[LC] 430. Flatten a Multilevel Doubly Linked List

You are given a doubly linked list which in addition to the next and previous pointers, it could have a child pointer, which may or may not point to a separate doubly linked list. These child lists may have one or more children of their own, and so on, to produce a multilevel data structure, as shown in the example below.

Flatten the list so that all the nodes appear in a single-level, doubly linked list. You are given the head of the first level of the list.

/*
// Definition for a Node.
class Node {
    public int val;
    public Node prev;
    public Node next;
    public Node child;
};
*/
class Solution {
    public Node flatten(Node head) {
        if (head == null) {
            return head;
        }
        Node cur = head;
        while (cur != null) {
            if (cur.child != null) {
                Node right = cur.next;
                cur.next = flatten(cur.child);
                cur.next.prev = cur;
                cur.child = null;
                
                while (cur.next != null) {
                    cur = cur.next;
                }
                if (right != null) {
                    cur.next = right;
                    cur.next.prev = cur;
                }
            }
            cur = cur.next;
        }
        return head;
    }
}