qa.rb

require 'set'

def pairs arr, n
  pairs = Set.new
  arr.each_with_index do |left, i|
    arr.each_with_index do |right, j|
      if left + right == n and i != j
        pair = [left, right].sort
        if !pairs.include? pair
          pairs << pair
        end
      end
    end
  end
  return pairs
end

p = pairs([1, 23, 55, 40, 45, 99, 99, 45, 50], 100)
puts p.inspect