text: """Topological Sort."""
  
  from __future__ import print_function
  #     a
  #    / \
  #   b  c
  #  / \
  # d  e
  edges = {'a': ['c', 'b'], 'b': ['d', 'e'], 'c': [], 'd': [], 'e': []}
  vertices = ['a', 'b', 'c', 'd', 'e']
  
  
  def topological_sort(start, visited, sort):
      """Perform topolical sort on a directed acyclic graph."""
      current = start
      # add current to visited
      visited.append(current)
      neighbors = edges[current]
      for neighbor in neighbors:
          # if neighbor not in visited, visit
          if neighbor not in visited:
              sort = topological_sort(neighbor, visited, sort)
      # if all neighbors visited add current to sort
      sort.append(current)
      # if all vertices haven't been visited select a new one to visit
      if len(visited) != len(vertices):
          for vertice in vertices:
              if vertice not in visited:
                  sort = topological_sort(vertice, visited, sort)
      # return sort
      return sort
  
  
  if __name__ == '__main__':
      sort = topological_sort('a', [], [])
      print(sort)
  