http://en.wikipedia.org/wiki/Tarjan's_strongly_connected_components_algorithm
http://www.ics.uci.edu/~eppstein/161/960220.html#sca
https://github.com/bwesterb/py-tarjan/
https://gist.github.com/chadhutchins/1440602
Author: Shi Shougang
Created: 2015-03-05 Thu 23:21
Emacs 24.3.1 (Org mode 8.2.10)
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