leetcode之Bulls and Cows

猜数字游戏。报出来每次有几个数字是在正确的位置的，多少是不正确的。先求前者，再求后者。代码如下：

class Solution(object):    def getHint(self, secret, guess):        """        :type secret: str        :type guess: str        :rtype: str        """        if secret == '':            return '0A0B'        a = b = 0        s = list(secret)        g = list(guess)        for i in range(len(s))[::-1]:            if s[i] == g[i]:                a = a + 1                del s[i]                del g[i]        # print s, g        for i in range(len(g))[::-1]:            if g[i] in s:                b = b + 1                                s.remove(g[i])                del g[i]                # print s, g, b        return str(a) + 'A' + str(b) + 'B'