new_contents: # http://www.geeksforgeeks.org/bfs-vs-dfs-binary-tree/
  
  
  class Node:
      def __init__(self, value):
          self.value = value
          self.left = None
          self.right = None
  
  
  def traverse_in_order(values, node):
      if not node:
          return
  
      if node.left:
          traverse_in_order(values, node.left)
      values.append(node.value)
      if node.right:
          traverse_in_order(values, node.right)
  
  
  if '__main__' == __name__:
      head = Node(1)
      head.left = Node(2)
      head.right = Node(3)
      head.left.left = Node(4)
      head.left.right = Node(5)
  
      values = []
      traverse_in_order(values, head)
      assert [4, 2, 5, 1, 3] == values
  
subject: add bfs