message: update flatten-binary-tree-to-linked-list.py
  fixed Do not return anything, modify root in-place instead.
diff: @@ -11,10 +11,10 @@ class Solution(object):
  # @param root, a tree node
  # @return nothing, do it in place
  def flatten(self, root):
  - return self.flattenRecu(root, None)
  + self.flattenRecu(root, None)
  def flattenRecu(self, root, list_head):
  - if root != None:
  + if root:
  list_head = self.flattenRecu(root.right, list_head)
  list_head = self.flattenRecu(root.left, list_head)
  root.right = list_head
  @@ -28,11 +28,9 @@ class Solution2(object):
  # @param root, a tree node
  # @return nothing, do it in place
  def flatten(self, root):
  - if root != None:
  + if root:
  self.flatten(root.right)
  self.flatten(root.left)
  root.right = self.list_head
  root.left = None
  self.list_head = root
  - return root
  -
  