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

算法

(Hash) O(n)

Java 代码

public class Solution {

    // Input: secret = "1807", guess = "7810"
    // Output: "1A3B"
    public String getHint(String secret, String guess) {
        int countBull = 0;
        int countMatch = 0;
        HashMap<Character, Integer> map = new HashMap<>();

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

        //check cows
        for (int i = 0; i < guess.length(); i++) {
            char c = guess.charAt(i);
            // secret包含guess中这个字符，并且出现次数>0
            if (map.containsKey(c) && map.get(c) > 0) {
                countMatch++;
                map.put(c, map.get(c) - 1);
            }
        }

        // 注意countMatch - countBull才是cow的数量
        return countBull + "A" + (countMatch - countBull) + "B";
    }

}