Given several doubly linked lists and a description of which node in which list each of the other lists hangs off of as a "child" list, flatten the whole structure into a single doubly linked list — visiting a node, then diving into its entire child subtree, then continuing to its own next node — and return the head. A child list can itself have a node with its own child list, nested arbitrarily deep — so this isn't a one-level splice. The recursive approach handles that naturally: flattening a node's child fully (however deep it goes) before splicing it in is exactly what a depth-first recursive call does, with no extra bookkeeping needed to track "how many levels deep" the walk currently is.