Hello Codeforces community!

I want to ask you to give links to problems, that you find interesting.

By "interesting" I mean problems that:

• The solution wasn't about to code some very difficult algorithm, but mainly targeted at idea
• The problem needs to think in different way. (i.e straightforward solution will be wrong)

It will be very good if the problem is on some OJ (like Codeforces, SPOJ).

Thanks!