Single Number II

题目要求：

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?

 

明确一下要求：

1.  线性时间复杂度 O(n)

2.  常量的空间复杂度O(1)

 

class Solution {
public:
    int singleNumber(int A[], int n) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        int ones = 0, twos = 0;
        int commonBitMask = 0;
        for (int i = 0; i < n; ++i) {
            twos |= (ones & A[i]); //  此时twos中包含二次和三次的位
            ones ^= A[i]; //此时ones中包含一次和三次的位
            commonBitMask = ~(twos & ones); // 过滤掉三次的位
            ones &= commonBitMask;
            twos &= commonBitMask;
        }
        return ones;
    }
};