How Mankiw did his PhD in 2 years

Mankiw writes;

Yes, I managed to earn a PhD in two years of residence at MIT. Here is how:

1. During my last year as an undergrad at Princeton, I took the standard graduate sequence in micro and macro. When I arrived at MIT, I was effectively a second-year PhD student.
2. After my first year at MIT, I went for a year to Harvard Law School, where I was more concerned about writing economics articles than being a diligent law student.
3. After the year in law school, I went to the CEA as a staff economist, where I worked for senior staff economist Larry Summers, who served also as an academic adviser. Because I worked on research between CEA tasks, when I returned to MIT for my second year, I had much of my dissertation already completed.