Link to problem, it is not that hard to find the solution, but I can't quite understand why the operation done by the players can always be sorted into one of the three types of the following when the root is unknown:

• The first player sets the root in the first turn;
• The first player sets one of the leaves in the first turn, while the second player sets the root (That is only valid when at least 1 leaf is unknown);
• The first player sets one of the other nodes in the first turn, while the second player sets the root (That is only valid when the number of available nodes that is neither the root nor the leaf odd).

What I don't understand is that why can't one player choose the root or something else midgame, and why the condition for the third thing exists?

I am now facing a difficult situation, the problems I find are either too easy or too hard, there are not many problems which I find interesting and a little difficult to solve.

Please help!

This problem has only Japanese editorials and Youtube videos.

People in mainland China are not allowed to visit youtube (VPN or similar is illegal) and I'm not studying Japanese (the translation is terrible), so can anyone translate or just post the written editorial here?

I'm a student studying German and I'm interested about CP in Germany.

If you're from Germany, Austria or Switzerland, how's CP there and how the government (or any national organization) is maintaining it? Write what you know pls :)

Hi, everyone!

I've just checked my rating graph and saw this.

Is it a normal codeforces bug or a browser bug or else?

Also, I found other rating graphs strange (someone taking part in a contest recently but had its rating graph updated until last year).

Many people may be worried about homework when participating in CF contests on weekdays, and Chinese users don't have a chance to compete. So, I think it is better to set the time of the contests to weekends.

Jerrlee was my classmate and he is struggling with Atcoder ABC C-D Problems. However, in last contest, he amazingly made to the 94-th! What's he doing and most importantly, is he cheating?

It's normal people won't like this, but I'm so confused that a person like this will do such a performance.