private Node getTail(Node head) {
        if (head == null) {
            return null;
        }
        while (head.next != null) {
            head = head.next;
        }
        return head;
    }
http://www.geeksforgeeks.org/flatten-a-linked-list-with-next-and-child-pointers/
Test case
0 node in the list
1 node in the list
All nodes with child
No nodes with child
src/com/interview/linklist/FlattenLinkList.java