137. Single Number II

1.描述

Given an array of integers, every element appears three times except for one, which appears exactly once. Find that single one.

Note:
Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

2.分析

想了很久没想出比较好的办法，参考了下面的解题技巧：
http://www.cnblogs.com/grandyang/p/4263927.html

3.代码

int singleNumber(int* nums, int numsSize) {
    if (NULL == nums || numsSize <= 0) exit(-1);
    int a = 0, b = 0;
    for (unsigned int i = 0; i < numsSize; ++i) {
        b = (b ^ nums[i]) & ~a;
        a = (a ^ nums[i]) & ~b;
    }
    return b;
}