public static DoublyLinkedNode ReverseDoublyLinkedList(DoublyLinkedNode head)
{
if (head == null) return null;
DoublyLinkedNode current = head;
DoublyLinkedNode temp;
while(current!=null)
{
temp=current.NextNode;
current.NextNode = current.PreNode;
current.PreNode = temp;
head = current;
current = temp;
}
return head;
}