First, relevant xkcd.

Nerd sniping is like a chain email; after being sniped, if you don’t nerd snipe 10 other people in 3 days, your IQ will drop by 30 points. – A wise man

Anyways here are three problems that sniped me throughout the years that I still remember. Solutions are in white font following the problems.

- There are 64 prisoners, 64 different types of hats, and one guard. The guard is going to play a game with these prisoners. First, the guard randomly picks a hat for each prisoner to put on, and any prisoner cannot see his/her own hat, but can see the hats of everyone else. Some of these hats might be of the same type. They all get one chance to guess their own hat type (individually without knowing other prisoners’ guesses), and if any prisoner gets it right, they all win the game. All prisoners can negotiate before starting the game, but cannot talk to each other afterwards. How do they win the game?
- There are 2 prisoners A and B, and one guard. The guard pulls out a chess board (8×8) and put one coin on each grid, either heads or tails. The guard points at a certain coin, and only A can see the choice (also the board with all coins). Then A gets to flip exactly one coin, then he leaves the room, and B comes in. By only looking at the 64 coins after A’s coin flip, can B figure out which coin the guard pointed at? Same conditions as before: negotiation only before the game starts.
- There are 2 prisoners A and B, and also the same guard. This time the guard pulls out 64 cards with numbers 1-64 written on each card, then scrambles the order and put them on a table, face up. A gets to see all cards and also can swap one pair of cards. Then all cards are concealed, keeping the order after A’s swap. B comes in, and the guard tells him a number between 1 and 64 inclusively, and B can try to find the corresponding card in 32 trials. In each trial, B can only reveal one card to see if it matches with the guard’s specified card. How can B find the card? Same conditions as before, blah blah blah.

Solutions:

Just kidding, I’m not going to write down the solutions. It wouldn’t exactly be nerd sniping if that’s the case :^)