After more than 2 years...
Yes that's true. Seems like the tests didn't cover that.

Your code doesn't work correctly with arrays that have only one argument.
For example, Extract(new int[] { 1 }) returns “1,1” even though it should return “1”.

bro you wont die from typing out args 1 time

Yea true. I was rushing some 7s and 8s so I didn't think of such things :D

Why is there no index-out-of-bounds error for e.g. when r=0?

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.

I think it's unlikely. Unfornutately it doesn't show me for which case it time out. If I'm not mistaken infinite loops are impossible for my code and incorrect inputs would result in an instant error. Unless it really exceeds the 8 digit pin it shouldn't be too inefficient.

See - this is exactly why I suggested List is the wrong datatype

nope: look at the tests

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