Short, but not very efficient - it first sorts the full string, even for strings like 'ba....'

Nice, but it could be shorter - you could convert the string from start to lowercase, and handle in map() only the case for lowercase (as initial case doesn't matter, anyway, in the end). And converting the even positioned chars in uppercase at the end seems a stretch done like this (instead of using Character.toLowerCase())

Nice, first recursive solution I see here (not super efficient, but nice). As a note, you could have skipped storing inhab as a separate variable and having if logic over it, at the cost of 1 more iteration :) - check my solution.. (I wrote it for fun in Scala then translated it in Java)

nice :).. using that toUpperCase at the end to avoid another 2 replacements...

I see this is voted as Best Practice. Yes, is clear and simple, but not very efficient - that final multiplication for the check is actually unnecessary, it would be enough to check that the square root s is a whole (int) number, would be faster at runtime.

Not so efficient to compute .sqrt(n) twice (instead of saving it in a variable first)