We can't diagnose professionally because an accurate diagnosis requires a corpse.

Trump also suffers from mental illness in my opinion. There was a group of pyschiatrists who basically violated their own professional oathes to issue a statement saying as much.

If you listen to how he talks now and compare it to how he spoke on, say, the Howard Stern show, you'll see a significant difference. Back then, he sounded like a jerk, but he was able to stay focused on the subject matter and not veer off every few seconds.

But it's also possible we underestimate him. It could all be an act.