Data Structure Graph: strong connectivity

如果为undirected graph就是dfs或者bfs，如果都能visit则为连通O(V+E).

如果为directed graph就先dfs或者bfs，再reverse direct，再dfs或者bfs，如果两次都连通则为strong connect

http://www.geeksforgeeks.org/connectivity-in-a-directed-graph/