DP on tree

0.5h	Anniversary party, POJ2342	提高	http://bailian.openjudge.cn/practice/2342
1h	AtCoder Independent Set	提高	https://www.luogu.com.cn/problem/AT4537
1.5h	CF1528A Parsa's Humongous Tree	提高	https://codeforces.com/problemset/problem/1528/A
0.5h	Balancing Act, POJ1655	提高	http://bailian.openjudge.cn/practice/1655
0.5h	Cow Marathon, POJ1985	提高	http://bailian.openjudge.cn/practice/1985