题目描述

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.

算法

(Hash) O(n)

1. 首先使用一遍循环统计 bulls 的数量，并记录下secret串中出现的数字的数量。
2. 第二次遍历guess串，统计guess中每个数字是否在第一个字符串中出现过。若出现过，则 cows 加 1，对应字符出现数字的数量减 1。
3. 最终的得到bulls数量和 cows = cows - bulls数量。

Java 代码

class Solution {
    public String getHint(String secret, String guess) {
        int countBull = 0;
        int countCow = 0;
        HashMap<Character, Integer> map = new HashMap<Character, Integer>();

        //check bulls
        for (int i = 0; i < secret.length(); i++) {
            char c1 = secret.charAt(i);
            char c2 = guess.charAt(i);
            map.put(c1, map.getOrDefault(c1, 0) + 1);
            if (c1 == c2) {
                countBull++;
            }
        }

        //check cows
        for (int i = 0; i < secret.length(); i++) {
            char c = guess.charAt(i);
            if (map.containsKey(c) && map.get(c) > 0) {
                map.put(c, map.getOrDefault(c, 0) - 1);
                countCow++;
            }
        }

        return countBull + "A" + (countCow - countBull) + "B";
    }
}