Your challenge is to find a route across a crocodile
infested swamp using just a handful of rather short planks. Fortunately
the planks are light enough to move around, and the swamp is
full of old tree-stumps which will support the planks to form
temporary bridges. So by careful planning, and re-use of planks,
you might just find a route. Needless to say you can only move
planks you can physically reach, so try not to leave any too
Update August 2007: Plank
puzzles is also available as a mechanical puzzle called River Crossing
produced by ThinkFun.
Four editions are now available. For an introduction to the whole
range click here.
New Sept 2003: If you
find plank-puzzles inexplicably tricky, you are not alone. Plank
puzzles have been proven 'hard' to solve. For more info see The
Complexity of Sliding Block Puzzles (and Plank Puzzles, too)
by Bob Hearn
Rules and introduction
The best way to understand the plank puzzle concept is to
imagine yourself stuck in a real swamp, with tree-stumps too
far apart to jump-between, and planks too heavy to move more
than one at a time. The remaining rules get a bit fiddley, but
here they all are:
- You can pick up and carry a plank provided you have access
to one end.
- You can only pick-up and carry one plank at a time.
- Planks must be supported both ends, and must be an exact
- You cannot jump between stumps or planks.
- You cannot move a plank if you are already carrying
- Planks cannot be used diagonally, only north-south
- Planks cannot support other planks, and cannot be
- Planks cannot bridge intermediate stumps or planks.
Try the following warm-up puzzles and see the rules in action:
Click on Restart to restart.
Click on Undo/Redo to revisit previous moves.
Click on Replay to restart (with redo active).
Click on the play area to pick up or put down a plank. Accessible
stumps will be automatically highlighted for you.
Two of Five
This applet features two of the original five puzzles which existed when
the plank puzzles first went on-line in July 2000. These puzzles are now
entitled Deep-end and Smash-hit.
Deep-end (as in in-at-the...) is a firm favourite. It illustrates
many of the best plank-puzzle features, in a tidy small package. If you're
new to plank puzzles, or only have time to try one, this is the one for
you. Smash-hit was my featured challenge for a long time and is
still pretty hard to beat.
Route-66 has the longest solution of all plank-puzzles
to date. It takes at least 66 moves (plank relocations) to solve
this one, that's over double any of its predecessors.
A little bit of luck helped with the design of Route-66
since the original layout had a planned solution of mere 48,
but a short-cut came to light which couldn't be blocked without
breaking the true-path. It was sometime before I noticed there
was a unplanned longer solution that was still intact. For an
interesting view of Route-66 see its span
The hexagon based swamp is a variation of the plank puzzle
concept, introduced by Graham. This one is called Four-by-four,
by virtue of being one of the first to feature four planks (lengths
1, 2, 3 and 4).
Four-by-four illustrates a few features unique to hex-swamps.
For instance, note the two approaches to the exit. Which one
is the right one to aim for? Later hex-swamps (see Henleymob)
all play on the word Twist, thanks to their unique ability
to almost spin you on one spot. A feature I felt a strong urge
to exploit, once discovered.
concept & maze designs
© Andrea Gilbert 2000-01
applet (& hex-puzzle concept) © Graham