nindex.rb

class Array
  def nindex(n)
    p self
    _index = self.index(n)
    if _index.nil?
      return []
    else
      if _index == (self.size - 1)
        return [_index]
      else
        return [
          _index,
          self[_index+1 ... self.size].nindex(n).map{|i| i+_index+1}
        ].flatten
      end
    end
  end
end

p [1, 2, 3, 4, 5, 3].nindex(3)