Use single quotes unless string interpolation is required.

Space bar - ::Not Found::

Yeah, String methods weren't allowed!

Yes, I also thought the same think. This solution absolutely is using string operation.

Should these solution not be failed as they use string methods?

That's cool.. No worries.
The statement is in the description.
"The interesting part of this task is that you have to do it without string operation (hey, it's not really interesting otherwise)"

I did not see the stipulation. Sorry, but the code should work. Just check for syntax errors!

I've tried this. But it did not pass. I thought string operation were not allowed.