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.
Sentinels are used as an alternative over using NULL as the path terminator in order to get one or more of the following benefits:
NULL it suffices to protect the data structure as a whole for “read-only” (if an update operation will not follow).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 NULL, and the second one a (pointer to the) sentinel node Sentinel, as the end-of-list indicator. The declarations of the singly linked list data structure and the outcomes of both subroutines are the same.
struct sll_node { // one node of the singly linked list
struct sll_node *next; // end-of-list indicator or -> next node
int key;
} sll, *first;
// global initialization
first = NULL; // before the first insertion (not shown)
struct sll_node *Search(struct sll_node *first, int search_key) {
struct sll_node *node;
for (node = first;
node != NULL;
node = node->next)
{
if (node->key == search_key)
return node; // found
}
// search_key is not contained in the list:
return NULL;
}
The for-loop contains two tests (yellow lines) per iteration:
node != NULL;if (node->key == search_key).The globally available pointer sentinel to the deliberately prepared data structure Sentinel is used as end-of-list indicator.
// global variable
sll_node Sentinel, *sentinel = &Sentinel;
// global initialization
sentinel->next = sentinel;
first = sentinel; // before the first insertion (not shown)
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.
struct sll_node *SearchWithSentinelnode(struct sll_node *first, int search_key) {
struct sll_node *node;
// Prepare the “node” Sentinel for the search:
sentinel->key = search_key;
for (node = first;
node->key != search_key;
node = node->next)
{}
// Post-processing:
if (node != sentinel)
return node; // found
// search_key is not contained in the list:
return NULL;
}
The for-loop contains only one test (yellow line) per iteration:
node->key != search_key;.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:
class Node:
def __init__(self, data, next=None, prev=None):
self.data = data
self.next = next
self.prev = prev
def __repr__(self) -> str:
return f'Node(data={self.data})'
class LinkedList:
def __init__(self):
self._sentinel = Node(data=None)
self._sentinel.next = self._sentinel
self._sentinel.prev = self._sentinel
def pop_left(self) -> Node:
return self.remove_by_ref(self._sentinel.next)
def pop(self) -> Node:
return self.remove_by_ref(self._sentinel.prev)
def append_nodeleft(self, node):
self.add_node(self._sentinel, node)
def append_node(self, node):
self.add_node(self._sentinel.prev, node)
def append_left(self, data):
node = Node(data=data)
self.append_nodeleft(node)
def append(self, data):
node = Node(data=data)
self.append_node(node)
def remove_by_ref(self, node) -> Node:
if node is self._sentinel:
raise Exception('Can never remove sentinel.')
node.prev.next = node.next
node.next.prev = node.prev
node.prev = None
node.next = None
return node
def add_node(self, curnode, newnode):
newnode.next = curnode.next
newnode.prev = curnode
curnode.next.prev = newnode
curnode.next = newnode
def search(self, value):
self._sentinel.data = value
node = self._sentinel.next
while node.data != value:
node = node.next
self._sentinel.data = None
if node is self._sentinel:
return None
return node
def __iter__(self):
node = self._sentinel.next
while node is not self._sentinel:
yield node.data
node = node.next
def reviter(self):
node = self._sentinel.prev
while node is not self._sentinel:
yield node.data
node = node.prev
Notice how the add_node() method takes the node that will be displaced by the new node in the parameter curnode. 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 curnode will be the sentinel node.
General declarations, similar to article Binary search tree:
struct bst_node { // one node of the binary search tree
struct bst_node *child[2]; // each: ->node or end-of-path indicator
int key;
} ;
struct bst { // binary search tree
struct bst_node *root; // ->node or end-of-path indicator
} *BST;
The globally available pointer sentinel to the single deliberately prepared data structure Sentinel = *sentinel is used to indicate the absence of a child.
// global variable
bst_node Sentinel, *sentinel = &Sentinel;
// global initialization
Sentinel.child[0] = Sentinel.child[1] = sentinel;
BST->root = sentinel; // before the first insertion (not shown)
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.
struct bst_node *SearchWithSentinelnode(struct bst *bst, int search_key) {
struct bst_node *node;
// Prepare the “node” Sentinel for the search:
sentinel->key = search_key;
for (node = bst->root;;) {
if (search_key == node->key)
break;
if search_key < node->key:
node = node->child[0]; // go left
else
node = node->child[1]; // go right
}
// Post-processing:
if (node != sentinel)
return node; // found
// search_key is not contained in the tree:
return NULL;
}