LeetCode.299 Bulls and Cows

题目：

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.

分析1（原创，时间复杂度高）：

class Solution {    public String getHint(String secret, String guess) {            //你和朋友在玩下面的猜数字游戏（Bulls and Cows）：你写下一个4位数的神秘数字然后让朋友来猜，你的朋友每次猜一个数字，你给一个提示，告诉他有多少个数字处在正确的位置上（称为"bulls" 公牛），以及有多少个数字处在错误的位置上（称为"cows" 奶牛），你的朋友使用这些提示找出那个神秘数字        //思路：使用HashMap记录secret中元素的位置和个数        //使用一个数组记录bull已经访问的        int bull=0,cows=0;        HashMap<Character,Integer> hm=new HashMap<Character,Integer>();        boolean [] flag=new boolean[secret.length()];        for(int i=0;i<secret.length();i++){            if(guess.charAt(i)==secret.charAt(i)){                //该位置不需要在再次判断了                bull++;                flag[i]=true;            }else{                //不相等就存入hm                if(hm.containsKey(secret.charAt(i))){                    //已经存在直接叠加                    hm.put(secret.charAt(i),hm.get(secret.charAt(i))+1);                }else{                    hm.put(secret.charAt(i),1);                }            }        }        //遍历查找cows        for(int i=0;i<secret.length();i++){            if(!flag[i]){                if(hm.containsKey(guess.charAt(i))&&hm.get(guess.charAt(i))>0){                    //减去一个                    cows++;                    hm.put(guess.charAt(i),hm.get(guess.charAt(i))-1);                }            }        }        return bull+"A"+cows+"B";    }}

分析2（参考答案）：

class Solution {    public String getHint(String secret, String guess) {            //你和朋友在玩下面的猜数字游戏（Bulls and Cows）：你写下一个4位数的神秘数字然后让朋友来猜，你的朋友每次猜一个数字，你给一个提示，告诉他有多少个数字处在正确的位置上（称为"bulls" 公牛），以及有多少个数字处在错误的位置上（称为"cows" 奶牛），你的朋友使用这些提示找出那个神秘数字               //参考答案：巧妙的使用两个数组记录请求次数。时间复杂度o(n)        int bull=0,cows=0;        int len=secret.length();        int [] set=new int[10];        for(int i=0;i<len;i++){            if(secret.charAt(i)==guess.charAt(i)){                bull++;            } else {                if(set[secret.charAt(i)-'0']<0){                    cows++;                }                set[secret.charAt(i)-'0']++;                if(set[guess.charAt(i)-'0']>0){                    cows++;                }                set[guess.charAt(i)-'0']--;            }        }        return new StringBuilder().append(bull).append("A").append(cows).append("B").toString();    }}