Flatten a linked list with side branches into one doubly linked list

Each node of a singly linked list has a forward link and may also have one side branch: a link to the head of another list, whose nodes may have side branches of their own. Flatten the whole structure into a single doubly linked list, with each branch spliced in right after the node it hangs from, and set every node's back link. Do it in place and discuss recursion depth versus an iterative approach.