24 Knight Swap
Puzzle 1
++++++
 B  B  B  W  W  1
++++++
 B  B  B  W  W  2
++++++
 B  B   W  W  3
++++++
 B  B  W  W  W  4
++++++
 B  B  W  W  W  5
++++++
a b c d e


Puzzle 2
++++++
 B  B  B  B  W  1
++++++
 B  B  B  W  W  2
++++++
 B  B   W  W  3
++++++
 B  B  W  W  W  4
++++++
 B  W  W  W  W  5
++++++
a b c d e


Place 12 black and 12 white knights in a 5x5 grid, with the center square
unoccupied. Place blacks left and above the center square and whites right
and below the center square.
Swap the colors in as few moves as possible.

Puzzle 2 is the actual original puzzle I was sent.
I misread the description,
so I just decided to make it a second puzzle.
It differs from the first by one square, making more of a triangular
arrangement of the knights.
Source: A reader, citing
Zillions of Games's KnightSwap
puzzles.
(If anyone has a better source, let me know.)
Update 4/10/01. Al Zimmermann improved upon both solutions.
Each of these solutions is computergenerated and is the shortest possible.
Here's a 40move solution for the 1st problem:
1b, 2d, 3b, 5c, 4e, 3c, 5d, 4b, 3d, 1c,
2a, 3c, 5b, 4d, 2c, 1a, 3b, 5a, 4c, 5e,
3d, 1c, 2e, 4d, 2c, 1e, 3d, 2b, 4c, 3a,
2c, 3e, 4c, 2d, 3b, 5c, 4a, 2b, 1d, 3c
Here's a 36move solution to the 2nd problem:
1b, 2d, 3b, 5c, 4e, 3c, 2e, 1c, 3d, 2b,
4c, 5e, 3d, 4b, 2a, 3c, 1d, 3e, 2c, 1a,
3b, 5a, 4c, 3a, 5b, 3c, 4a, 5c, 3b, 4d,
2c, 1e, 3d, 4b, 5d, 3c
These solutions were received from Colin Bown.
This may not be minimum but it gets the job done in 64 moves.
5d, 4b, 3d, 1c, 2e, 4d, 2c, 3e, 1d, 2b,
4c, 1a, 5d, 4b, 3d, 1c, 2e, 4d, 2c, 3e,
1d, 2b, 4c, 3a, 5b, 3c, 1b, 2d, 3b, 5c,
4a, 3c, 4e, 5c, 4a, 3c, 2a, 1c, 2e, 3c,
5d, 3e, 1d, 3c, 5d, 3e, 4c, 3a, 2c, 1e,
3d, 5e, 4c, 5a, 3b, 1a, 2c, 4b, 3d, 5c,
3b, 1c, 2a, 3c
Here's a 46 move solution to part 2:
5d 4b 3d 1c 2e 4d 2c 3e 1d 3c
1b 2d 3b 5c 4a 2b 4c 3a 5b 3c
4e 5c 4a 3c 2a 1c 2e 3c 5d 3e
4c 5a 3b 1a 2c 1e 3d 5e 4c 2d
3b 1c 3d 5c 4e 3c
Thanks to Al Zimmermann for checking these solutions.
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