old_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 check_dag():
      """Check Directed Acyclic Graph (DAG)."""
      pass
  
  
  def main():
      # DAG.
      dag_adj_d = {
          '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 check_dag():
      """Check Directed Acyclic Graph (DAG)."""
      pass
  
  
  def main():
      # Graph adjacency dictionary for DAG.
      dag_adj_d = {
          'A': ['D'],
          'B': ['D'],
          'C': ['D'],
          'D': ['E', 'G'],
          'E': ['J'],
          'F': ['G'],
          'G': ['I'],
          'I': ['J'],
          'J': []
      }
  
      # Graph adjacency dictionary for non-DAG.
      nondag_adj_d = {
          'A': ['B'],
          'B': ['C', 'E'],
          'C': ['C', 'F'],
          'D': ['B', 'G'],
          'E': ['A', 'D'],
          'F': ['H'],
          'G': ['E'],
          'H': ['I'],
          'I': ['F']
      }
  
  if __name__ == '__main__':
      main()
  
subject: Add DAG & non-DAG adjacency dicts