I know codeforces, timus, uva (to some extent), codechef, lightoj have option to sort problems by categories/tags. Any other online judges with this feature, like HDU, POJ etc?

I was trying to solve this problem for several days. I failed and read the editorial. But I couldn't understand the solution. Can someone please explain how they came up with that formula?

Thanks in advance :)

Can anyone please provide an implementation of block-cut tree?