reverse topological ordering (#638)

Add reverse topological ordering for `Data.Graph`.

Author: jprider63

containers/src/Data/Graph.hs

@@ -67,6 +67,7 @@ module Data.Graph (
     , dfs
     , dff
     , topSort
+    , reverseTopSort
     , components
     , scc
     , bcc
@@ -617,6 +618,10 @@ postOrd g = postorderF (dff g) []
 topSort      :: Graph -> [Vertex]
 topSort       = reverse . postOrd
 
+-- | Reverse ordering of `topSort`.
+reverseTopSort :: Graph -> [Vertex]
+reverseTopSort = postOrd
+
 ------------------------------------------------------------
 -- Algorithm 3: connected components
 ------------------------------------------------------------