DList-The actual linked lists built into C++ work almost exactly like the doubly linked lists described

in class, except that they use a very cool heuristic. Instead of maintaining head and tail references such that head.previous = null and tail.next = null, they maintain a reference to one special node called “the sentinel node” or “nil.” This node replaces the null references so that a NullPointerException can never be thrown. The sentinel node has the following properties: sentinel.previous = what we used to call the tail sentinel.next = what we used to call the head what we used to call the tail . next = sentinel what we used to call the head . previous = sentinel In effect this node replaces null, and maintaining a reference to it replaces having to maintain separate head and tail references.