How to get difference of top 2 records of each group in SQL Server order by date

I went though today Codility online exam for one of the top investment banking client interview, and I found one interesting question on SQL as below,

Question: 

Get top 2 records of each group /category in SQL Server, order by date and then get difference between them by category wise

In SQL Terms:

Retrieve difference between top 2 records of each group in sql server, order by date OR

Get difference of top 2 records of each group/category in SQL Server, order by date OR

Select top 2 records for each category in sql server order by date

Table Given:

CREATE TABLE [dbo].[test](

[event_type] [int] NOT NULL,
[value] [int] NOT NULL,
[date] [datetime] NOT NULL
) ON [PRIMARY]

Table Values 

Solution:

;WITH cte AS

(
   SELECT *,
         ROW_NUMBER() OVER (PARTITION BY event_type ORDER BY date DESC) AS rn
   FROM test
)

-- Now select from cte.

now retrieve top 2 elements of each group and get difference between them as below,

SELECT event_type, (max(value)-min(value)) as v FROM cte
WHERE rn <= 2
group By event_type

And here goes,