Please use spoiler flag next time.

I had a very similar solution, though yours looks a bit more memory effective than mine. This fails case-sensitivity though ;) (even if it wasnt a rule.)

I did the same thing with a switch, I like how he extruded the comparator to its own function. I didn't even think to just use a char array for memory then convert back to a string with the return line. I used a string array. (I also haven't coded in years xD) Very elegant!

I did this but added strings to a knew string. This is more efficient

Liked how you even created a function, such an efficient code!

I we know the library methods, then why not?

I'm curious, does converting it to an array somehow make it more efficient? I just appended to a StringBuilder using a case statement which seems less complicated.

check my solution.
you made it really diffucult.

Finally a kata where a very efficient solution is considered best practice. :D