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.

Solution:

I am using an array to keep track the occurrence of each character.

Bull case: If secret and guess share the same char at the same position, it is a bull math, we increment bull and we do nothing to the counter array, because they cancel each other. 

Cow case: If the occurrence of current secret char is negative, it means this character must occur in the guess in some previous position, so increment cow.

Similar If the occurrence of current guess char is positive, it means this character must occur in the secret in some previous position, so increment cow.

On each position, we do increment for occurrence of character in secret and decreament for occurrence of character in guess. 

So we can get bull and cow in one pass.

 public String getHint(String secret, String guess) {  
     int bull=0;  
     int cow=0;  
     int[] count=new int[10];  
     for(int i=0;i<secret.length();i++){  
       int ind1=secret.charAt(i)-'0';  
       int ind2=guess.charAt(i)-'0';  
       if(ind1==ind2) bull++;  
       else{  
         if(count[ind1]<0) cow++;  
         if(count[ind2]>0) cow++;  
         count[ind1]++;  
         count[ind2]--;  
       }  
     }  
     return bull+"A"+cow+"B";  
   }