Definitely an introvert. I think all writers must be introverts, because writing is a very lonely occupation. At the same time, however, you must study people if you're going to make believable characters. In almost all of my characters, I can see elements of people I know.
Do you know the difference between an introverted math major and an extroverted math major? An introverted math major looks at his feet when he's talking to someone; an extroverted math major looks at the other person's feet when he's talking.