good idea

I hope you enjoy this solution ;)

Using arr.toSet() instead of just arr as optimization for really long arrays. Usually, it won't be necessary.

Hi, I've just submitted a solution knowing it won't work with negative integers.

I suggest you update either the definition to exclude negative integers or the test cases to test negative integers as well.

It's part of the task requirement

The warnings appeared when C was automatically updated to CLang8/C18. These warnings don't prevent you to pass the kata (they are only warnings) so it is not an issue (you can notice that 805 people passed the C kata). Nevertheless I modified the translation so that there are no more warnings but due - it seems - to CW problems I have not been able to republish the C translation. I will try later.

Update: at last C translation was re-published.

There's an issue with the C translation. A warning in setup.c:3:13:

setup.c:3:13: warning: implicitly declaring library function 'strlen' with type 'unsigned long (const char *)' [-Wimplicit-function-declaration]
    int l = strlen(s);
            ^
setup.c:3:13: note: include the header <string.h> or explicitly provide a declaration for 'strlen'
1 warning generated.

I think that it is supposed to be fixed by replacing

// setup.c
// ...
int l = strlen(s);
// ...

with

// setup.c
#include <string.h>
// ...
unsigned long l = strlen(s);
// ...

Wouldn't it be better to accept any iterable like range or tuple?, for me they are good options too

It seems that every word in the tests is lowercase. Is this a feature or a bug?