Guy who thinks he's Napoleon really is

[Brightonian]

IN this story I read a few decades ago, there's a dialogue something like this:

(scene: Mental hospital, somewhere in US, 20th century)
- Why are you in here?
- I think I'm Napoleon.
- And are you?
- Yes I am, as a matter of fact.
- That's too bad. They'll never let you out.

The twist is that he really is Napoleon; at the end of the story he goes insane and forgets who he is, the shrinks think they've cured him and he's released into the community.