Santa Claus must still be waiting for this code to run

I assume that I can transfer the characters correctly from the browser to VS, i.e. converting from UTF8 to Unicode. `

"ẞtß".ToLower() gives hex:"DF-00-74-00-DF-00", therefore "t" is the single non-repeating char.

I make the same steps with "σtς" which gives: hex	"C3-03-74-00-C2-03".
From this result I would conclude that "σ" is the first non-repeating character. What is my mistake?

VS displays your 3rd string with an obviously different character than the browser.

Yes, Thanks !

Your function should return the first non-repeated char of the string, so, if the string has only 1 char, it should return that one. It's not repeated, is it? At least I think that's the way of understanding it.

Looks at the minimal test suite (in Python) :

test.assert_equals(first_non_repeating_letter('a'), 'a')

I may have misunderstood the point, so I've added a specific check on len(s) == 1 => return s

What are you saying there? 'a' is not repeated, so, yes, dups('a') => '' of course.

If a string contains all repeating characters, it should return an empty string ("");

dups('a') => '' and not 'a'

From the docs: https://docs.codewars.com/training/troubleshooting#your-solution-seems-to-work-or-is-close-to-working-but-still-fails-tests

Check if your solution passes two successive calls with different inputs. It's sometimes the case that the solution contains some global variables or static members and a stale state is carried over from one invocation to another.

This is not true and your oslution has a bug because it uses global variables.

all random tests return an incorrect result, although I have checked my code in another compiler and it returns the correct answer with these values
of the "number" parameter. (python)

Same bro

For C# the opening and closing braces of the class should be at the line start.

Typo in the description in "The digits are sequential, incementing†: 1234" : "incementing" instead of "incrementing".

It also took me so long for such an easy solution. But I got it in the end!

Wow