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.

For example:

Secret number:  "1807"
Friend's guess: "7810"
Hint: 1 bull and 3 cows. (The bull is 8, the cows are 0, 1 and 7.)
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. In the above example, your function should return "1A3B".

Please note that both secret number and friend's guess may contain duplicate digits, for example:

Secret number:  "1123"
Friend's guess: "0111"
In this case, the 1st 1 in friend's guess is a bull, the 2nd or 3rd 1 is a cow, and your function should return "1A1B".
You may assume that the secret number and your friend's guess only contain digits, and their lengths are always equal.

创建两个变量A, B记录bulls和cows的个数，bulls的个数很简单，记录对应位置字符相等的个数。创建两个数组，保存对应位置不相等时的字符个数。最后累加两个数组中相同位置值小的元素就是B的值。代码如下：

public class Solution {
    public String getHint(String secret, String guess) {
        int[] a = new int[10];
        int[] b = new int[10];
        int A = 0;
        int B = 0;
        for(int i = 0; i < secret.length(); i++) {
            if(secret.charAt(i) == guess.charAt(i)) {
                A ++;
            } else {
                a[secret.charAt(i) - '0'] ++;
                b[guess.charAt(i) - '0'] ++;
            }
        }
        for(int i = 0; i < a.length; i++) {
            B += Math.min(a[i], b[i]);
        }
        String s = A + "A" + B + "B";
        return s;
    }
}