There is a problem that when you select Mono 4.3.2 version of C# runtime, it's ~3 times slower than 7.3 version, and quite often times out. I published a fork with rebalanced random tests so you can check again when it gets accepted, but it's highly probable that your solution will pass even currently, when you select C# 7.3 version before you submit your solution.

Let me know if it worked for you, and mark question as resolved if so.

Regards!

I've done this in Python/JS/Ruby/C++, and did not encounter inputs longer than 5 characters, this is probably true for other languages as well.

You can post your code in a comment with a spoiler tag, so somebody could review it if you want.

Hmmm, have you considered the possibility your code make have a bug? (e.g. infinite loop for some incorrectly handled inputs)