http://www.math.ucla.edu/~pak/hidden/papers/Moon-counting_labelled_trees.pdf
http://math.stackexchange.com/questions/689526/how-many-connected-graphs-over-v-vertices-and-e-edges
Level 1.
http://www.matrix67.com/blog/archives/682
http://www.spoj.com/problems/PT07D
http://www.shuizilong.com/house/archives/spoj-1480-let-us-count-1-2-3/
Level 2.
http://poj.org/problem?id=1737
Level 3.
http://www.spoj.com/problems/KPGRAPHS/
https://www.hackerrank.com/contests/infinitum-sep14/challenges/strongly-connected-digraphs
Other:
http://user.qzone.qq.com/251815992/blog/1380986878 (仙人掌图计数。。？？？)
https://projecteuler.net/problem=434
http://hihocoder.com/contest/challenge2/problems
http://hihocoder.com/discuss/question/591
“ 我十分有幸参加了这场500人斩比赛并贡献一WA ”。。
