public void inorder(Node head){
        if(head == null){
            return;
        }
        inorder(head.before);
        System.out.print(head.data + " ");
        inorder(head.next);
    }
http://www.geeksforgeeks.org/given-linked-list-representation-of-complete-tree-convert-it-to-linked-representation/
Test cases
Zero, One or more nodes in link list
src/com/interview/linklist/LinkListToCompleteBinaryTree.java