// Find the least common ancestor or lca()
    public static Node lca(Node n1, Node n2, Node r){
        if (r != null){
            if (r.data == n1.data || r.data == n2.data){
                return r;
            }else {
                Node left = lca(n1, n2, r.left);
                Node right= lca(n1, n2, r.right);
                if (left != null && right != null){
                    return r;
                }else if(left != null && right == null){
                    return left;
                }else if(left == null && right != null){
                    return right;
                }
            }
        }
        return null;
    }

Least Common Ancestor:
5 is the LCA of 1 and 9
5 is the LCA of 5 and 1
10 is the LCA of 9 and 15