// Runtime: 0 ms, faster than 100.00% of Java online submissions for Flatten a Multilevel Doubly Linked List.
class Solution {
    public Node flatten(Node head) {
        flatten(head, null);
        return head;
    }

    private Node flatten(Node head, Node rest) {
        if (head == null) return rest;

        Node c = head;
        while (c.next != null && c.child == null) c = c.next;

        c.next = flatten(c.child, flatten(c.next, rest));
        if (c.next != null) c.next.prev = c;
        c.child = null;

        return head;
    }
}