Array of arrays will be flattened.

flattener.rb

class Array
  def flattener
    arr = []

    self.each do |x|
      if x.is_a? Array
        arr = arr + x.flattener
      else
        arr << x
      end
    end
    return arr
  end
end

p [3, [4, [5, 6]], 7].flattener
#ouput [3, 4, 5, 6, 7]