Happend to me as well, because I forgot to include - 0^2 - , so my counter did not count the first 0.
Also make sure to count the upper-bound - n^2.

It's a difficult question. What you need to learn depends on challenges you will face during the trial, your background in computer science, and programming skills. I don't know anything about that.

It would be fine to address this question to the supervisor/manager/administrator/etc. of your preliminary trial. If it isn't possible, you may read any short guide about the C language, data structures and algorithms, for example: http://www.cs.yale.edu/homes/aspnes/classes/223/notes.html, chapters 4, 5, may be 3 (if you use Linux).

But again, it's very difficult to give an advice without any knowledge about the trial, your background and skills.

Hi,

Your description of the solution is correct.

Feel free to ask any questions, and good luck with learning the C language.

Done.