private BSTNode removeRootOf(BSTNode subroot){
        if (subroot.left ==null && subroot.right == null)
            return null;
        if(subroot.left == null || subroot.right == null){
            if(subroot.left == null){
                return subroot.right;
            }
            else { return subroot.left;}
        }

        BSTNode rightchild = subroot.right;

        if(rightchild.left == null){
            rightchild.left = subroot.left;
            return rightchild;

        }

        BSTNode parent = rightchild;

            while (parent.left.left != null) {
            parent = parent.left;
            }

                     BSTNode leftmostNode = parent.left;
                        parent.left = leftmostNode.right;
                        leftmostNode.right = rightchild;
                         leftmostNode.left = subroot.left;
                         return leftmostNode;

    }