BST to Circular DLL

 public Node concatenate(Node leftList,Node rightList) 
    { 
        // If either of the list is empty, then 
        // return the other list 
        if (leftList == null) 
            return rightList; 
        if (rightList == null) 
            return leftList; 
  
        // Store the last Node of left List 
        Node leftLast = leftList.left; 
  
        // Store the last Node of right List 
        Node rightLast = rightList.left; 
  
        // Connect the last node of Left List 
        // with the first Node of the right List 
        leftLast.right = rightList; 
        rightList.left = leftLast; 
  
        // left of first node refers to 
        // the last node in the list 
        leftList.left = rightLast; 
  
        // Right of last node refers to the first 
        // node of the List 
        rightLast.right = leftList; 
  
        // Return the Head of the List 
        return leftList; 
    } 
  
    // Method converts a tree to a circular 
    // Link List and then returns the head 
    // of the Link List 
    public Node bTreeToCList(Node root) 
    { 
        if (root == null) 
            return null; 
  
        // Recursively convert left and right subtrees 
        Node left = bTreeToCList(root.left); 
        Node right = bTreeToCList(root.right); 
  
        // Make a circular linked list of single node 
        // (or root). To do so, make the right and 
        // left pointers of this node point to itself 
        root.left = root.right = root; 
  
        // Step 1 (concatenate the left list with the list  
        //         with single node, i.e., current node) 
        // Step 2 (concatenate the returned list with the 
        //         right List) 
        return concatenate(concatenate(left, root), right); 
    }