You're playing a number-guessing game. You have a secret number and your friend's guess, both as strings of digits. A bull is a digit in the correct position. A cow is a digit that exists in secret but is in the wrong position. This is a classic Google interview question that tests your ability to count matches precisely using hash maps.
Return a hint string in the format "xAyB" where x is the bull count and y is the cow count.
For secret = "1807" and guess = "7810", the answer is "1A3B". One bull (the 8), three cows (1, 0, 7).
Constraints: . Both strings have the same length and contain only digits.