Cool!

Very smart solution.

I like it. I am not sure about the performance of this solution. Are you sure that usage of sorting is the best practice? I am a new user on Codewars. It is not clear for me should we take the performance into consideration?

Anyway, I use GroupBy for word and words. And compare groups. May it provide less execution time?

Resolved.

Merge conflict.

Merge conflict. Fork and update the description.

Crashes on any input string with the letter 'z' in it, because the boolean array is initialized with ['z'-'a'] => [ 90 - 65 ] => [25] instead of [26].

Merge conflict; fork and update the description.

n == 0 is an edge case that should be tested.

Merge conflict, fork and update the description.

It is a normal order for F#.

[|42;12;34;56;78;42|]
|> arrayDiff [|42|]

[42;12;34;56;78;42]
|> List.except [42]

Why are the arguments swapped?

Only 1 test for case-insensitiveness?..

null is not a string, why does one even use null in a functional language (that doesn't use it for None/Nothing)?