new_contents: # https://www.hackerrank.com/challenges/tree-postorder-traversal
  # Python 2
  
  """
  Node is defined as
  self.left (the left child of the node)
  self.right (the right child of the node)
  self.data (the value of the node)
  """
  
  def postOrder(tree):
      if tree is None: return
      postOrder(tree.left) or postOrder(tree.right)
      print(tree.data),
  
  """
  class Node:
      def __init__(self, left=None, right=None, data=None):
          self.left = left
          self.right = right
          self.data = data
  
  
  one = Node(data=1)
  four = Node(data=4)
  six = Node(data=6)
  
  five = Node(left=one, right=four, data=5)
  two = Node(left=six, data=2)
  three = Node(left=five, right=two, data=3)
  
  postOrder(three)
  """
  
subject: Implement tree postorder traversal