message: No need to nested method and no need for +1
  It looks much cleaner without nested method and +1 in the end.
diff: @@ -29,10 +29,11 @@ class Solution(object):
  :type root: TreeNode
  :rtype: int
  """
  - def depth(root, diameter):
  - if not root: return 0, diameter
  - left, diameter = depth(root.left, diameter)
  - right, diameter = depth(root.right, diameter)
  - return 1 + max(left, right), max(diameter, 1 + left + right)
  + return self.depth(root, 0)[1]
  - return depth(root, 1)[1] - 1
  + def depth(self, root, diameter):
  + if not root:
  + return 0, diameter
  + left, diameter = self.depth(root.left, diameter)
  + right, diameter = self.depth(root.right, diameter)
  + return 1 + max(left, right), max(diameter, left + right)
  