gistfile1.rb

#!/usr/bin/env ruby

D = Struct.new(:month, :day)

dates = <<EOC
May 15 May 16 May 19
June 17 June 18
July 14 July 16
August 14 August 15 August 17
EOC

candidates = dates.scan(/(\w+) (\d+)/).map {|mon, day| D.new(mon, day)}

def candidates.group(how)
  group_by(&how).select {|_, dates| dates.size == 1 }.map {|_, dates| dates.first }
end

# Albert: I don't know. Neither does Bernard.
months = candidates.group(:day).map(&:month)
candidates.reject! {|x| months.include? x.month }

# Bernard: I didn't know, but now I do.
days = candidates.group(:day).map(&:day)
candidates.select! {|x| days.include? x.day }

# Albert: Woo.
months = candidates.group(:month).map(&:month)
puts candidates.select! {|x| months.include? x.month }.map(&:to_a).flatten.join(' ')