Not to mention the performance characteristics on finite lists.

O(n) is possible, why vote an O(n log n) solution Best Practice?

I stand corrected

It does.

When both letters are in upper case, both calls to char.IsLower() will return false.
false == false resolves to true, so the function returns 1.

This does not account for the case when both letters are upper and should return 1.

Please use spoiler flag next time.

Impressive

Of course, I was just commenting because for me, as a mathematician, it was on the first glance confusing that this actually works :D

To be fair, the type definition is Int -> Int, not (Num a) => a -> Int, so a Double would never be allowed into the function.

Note that this definition would not work for Double, e.g. 2.25=1.5^2 would be sent to 2^2=4, because 1.5 gets rounded to 2, although 2.25 is closer to 1 than to 4.

Unfortunately, it's O(n²) when O(n log n) is possible.

Fixed.

Done.

True, I don't remember why I didn't do that