题目：
Given an array of integers, every element appears three times
except for one. Find that single one.
Note:
Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?
我的解法：
（1）算法思想：
本题是Single Number 的升级版（见博文《Single Number》）。先用快排算法将数组排序，然后遍历一次数组即可：对于i，如果A[i]=A[i+1]，则将i加上3，否则返回A[i].
（2）代码如下：
原文：http://blog.csdn.net/littlebob180/article/details/42834255