15、Single-number-ii
现在有一个整数类型的数组，数组中只有一个元素只出现一次，其余元素都出现三次。
注意：
你需要给出一个线性时间复杂度的算法，你能在不使用额外内存空间的情况下解决这个问题么？
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?
思路：设计出现3次即将归0的位运算，详情参考 https://leetcode-cn.com/problems/single-number-ii/solution/single-number-ii-mo-ni-san-jin-zhi-fa-by-jin407891/