Flatten a multilevel doubly Linked List - Java recursion

Can someone explain to me why I'm getting:

The linked list [1,2,3,7,8,11,12,9,10,4,5,6] is not a valid doubly linked list.

class Solution { Node flattenedList = new Node(-1); Node flattenedListPointer = flattenedList; public Node flatten(Node head) { if (head == null){ return flattenedList.next; } flattenedListPointer.next = new Node(head.val,head.prev,null,null); flattenedListPointer = flattenedListPointer.next; if (head.child != null){ flatten(head.child); } if (head.next != null){ flatten(head.next); } return flattenedList.next; } }

[1,2,3,7,8,11,12,9,10,4,5,6] is the expected answer - and I did make all the child nodes to be null (as require). I also made sure all the prev node are null.

Thanks!