A prison guard has a unique challenge for a prisoner. There are seven rooms in a row with doors opening onto a hallway. Inside one of the rooms is a key that will let the prisoner escape. Each day, the prisoner may open one of the doors, and if the key is there, he may leave. If not, that night the warden will move the key to an adjacent room, and the prisoner may try again the next day.
How long might it take the prisoner to escape using his best strategy, and what is that strategy?