public boolean isBSTree(){
        return isBSTree(root,Double.NEGATIVE_INFINITY,Double.POSITIVE_INFINITY);

    }

    private boolean isBSTree(BSTNode<T> node,double min,double max ){
        if(node==null)
            return true;
        int i= (Integer) node.val;
        if(i<= min || i>= max)
            return false;
        return isBSTree(node.left,min,i)&&isBSTree(node.right,i,max);
    }
    }