Problem:
 Input: a directed graph G
 Output: is an ordering V1, ..., Vn of Vertices(G) s.t.
  forall all Vi --> Vj  (i.e. (Vi,Vj) \in Edges(G))
  i < j

// DFS implemented using post-visit
TopoSort(G)
   out := EmptyList()
   for each v in Vertices(G)
      if not(marked(v)) then
         Visit(G,v,out)
   return out

Visit(G,v,out)
 mark(v)
 for each w in AdjSet(v)
   if not(marked(w))
      Visit(G,w,out)
 return Push(out,w)