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!