# Definition for a binary tree node.

#

# defmodule TreeNode do

# @type t :: %__MODULE__{

# val: integer,

# left: TreeNode.t() | nil,

# right: TreeNode.t() | nil

# }

# defstruct val: 0, left: nil, right: nil

# end

defmodule Solution do

@spec invert_tree(root :: TreeNode.t | nil) :: TreeNode.t | nil

def invert_tree(nil), do: nil

def invert_tree(root) do

invert(root, root.left, root.right)

end

defp invert(parent, nil, nil), do: parent

defp invert(parent, left, nil) do

%TreeNode{parent | left: nil, right: invert(left, left.left, left.right)}

end

defp invert(parent, nil, right) do

%TreeNode{parent | left: invert(right, right.left, right.right), right: nil}

end

defp invert(parent, left, right) do

tmp = left

%TreeNode{parent |

left: invert(right, right.left, right.right),

right: invert(tmp, tmp.left, tmp.right)}

end

end