A room is filled by dwarves with coloured hats.
Some hats are blue and the other are green.
The dwarves can't communicate and can't see the colour of their own hat (but they see the colour of the other hats).
They have to stand against one wall of the room ; all dwarves with blue hats must be on a side and the dwarves with green hats on the other side (all dwarves will stand against the same wall).
How can they do?
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The first dwarf go against the wall.
The second dwarf goes beside the first dwarf (on the left or on the right, it doesn't matter).
Each following dwarf stands up agains the wall one by one in the following way :
as long as there is only one color against the wall, the dwarf goes at an end of the group.
As soon as the two colors are present, the dwarf stands up himself between the two adjoining dwarves of different colors.