Isn't this question misplaced in 'easy' category?

I just want to know whether is it just me or also some of the users here believe that this question is really not 'easy'. I spent 15-20 minutes on it but still I'm not able to deduce an algorithm for it. I tried looking at the successful submissions of other users but majority of them use some 'countOne', 'countThree' variables in their code which I'm not able to understand.

Is it some famous optimized pattern that I'm not be aware of?