Flatten Multilevel Double 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.



Example:

Input:
 1---2---3---4---5---6--NULL
         |
         7---8---9---10--NULL
             |
             11--12--NULL

Output:
1-2-3-7-8-11-12-9-10-4-5-6-NULL
*/
class Node {
    public int val;
    public Node prev;
    public Node next;
    public Node child;

    public Node() {}

    public Node(int _val,Node _prev,Node _next,Node _child) {
        val = _val;
        prev = _prev;
        next = _next;
        child = _child;
    }
}
public class FlattenMultiLevelDoublyLinkedList{

Node prev;

public Node flatten(Node head){
prev = null;
traverse(head);
return head;
}

public void traverse(Node head){
if(head==null){
return;
}
if(prev!=null){
prev.next = head;
head.prev = prev;
}
prev = head;

Node child = head.child;
Node next = head.next;
head.child = null;
head.next = null;
traverse(child);
traverse(next);
}

}