299. Bulls and Cows (1)

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Description

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.

Solution:

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.

class Solution {
    public String getHint(String secret, String guess) {
        int bulls = 0;
        int cows = 0;
        int[] nums = new int[10];
        //to record count of number 0-9
        // add 1 if the number is proposed by secret
        // decrease 1 if the number is proposed by guess
        for(int i = 0 ; i < secret.length(); i++){
            if(secret.charAt(i) == guess.charAt(i)){
                bulls++;
            }
            else{
                //we shall check the nums to know if they have been proposed before
                if(nums[secret.charAt(i) - '0'] < 0){
                    //the number from secret has been proposed by guess before 
                    cows++;
                }
                if(nums[guess.charAt(i) - '0'] > 0){ 
                    //the number from guess has been proposed by secret before 
                    cows++;
                }
                nums[secret.charAt(i) - '0']++;
                nums[guess.charAt(i) - '0']--;
            }
        }

        return  bulls + "A"  + cows + "B";
    }
}