Given three posts (towers) and n disks of decreasing sizes, move the disks from one post to another one at a time without putting a larger disk on a smaller one. The minimum is 2ⁿ-1 moves. The "ancient legend" was invented by De Parville in 1884.

A solution using recursion is: to move n disks from post A to post B 1) recursively move the top n-1 disks from post A to C, 2) move the n^th disk from A to B, and 3) recursively move the n-1 disks from C to B. A solution using iteration is: on odd-numbered moves, move the smallest disk clockwise. On even-numbered moves, make the single other move which is possible.