In computer programming, a sentinel node is a specifically designated node used with linked lists and trees as a traversal path terminator. This type of node does not hold or reference any data managed by the data structure.

Benefits

Sentinels are used as an alternative over using as the path terminator in order to get one or more of the following benefits:

Drawbacks

Examples

Search in a linked list

Below are two versions of a subroutine (implemented in the C programming language) for looking up a given search key in a singly linked list. The first one uses the sentinel value, and the second one a (pointer to the) sentinel node , as the end-of-list indicator. The declarations of the singly linked list data structure and the outcomes of both subroutines are the same.

First version using NULL as an end-of-list indicator

The -loop contains two tests (yellow lines) per iteration:

Second version using a sentinel node

The globally available pointer to the deliberately prepared data structure is used as end-of-list indicator. Note that the pointer sentinel has always to be kept at the end of the list. This has to be maintained by the insert and delete functions. It is, however, about the same effort as when using a NULL pointer. The -loop contains only one test (yellow line) per iteration:

Python implementation of a circular doubly-linked list

Linked list implementations, especially one of a circular, doubly-linked list, can be simplified remarkably using a sentinel node to demarcate the beginning and end of the list. Following is a Python implementation of a circular doubly-linked list: Notice how the method takes the node that will be displaced by the new node in the parameter. For appending to the left, this is the head of a non-empty list, while for appending to right, it is the tail. But because of how the linkage is set up to refer back to the sentinel, the code just works for empty lists as well, where will be the sentinel node.

Search in a binary tree

General declarations, similar to article Binary search tree: The globally available pointer to the single deliberately prepared data structure is used to indicate the absence of a child. Note that the pointer sentinel has always to represent every leaf of the tree. This has to be maintained by the insert and delete functions. It is, however, about the same effort as when using a NULL pointer.