Strongly Connected Components

In the mathematical theory of directed graphs, a graph is said to be strongly connected 
if every vertex is reachable from every other vertex. The strongly connected components of an 
arbitrary directed graph form a partition into subgraphs that are theselves strongly connected. 

Kosaraju's Algorithm

In computer science, Kosaraju's algorithm, also known as the Kosaraju-Sharir algorithm, is a linear time algorith to find the strongly connected components of a directed graph.

Demo: https://ideone.com/AhiTUU

References

• http://en.wikipedia.org/wiki/Strongly_connected_component
• http://en.wikipedia.org/wiki/Tarjan%27s_strongly_connected_components_algorithm
• http://en.wikipedia.org/wiki/Kosaraju%27s_algorithm