Tower of Hanoi
- You may only move the top most ring on a post.
- You may not move a ring to a post that has a ring that is smaller than the ring you are moving.
The puzzle was invented by the French mathematician Edouard Lucas in 1883. There is a legend about an Indian temple which contains a large room with three time-worn posts in it surrounded by 64 golden discs. The priests of Brahma, acting out the command of an ancient prophecy, have been moving these discs, in accordance with the rules of the puzzle. According to the legend, when the last move of the puzzle iscompleted, the world will end. The puzzle is therefore also known as the Tower of Brahma puzzle. It is not clear whether Lucas invented this legend or was inspired by it.
If the legend were true, and if the priests were able to move discs at a rate of 1 per second, using the smallest number of moves, it would take them 2^64 - 1 seconds or roughly 585 billion years. The universe is currently about 13.7 billion years old.
There are many variations on this legend. For instance, in some tellings, the temple is a monastery and the priests are monks. The temple or monastery may be said to be in different parts of the world - including Hanoi, Vietnam, and may be associated with any religion. In some versions, other elements are introduced, such as the fact that the tower was created at the beginning of the world, or that the priests or monks may make only one move per day.