Flatten multilevel list

A multilevel doubly linked list is given as a nested structure: a list of nodes where each node is [val, child] and child is either null (encoded as None / empty) — represented here as another nested list of the same shape, or [] for no child. Flatten it depth-first so a node's child sublist is spliced in right after that node and before the rest of the level. Return the flattened list of values in order.