Given the head of a doubly linked list, reverse it in place — the new head should be the old tail — and return the new head. Unlike a singly linked list, where reversal means redirecting a forward-only pointer at every node, a doubly linked list already carries both directions on every node: reversing it is simply swapping each node's own next and prev pointers. Whatever a node used to consider "forward," it now considers "backward," and vice versa — no traversal order needs to be rebuilt, just the meaning of the two pointers each node already had.