The project has been started at autumn 2015. In order to motivate my readers, no solutions were published. Instead, I checked solutions by email (like a school or uni teacher).
I can't do this anymore due to extreme load. Also, the whole RE4B book and challenges need a major overhaul. (I don't know when I would be able to do it.)
Meanwhile, there are all solutions (see "solution*" folders and files).
And thanks to all of you, my readers, who has helped me to make these challenges better, to fix bugs, etc.