longest_substring.rb

module LongestSubstring

	def self.find(first, second)
		return nil if first.nil? || second.nil?
		return nil if first != second
  		x, xs, y, ys = first[0..0], first[1..-1], second[0..0], second[1..-1]
  		if x == y
    		x + self.find(xs, ys)
  		else
    		[self.find(first, ys), self.find(xs, second)].max_by {|x| x.size}
  		end
	end

end