299-medium-Bulls_and_Cows.js

/**
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.
*/

/**
 * @param {string} secret
 * @param {string} guess
 * @return {string}
 */
var getHint = function(secret, guess) {
  var bull = cows = 0;
  var len = secret.length;

  const record = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0];

  for (var i = 0; i < len; i++) {
    if (secret[i] === guess[i]) bull++;
    else {
      if (record[secret[i]] < 0) cows++;
      if (record[guess[i]] > 0) cows++;
      record[secret[i]]++;
      record[guess[i]]--;
    }
  }

  return (bull + 'A' + cows + 'B');
};

console.log(getHint('0103', '3160'));

/**
解题思路：
设置一个10位的数组 record 分别记录 0-9 10个数字出现的频次
在遍历字符串过程中，记录每个数字出现的次数，而且 secret 和 guess 的记录过程是相反的；
 */