Previous replies are all correct and reasonable. However, Positano is not an ideal sleeping location for visiting Pompeii. It's an hour on the bus, an hour on the train, and an average 15 minute wait for each of them. That doesn't count late public transportation service, which does happen.
Early in April, the bus may not be too crowded. But by mid May you can expect all busses to be full, with a line to board that may be larger than the capacity of one bus.
You need to take this into consideration on your departure home, if you are flying home from Rome. It is essentially impossible to wake up in Positano and fly to the US from Rome that day. A common recommendation is to shift some "Rome days" to the very end of such a trip.