299. Bulls and Cows

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.

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.) 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".

Please note that both secret number and friend's guess may contain duplicate digits, for example:

Secret number: "1123"

Friend's guess: "0111"

In this case, the 1st 1 in friend's guess is a bull, the 2nd or 3rd 1 is a cow, and your function should return "1A1B". You may assume that the secret number and your friend's guess only contain digits, and their lengths are always equal.

The question:

if both string has same chars at the same position, it counts as a bull,

if a char instance in guess shows in the secret, it counts as a cow, it behaves like offset this char from both strings ,if there is another instance of same char shows both in guess and secret, still counts as a cow.

1, use a int[256] as char set.
2, foreach char in string:
    if they are equal, bulls++;
    else increase count in the set for char at secret
3, for each char in guess and secret:
    if they are not equal, and set has char in guess:
       cow++, decrease count in set for char.
return bulls + 'A' + cows + 'B'

Here is a nicer solution, worth remembering and learning from this method.

public class Solution {
    public String getHint(String secret, String guess) {
        int[] set = new int[256];
        int bulls =0, cows = 0;
        for( int i=0; i< secret.length();i++){
            char p = secret.charAt(i);
            char q = guess.charAt(i);
            if(p == q){
                bulls++;
            }else{
                if(set[p]++ < 0) cows++; // if set[char] < 0, which means in guess already show once. cows++, offset by ++
                if(set[q]-- > 0) cows++;// if set[char] >0 which means in secret already show once, cow++, offset by --;
            }
        }
        return "" + bulls + "A" + cows + "B";
    }
}