Imagine an island composed of several connected countries with no enclaves or exclaves. We are concerned with coloring a map of the island with as few colors as possible.

If two neighboring countries merge their border dissolves and they become the same color.

Begin with an island that can be two colored.

One or more pairs of countries merge and now the new map of the island can only be three colored.

Again, one or more countries merge and now the map of the island requires four colors.

Find the smallest number of starting countries for which this is possible as well how they are joined.

A diagram like this example of a 3 color map should be easy enough to read:

1122
1332
1122

Stumped by a D2 problem!  I could not find a 6 country starting point that meets the puzzle requirement.  I am assuming that the 6 countries must merge into 5, and that those 5 must merge into 4, but perhaps I am misreading the problem.  Or perhaps it makes no difference.

Posted by Steve Herman on 2015-06-05 20:23:52