private Node join(Node r1, Node r2){

        if(r1 == null){
            return r2;
        }
        if(r2 == null){
            return r1;
        }
        Node t1 = r2.left;
        
        r1.left.right = r2;
        r2.left = r1.left;
        r1.left = t1;
        t1.right = r1;
        return r1;
    }
http://cslibrary.stanford.edu/109/TreeListRecursion.html
Test cases
Null tree
src/com/interview/tree/BinaryTreeToCircularLinkList.java