This solution has me stumped. I totally don't get how this works.

I feel like as a kotlin exercise this misses the point of nullability in kotlin. for any type T whe can have T? or nullable T and this is a core feature of kotlin the the language is built to support. so in using optional you are using javas hacky tool that is designed to do what kotlin natively does.

Same, i used Continuation.resumeWithException, and the programme always truns wrong untill i removed it.

dont_use_try_and_catch

Such information should be present in the description.

There is a typo in the empty solution.

Line 11: implemention -> implementation

Yes.

It can consume huge amount of memory, right?

I find your solution of pred to be both amazingly simple and elegant.

Approved

What's the problem with the package statement? I don't think it can cause any problem.

You can use

object solution {
  // things originally in your package
}

though.