Lets list all the good problems from acm.sgu.ru in this post.
1) 507. Treediff
Spoiler
Please comment the problems you solved and think to be good or interesting, or the problems that you think to be "must solve". I will add them to the list of this post.
You may give the category/type or hint or algorithm/data structure needed to solve the problem in spoiler.
A certain man kills corrupt rich people and gives away their wealth to the poor. Is he a good man?
There is no one absolute definition of good. We define what we consider good or not...
paradox) for me yes, he's good