Problem:
 Input: a Digraph(G)  (Digraph = directed graph)
 Output: a list of lists of Vertices(G) s.t. each list
  is a strongly directed component of G

StronglyDirectedComponents(G)
   L := TopoSort(G)
   G' := G where I reverse all edges
   /* another DFS visit of G' */
   out := EmptyList()
   for each v in L  // I visit according to L, not Vertices(G)
      if not(marked(v)) then
         S := EmptyList()
         Visit(G',v,S)
         push(out,S)
   return out

Visit(G',v,S)  // same used in TopoSort etc.
   mark(v)
   for each w in AdjSet(G',v)
      if not(marked(v)) then
          Visit(G',w,S)
          push(S,w)


Main ideas:
1) TopoSort clusters together the nodes in the strongly connected
   component and topologically sorts the components
2) The topological order for the reversed graph lists first
   the components at the "bottom" of the DAG, i.e. those with
   incoming edges only; the ones that can reach it are listed later
3) Thus, every call to Visit in the algorithm above can just reach the
   nodes in the connected components of v (plus the ones in the
   connected components "below", but those are already marked)