# Definition for a binary tree node.
# class TreeNode
#   attr_accessor :val, :left, :right
#   def initialize(val)
#     @val = val
#     @left, @right = nil, nil
#   end
# end

# @param {TreeNode} root
# @return {TreeNode}
def invert_tree(root)
  return nil if root.nil?

  temp = root.left
  root.left = invert_tree(root.right)
  root.right = invert_tree(temp)
  root
end