LeetCode - Refresh - Single Number II

class Solution {
public:
    int singleNumber(int A[], int n) {
        int ones = 0, twos = 0, threes = 0;
        for (int i = 0; i < n; i++) {
            twos |= (ones & A[i]);
            ones ^= A[i];
            threes = ~(ones & twos);
            ones &= threes;
            twos &= threes;
        }
        return ones;
    }
};