[LeetCode] 103: Single Number II

[Problem]

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?

[Solution]

class Solution {
public:
int singleNumber(int A[], int n) {
// Note: The Solution object is instantiated only once and is reused by each test case.

// invalid
if(n <= 0)return 0;

int ones = 0, twos = 0, mask = 0;
for(int i = 0; i < n; ++i){
twos |= (ones & A[i]);
ones ^= A[i];
mask = ~(ones & twos);
ones &= mask;
twos &= mask;
}

// the one could occurs one time or two times, but no more than three times
return ones == 0 ? twos : ones;
}
};

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