430-flatten-a-multilevel-doubly-linked-list.java

/*
// Definition for a Node.
class Node {
    public int val;
    public Node prev;
    public Node next;
    public Node child;
};
*/

class Solution {
    Node tail=null;
    public Node flatten(Node head) {
        
        if(head==null)
            return null;
        head.prev=tail;
        tail=head;
        
        Node nextNode=head.next;
        head.next=flatten(head.child);
        head.child=null;
        
        tail.next=flatten(nextNode);
        return head;
    }
}