o(n2) without any temporary buffer

Remove duplicates of linked list

 def remove_duplicates
    unless head && head.next
      return self
    end
    current  = head
    while(current)
      runner = current.next
      _previous = current
      while(runner)
        if runner.value == current.value
          _previous.next = runner.next
        else
          _previous = runner
        end

        runner = runner.next
      end
      current = current.next
    end
  end