Depends on who you ask. For a simple conditional like this I find it more readable than having two return statements, but that's just my opinion

doesn't really make it any more readable though, does it?

could've one-lined this with the ternary operator

how is this working without sorting the vector first?

oh, I like this one, linear time and O(n) less multiplications/divisions than in my solution

4-liner, cheating on whitespace doesn't count.

Use spoiler flag.

The replaceAll method in Java takes in a regex as the first argument. Thus, . means "any character", so we replace any character with #

The negative sign is not a digit, it is just a sign, therefore it shouldn't make a number to win

that's a best practice to remember ;)

well mutation was not explicitly prohibited

You should considere it the other way around: if mutation isn't explicitly allowed, it's forbidden ;)

well mutation was not explicitly prohibited, although you are right, of course, this can produce side effects which would be not so easy to debug