Smallest positive integer that does not occur in Array in Ruby

smallest_positive.rb

class Array
  def sum
    inject(0) { |sum, x| sum + x }
  end
end

def solution(a)
  # write your code in Ruby 2.2
  return unless a.is_a? Array

  a = a.sort.uniq
  last_elem = a.last
  return 1 if last_elem <= 0
  first_elem = a.first
  
  b = (first_elem..last_elem).to_a
  
  
  c = b.sum - a.sum
  c > 0 ? c : last_elem+1
  
end

def slow_solution_2(a)
  
  a = a.sort.uniq
  b = (a.first.to_i..a.last.to_i+1).to_a

  c = (b-a).first.to_i #=> [4, 6]

  c > 0 ? c : 1

end


def slow_solution(a)
  return unless a.is_a? Array
  sorted_array = a.sort.uniq
  first_elem = sorted_array.sort.uniq.first
  first_from = first_elem >= 0 ? first_elem : 0
  n = 0
  b = 0

  loop do
    n += 1
    b = first_from + n
    break if !sorted_array.include?(b) && b > 0
  end

  b
end