299. Bulls and Cows

You are playing the following Bulls and Cows game with your friend: You write down a number and ask your friend to guess what the number is. Each time your friend makes a guess, you provide a hint that indicates how many digits in said guess match your secret number exactly in both digit and position (called "bulls") and how many digits match the secret number but locate in the wrong position (called "cows"). Your friend will use successive guesses and hints to eventually derive the secret number.

Write a function to return a hint according to the secret number and friend's guess, use A to indicate the bulls and B to indicate the cows.

Please note that both secret number and friend's guess may contain duplicate digits.

Example 1:

Input: secret = "1807", guess = "7810"

Output: "1A3B"

Explanation: 1 bull and 3 cows. The bull is 8, the cows are 0, 1 and 7.

Example 2:

Input: secret = "1123", guess = "0111"

Output: "1A1B"

Explanation: The 1st 1 in friend's guess is a bull, the 2nd or 3rd 1 is a cow.

Note: You may assume that the secret number and your friend's guess only contain digits, and their lengths are always equal.

解题要点：

用一个数组长度为10的来记录bull和cow的情况，如果当前secret和guess数字相等，bull加1。在不相等的情况下，遇到secret对应的index会加1， 遇到guess会减1。所以当在secret的情况里有小于0的情况，说明这个数字有在guess里出现过，就要在cow上加1；在guess的情况有大于0的情况，说明这个数字在secret里存在，也要cow加1。

class Solution(object):
    def getHint(self, secret, guess):
        """
        :type secret: str
        :type guess: str
        :rtype: str
        """
        bulls = cows = 0
        num = [0 for i in range(10)]
        for i in range(len(guess)):
            if secret[i] == guess[i]:
                bulls += 1
            else:
                if num[int(secret[i])] < 0: cows += 1
                if num[int(guess[i])] > 0: cows += 1
                num[int(secret[i])] += 1
                num[int(guess[i])] -= 1
        return str(bulls) + "A" + str(cows) + "B"