Numbering, also known as BullsandCows, is an ancient puzzle game, which originated in the mid-20th century and is usually played by two or more people.
Usually two people play, one gives the numbers and the other guesses. The person who gives the number must think of a four-digit number, and can't repeat the number, and can't let the guesser know. Every time you guess a number, the molecule will give a or b according to the number, where the number before A indicates the number in the right position and the number before B indicates the number in the right position.