Bulls - Cows

Game rules are simple: Computer thinks of a 4-digit secret number with non-repeating digits.

Then, player tries to guess the number. If the matching digits are in their right positions, they are "bulls", if in different positions, they are "cows".

Example: If secret number is 4271 and guess is 1234, then we get 1 bull and 2 cows (Bull is 2, cows are 4 and 1).

Game stops when player guesses the number correctly.