Flatten a Linked List with Down Pointers

Problem You are given a linked list where each node contains: val: integer value next: pointer to the next node on the same level (nullable) down: pointer to the head of a child sublist (nullable) Flatten the multilevel list in-place into a single-level list with the following rules: If a node x has a non-null down, insert the down list between x and x.next. After flattening, all down pointers must be null. The resulting order corresponds to a DFS traversal (expand down first, then continue with next). Implement: Node* flatten(Node* head) Note: You may provide a clear recursive or iterative approach. Example Input # Structure-based linked list input is omitted. Output # Flattened order follows DFS; down pointers all null.