Category Post specific single.php

Hi,

I’m right now having quite a problem and I need help.
We all know that most themes are having a single.php file to display single posts, if not, then they have a index.php file to do that sometimes.
My problem is that, I would like to show a different single post style for a specific category.

Let’s say for example, all regular posts have the single post file shown as usual
While I want posts from another category to for example, display advertisements or a specific text.

Help would be very much appreciated,

Thanks,