked list, traversing the list to find a particular node or to delete a node requires checking whether the next node is null. However, if a sentinel node is used, this check can be avoided as the sentinel node can be placed at the end of the list, ensuring that no node's next pointer is null. This can significantly simplify the code and reduce the chances of null pointer errors.

Similarly, in a doubly linked list, a sentinel node can be used to mark both the start and the end of the list. This can make it easier to add or remove nodes at both ends of the list, as there is no need to check whether the previous or next node exists.

Moreover, sentinel nodes can also be used in circular linked lists, where the last node in the list points back to the sentinel node at the start of the list, rather than being null. This can make it easier to detect the end of the list during traversal.

In all these cases, the key advantage of using a sentinel node is that it can simplify the code and make it more robust by eliminating the need to handle special cases. However, it's important to remember that the sentinel node itself does not hold any meaningful data and is only used as a marker or placeholder.