code: def DFS_prefix(self, root=None):
          
  
          if not root:
              root = self._root
  
          return self._DFS_prefix(root)
docs: Depth-first search.
  
          .. seealso::
             `Wikipedia DFS descritpion <http://en.wikipedia.org/wiki/Depth-first_search>`_
  
          :param root: first to start the search
          :return: list of nodes