# Dominoes

Make a chain of dominoes.

Compute a way to order a given set of dominoes in such a way that they form a
correct domino chain (the dots on one half of a stone match the dots on the
neighbouring half of an adjacent stone) and that dots on the halfs of the stones
which don't have a neighbour (the first and last stone) match each other.

For example given the stones `[2|1]`, `[2|3]` and `[1|3]` you should compute something
like `[1|2] [2|3] [3|1]` or `[3|2] [2|1] [1|3]` or `[1|3] [3|2] [2|1]` etc, where the first and last numbers are the same.

For stones `[1|2]`, `[4|1]` and `[2|3]` the resulting chain is not valid: `[4|1] [1|2] [2|3]`'s first and last numbers are not the same. 4 != 3

Some test cases may use duplicate stones in a chain solution, assume that multiple Domino sets are being used.

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