A doubly linked list extends the singly linked node with a second reference to its predecessor, so every adjacency is recorded twice and traversal becomes possible in both directions; the cost of this is that every structural mutation must maintain both links or the reverse chain silently diverges from the forward one. Head insertion accordingly rewires three references rather than one — the existing head's previous pointer, the new node's next pointer, and the head reference itself — with the empty list as the degenerate case where no existing head exists to back-link. The lesson sits in data structures taught through the C/C++ storage model, where the decisive constraint is that nodes must outlive the call that creates them and therefore must be allocated on the heap, reachable only through pointers, rather than in an automatically reclaimed stack frame.
A doubly linked list extends the singly linked node with a second reference to its predecessor, so every adjacency is recorded twice and traversal becomes possible in both directions; the cost of thi…