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12 Feb 2018 : Countdown #
I'm not convinced it was good use of my time, but I spent the weekend writing some code to solve the Countdown numbers game. In case you're not familiar with Countdown, here's a clip.
 

There are lots of ways to do this, but my solution hinges on being able to enumerate all binary trees with a given number of nodes. Doing this efficiently (both in terms of time and memory) turned out to be tricky, and there's a hinge for this too, based on how the trees are represented. The key is to note that each layer can't have more than n nodes, where n is the number of nodes the tree can have overall.

Each tree is stored as a list, with each item in the list representing the nodes at a given depth in the tree (a layer). Each item is a bit sequence representing which nodes in the layer have children.

These bit sequences would get long quickly if they represented every possible node in a layer (since there are 2, 4, 8, 16, 32, ... possible nodes at each layer). Instead, the index of the bit represents the index into the actual nodes in the layer, rather than the possible nodes. This greatly limits the length of the bit sequence, because there can no more than n actual nodes in each layer, and there can be no more than n layers in total. The memory requirement is therefore n2.

Here's an example:
T = [[1], [1, 1], [1, 1, 0, 0], [0 ,1, 0, 0]]
which represents a tree like this:

A binary tree

It's really easy to cycle through all of these, because you can just enumerate each layer individually, which involves cyclying through all sequences of binary strings.

It's not a new problem, but it was a fun exercise to figure out.

The code is up on GitHub in Python if you want to play around with it yourself.
 

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