Question: Suppose you create a doubly linked list DLL that has three instance variables: head, tail, length. The data is stored in Node objects with next and prev pointers. For each implementation below, give all the class invariants that must be satisfied for a DLL to be well-formed.
a. Implementation 1: The head and tail pointers point to null if the list is empty.
b. Implementation 2: The head and tail pointers always point to special dummy Nodes that are just placeholders.