New tenants move into a new apartment.
Six rooms are on the same floor, all communicating, as shown in the diagram. 
The rooms numbers 4, 5, and 6, all facing the sea. 
 
The tenants insisted that the piano and the bookcase should change rooms.
Now, the rooms were very small and the pieces of furniture indicated were very big, so that no two of these articles could be got into any room at the same time. 

How was the exchange to be made with the least possible labour?

See answer