Find the Missing Integer

missing.rb

def missing(list)
  min, max, sum = list.reduce([f = list.first, f, 0]) do |(l, h, s), n|
    [n < l ? n : l, n > h ? n : h, s + n]
  end
  max * (max + 1) / 2 - min * (min - 1) / 2 - sum
end