How does this have anything to do with math?

perfect example of implemention of math skills

Java: Exactly one random test and no test expecting null.

I feel silly now

approved

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I've revised the description, I hope it's clearer now. Thanks!

Fixed, thanks!

doh! I hate when I leave the redundant bits

No tests with all arrays being full of zeros.

Such input is sometimes generated randomly, but the reference solution is wrong, and it fails in such cases.

One random test is not enough.

The tests are showing warnings.

??? Expected: 'fi', instead got: 'i'
!!! only this problem
##CODE##
function firstNonRepeatingLetter(s) {
let arr = [...s.toUpperCase()].find(i => s.toUpperCase().indexOf(i) == s.toUpperCase().lastIndexOf(i)) || []
if(arr.length==0){
return ''}
else{
return s.indexOf(arr[0])>-1?arr[0]:arr[0].toLowerCase();
}
}

Thanks a lot! Fixed.