One solution is to formulate the contract so that guessers submit a hash of (guess, nonce). The contract records each submission. A few blocks after the submi

One solution is to formulate the contract so that guessers submit a hash of (guess, nonce). The contract records each submission. A few blocks after the submission deadline, Alice reveals the correct answer. Then submitters of correct guesses send another message to the contract with their nonce. The contract gives the reward to the person who submitted a correct guess the earliest.