Cleanest solution :)

beautiful :)

Very clever. I had to debug it to wrap it around my head.
For future readers:

• i is the index (number-1) of the room, thus i+1 is room number
• rooms is holding for each room, the next day available (the last departure for each room)
• the map {} is checking for each arrival, the first room available at this arrival date, and puts the departure date in this room

.joinToString(" ")

Are you kidding ? -_-

Better solution than everyone else :-)