[leetcode] Bulls and Cows

题目：

You are playing the following Bulls and Cows game with your friend: You write a 4-digit secret number and ask your friend to guess it, each time your friend guesses a number, you give a hint, the hint tells your friend how many digits are in the correct positions (called "bulls") and how many digits are in the wrong positions (called "cows"), your friend will use those hints to find out 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.)
According to Wikipedia: "Bulls and Cows (also known as Cows and Bulls or Pigs and Bulls or Bulls and Cleots) is an old code-breaking mind or paper and pencil game for two or more players, predating the similar commercially marketed board game Mastermind. The numerical version of the game is usually played with 4 digits, but can also be played with 3 or any other number of digits."

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.

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

解题思路：

1. guess中总共猜对多少个secret中的字符(包括下标不对应的情况)，记为sum.

2. 统计下标相同时，guess猜对的字符数，记为numA.

3. 统计 wrong position情况下，guess猜对的字符数，为sum-numA.

Java代码如下：

    public String getHint(String secret, String guess) { 
        HashMap<Character, Integer> hm = new HashMap<Character, Integer>();
        for (int i = 0; i < secret.length(); i++) { //借助HashMap，统计secret中每个字符出现的次数
            if (! hm.containsKey(secret.charAt(i))) {
                hm.put(secret.charAt(i), 1);
            } else {
                hm.put(secret.charAt(i), hm.get(secret.charAt(i)) + 1);
            }
        }
        for (int i = 0; i < guess.length(); i++) { //分析guess，看猜对了多少个secret中的字符
            if (hm.containsKey(guess.charAt(i)) && hm.get(guess.charAt(i)) > 0) {
                hm.put(guess.charAt(i), hm.get(guess.charAt(i)) - 1);
            }
        }
        int tmp = 0;
        for (Integer i : hm.values()) {
            tmp += i;
        }
        int sum = secret.length() - tmp; //guess中总共猜对多少个secret中的字符(包括下标不对应的情况)
        
        int numA = 0;
        for (int i = 0; i < secret.length(); i++) { //统计下标相同时，guess猜对的字符数
            if (secret.charAt(i) == guess.charAt(i)) {
                numA++;
            }
        }
        int numB = sum - numA; //统计 wrong position情况下，guess猜对的字符数
        return "" + numA + "A" + numB + "B";
    }