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.





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.


https://www.cnblogs.com/yrbbest/p/5047102.html



Input:
"1123"
"0111"
Output:
"1A2B"
Expected:
"1A1B"
    
    key    value 
    1    0
    2     2 
    3      3
    
    
    a++, a =  1
    
    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.
    
    
    
    
    
// wrong 
class Solution {
    public String getHint(String secret, String guess) {
        // A: same char, same location
        // B: still has this char , but at differnt location
        // use hashmap to store , key : char, value : index 
        // traverse another string, check if the map has this char , if yes
        // if the char's index is the same as the the char's index in the map. A++
        // if not the same , B++
        int A = 0;
        int B = 0;
        HashMap<Character, HashSet<Integer>> map = new HashMap<>();
        for(int i = 0; i < secret.length(); i++){
            char c = secret.charAt(i);
            if(!map.containsKey(c)){
                map.put(c, new HashSet<>());
            }
            HashSet<Integer> set = map.get(c); // is it necessary to put HashSet<Integer> 
            set.add(i);  
        }
        
        // traverse the guess string 
        for(int i = 0; i < guess.length(); i++){
            char c = guess.charAt(i);
            if(map.containsKey(c)){
                HashSet<Integer> set = map.get(c);
                if(set.contains(i)){
                    A++;
                    set.remove(i);
                }else{
                    B++;
                    set.remove(i);
                }
                if(set.size() == 0) map.remove(c);
            }
            
        }
        StringBuilder sb = new StringBuilder();
        sb.append(A).append('A');
        sb.append(B).append('B');
        return sb.toString();
        
    }
}

 

// other's code

The idea is to iterate over the numbers in secret and in guess and count all bulls right away. For cows maintain an array that stores count of the number appearances in secret and in guess. Increment cows when either number from secret was already seen in guest or vice versa.

public String getHint(String secret, String guess) {
    int bulls = 0;
    int cows = 0;
    int[] numbers = new int[10];
    for (int i = 0; i<secret.length(); i++) {
        int s = Character.getNumericValue(secret.charAt(i));
        int g = Character.getNumericValue(guess.charAt(i));
        if (s == g) bulls++;
        else {
            if (numbers[s] < 0) cows++;
            if (numbers[g] > 0) cows++;
            numbers[s] ++;
            numbers[g] --;
        }
    }
    return bulls + "A" + cows + "B";
}
A slightly more concise version:

public String getHint(String secret, String guess) {
    int bulls = 0;
    int cows = 0;
    int[] numbers = new int[10];
    for (int i = 0; i<secret.length(); i++) {
        if (secret.charAt(i) == guess.charAt(i)) bulls++;
        else {
            if (numbers[secret.charAt(i)-'0']++ < 0) cows++;
            if (numbers[guess.charAt(i)-'0']-- > 0) cows++;
        }
    }
    return bulls + "A" + cows + "B";
}