def topological_sorting(graph):
<EOL>order = depth_first_search(graph)[<NUM_LIT:2>]<EOL>order.reverse()<EOL>return order<EOL>
Topological sorting.

@attention: Topological sorting is meaningful only for directed acyclic graphs.

@type  graph: digraph
@param graph: Graph.

@rtype:  list
@return: Topological sorting for the graph.