The problem that I have with those problems is, they always assume it's easy to answer after seeing the solution. But if you're working on them for the first time, it's a terrible environment. The minute that they stop asking you, as the canidate, are on a timeline to finish. (Also, to finish as well)
The answer to that item would be: (with scala)
> datasource.window(3,1).filter((a,b,c)=> a+b == c)
I mean if you knock it out of the park and know the internals of window.. then asking the bar raiser question is appropriate as a curiousity.
There are other problems as well: Those are the ones where they have an expected answer, response, and reiterate. I've seen that with tree problems. That's where they would low-key ask for the recursive version and then get you to say stackoverflow exception.