CiSRA Puzzle Competition 2009 - Solutions

This is the archive of the 2009 Puzzle Competition. Please visit the current competition site for information about the latest Puzzle Competition.

A.1 Tooth and Claw

There is a lot of biting going on. Some animals are biting others. Some are being bitten. Some are biting and being bitten.

Start by drawing a directed graph of the animals based on which is biting which. In other words, draw a box for each animal and an arrow pointing from it to the box of each animal it is biting (if any).

It turns out the digraph is acyclic; i.e. there are no loops, and all of the boxes can be arranged so that all the arrows point downwards.

If an animal is biting more than one other, arrange its child nodes (the animals it is biting) from left to right in the order in which they are being bitten. For example, the horse is biting the owl first and the kangaroo second (since the owl is further back in its mouth), so owl appears to the left and kangaroo to the right.

The animals should be named given the amount of detail provided in the drawing. For example, the two different snakes are drawn with distinguishing features to allow them to be identified as a rattlesnake and a cobra.

Only the wolf is not bitten by any other animal. Using the wolf as the starting point, perform a pre-order traversal of the graph. This means that a node should be visited first, and then each of its children should be visited from left to right in order.

Start at the wolf. Go down to the first animal it is biting (horse). Go down to the first animal the horse is biting (owl). The owl is not biting anything, so go to the next animal the horse is biting (kangaroo). The kangaroo is not biting anything, and the horse is not biting anything further, so go to the next animal the wolf is biting (iguana), then the first animal the iguana is biting (lion), and so on. See the red path in the diagram.

As you visit each animal, record the initial letter in its name. Note that an animal can be visited more than once if it has more than one animal biting it.

The letter sequence generated is "WHOKILLEDCOCKROBIN".

The question "Who killed Cock Robin?" comes from the nursery rhyme of the same name. The first two lines are:

Who killed Cock Robin?
I, said the Sparrow,