Permutations, this time with ruby.

permutations.rb

class Array
  def interleave(n)
    (0...length).map do |i|
      (0...length).map do |j|
        i == j ? [n, self[j]] : self[j]
      end.flatten
    end.concat([self + [n]])
  end

  def combine(n)
    dup.reduce([]) { |a, e| a + e.interleave(n) }
  end

  def rest
    self[1..-1]
  end

  def permutations
    empty? ? [[]] : rest.permutations.combine(first)
  end
end

puts [1,2,3].permutations.inspect