[Ernie Cohen told me this problem.]
100 persons are standing in line, each facing the same way. Each person is wearing a hat, either red or blue, but the hat color is not known to the person wearing the hat. In fact, a person knows the hat color only of those persons standing ahead of him in line.
Starting from the back of the line (that is, with the person who can see the hat colors of all of other 99 persons), in order, and ending with the person at the head of the line (that is, with the person who can see the hat color of no one), each person exclaims either "red" or "blue". These exclamations can be heard by all. Once everyone has spoken, a score is calculated, equal to the number of persons whose exclamation accurately describes their own hat color.
What strategy should the 100 persons use in order to get as high a score as possible, regardless of how the hat colors are assigned? (That is, what strategy achieves the best worst-case score?)
For example, if everyone exclaims "red", the worst-case score is 0. If the first 99 persons exclaim the color of the hat of the person at the head of the line and the person at the head of the line then exclaims the color he has heard, the worst-case score is 1. If every other person exclaims the hat color of the person immediate in front and that person then repeats the color he has just heard, then the worst-case score is 50. Can you do better?
[Here’s a generalization of the problem.]
Instead of using just red and blue as the possible hat colors and exclamations, use N different colors.