gistfile1.rb

class Array
  def pairs_for(k)
    seen = Hash.new(0)
    each_with_object([]) do |i, result|
      complement = k - i
      seen_count = seen[complement]
      if seen_count > 0
        result << [complement, i]
        seen[complement] -= 1
      else
        seen[i] += 1
      end
    end
  end
end