Use java 7. I tried below code, it is passing with custom test case with SAME DATA but fails for same data( 5 test failing) in their automation. Not sure why hacker rank guys are not fixing it...

boolean checkBST(Node root) {
    if (null == root){
        return true;
    }
    //System.out.println("Processing..." + root.data);
    boolean ret = true;
    if (null != root.left){
        if (root.left.data >= root.data){
            ret = false;
            //System.out.println("NOT_OK:Left is bigger or equal:" + root.left.data);
        } else {
            //System.out.println("OK:Left is small:" + root.left.data);
        }
    }
    if (null != root.right){
        if (root.right.data <= root.data){
            ret = false;
            //System.out.println("NOT_OK:Right is smaller or equal:" + root.right.data);
        } else {
            //System.out.println("OK:Right is big:" + root.right.data);
        }
    }
    //if false return here
    if(!ret){
        return false;
    }

    return checkBST(root.left) && checkBST(root.right);
}