def is_balanced(root)
  $balance = true
  balanced?(root)
  $balance
end

def balanced?(root)
  return -1 if root.nil?

  left_height = 1 + balanced?(root.left)
  right_height = 1 + balanced?(root.right)
  $balance = false if (left_height - right_height).abs > 1

  left_height > right_height ? left_height : right_height
end