I also recommend that you wait that someone else solves your problem before posting your solution. Of course, you can solve a problem that you've created, and it is nice to share your view on it (since, in this case, there is probably more than one function that maps to these same values). However, posting this many problems and solving all of them reinforces the theory of spam.

So sorry, I didn't realize that you were creating a problem group. I still recommend that you wait that someone else solves your problem before posting your solution.

Moreover, problem 16 needs to be have more test cases or else it becomes trivial.