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:


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:


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.
思路
这是一个猜数字的游戏。如果一个人猜到了该数字，且数字的位数和数字都正确，便称作“Bull”，如果位数不正确，但是数字正确，称作”Cow“。
理解了题意之后，我们可以考虑利用哈希表的方法来解决这道题。根据题意统计Bull和Cow的个数即可。注意，已经是Bull的数，就不再是Cow了。

