missing int

missing_int_solution.rb

# Missing int from unsorted array and with arbitrary starting int

def solution(a=[])
  sum = 0
  min = a.first
  max = a.last

  a.each do |e|
    sum += e
    min = e if e < min
    max = e if e > max
  end

  min_sum = min > 0 ? ((min) * ( min - 1 )) / 2 : 0

  a_sum = (((max + 1) * ( max )) / 2) - min_sum

  a_sum - sum
end