old_contents: from __future__ import absolute_import
  from __future__ import print_function
  from __future__ import division
  
  
  def topological_sort():
      """Topological Sorting for Directed Acyclic Graph (DAG)."""
      pass
  
  
  def main():
      # DAG.
      dag_adjacency_dict = {
          'A': ['D'],
          'B': ['D'],
          'C': ['D'],
          'D': ['E', 'G'],
          'E': ['J'],
          'F': ['G'],
          'G': ['I'],
          'I': ['J'],
          'J': []
      }
  
  if __name__ == '__main__':
      main()
  
new_contents: from __future__ import absolute_import
  from __future__ import print_function
  from __future__ import division
  
  
  def _previsit():
      pass
  
  def _postvisit():
      pass
  
  def _dfs_explore():
      pass
  
  def topological_sort():
      """Topological Sorting for Directed Acyclic Graph (DAG).
  
      To topologically sort a DAG, we simply do depth first search,
      then arrange DAG's vertices in decreasing postvisit order.
      """
      pass
  
  
  def main():
      # DAG.
      dag_adjacency_dict = {
          'A': ['D'],
          'B': ['D'],
          'C': ['D'],
          'D': ['E', 'G'],
          'E': ['J'],
          'F': ['G'],
          'G': ['I'],
          'I': ['J'],
          'J': []
      }
  
  if __name__ == '__main__':
      main()
  
subject: Add helper methods and revise doc string