Flatten a Multilevel Doubly Linked List (Skip Empty Nodes)

LC 430 (Flatten a Multilevel Doubly Linked List) with one delta: nested nodes carry empty values and must be excluded from the flattened output. Asked alongside a binary-tree max-path-sum question in the same IC round. Requirements Flatten a multilevel doubly linked list — nodes carry prev / next pointers plus a child pointer to a nested sub-list — into a single-level doubly linked list. Per the candidate, the base problem is the canonical LC 430 unchanged. Variant twist: the nested nodes carry an empty value, and empty-valued nodes must be excluded from the flattened result while the structure reachable through them is preserved. Notes The empty-node exclusion is the only delta from the canonical LC 430 problem. The exclusion breaks the memorized splice template: skipping a node mid-splice is where dangling prev / next pointers creep in, so decide up front whether you filter during the flatten or in a second pass. Clarify at the start exactly which nodes can be empty — in this variant, emptiness co-occurs with nesting, and that assumption changes how you stitch the sub-list back in. The same IC-led round pairs this with the binary-tree maximum-path-sum problem (DFS plus DP), so budget time for two problems. Preparation Re-implement the canonical multilevel flatten from scratch (iterative with a stack, then recursive), verifying prev / next symmetry with a list printer. Add a value filter to your template and re-test around the edge cases: excluded head, excluded tail, and consecutive excluded nodes.