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:

生成树计数
https://icpcarchive.ecs.baylor.edu/index.php?option=com_onlinejudge&Itemid=8&category=648&page=show_problem&problem=5150

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 ”。。