merge two binary trees sloved in ruby

class TreeNode
  attr_accessor :val, :left, :right
  def initialize(val=0, left=nil, right=nil)
    @val = val
    @left = left
    @right = right
  end
end

def merge_trees(t1, t2)
  return t2 if t1.nil?
  return t1 if t2.nil?
  t1.val += t2.val
  t1.left = merge_trees(t1.left, t2.left)
  t1.right = merge    trees(t1.right, t2.right)
  t1
end