LeetCode: Single Number II

LeetCode: Single Number II

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?

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public class SingleNumberII {
    public int singleNumber(int[] nums) {
        int ones = 0, twos = 0, threes = 0;
        for (int num : nums) {
            twos |= ones & num;
            ones ^= num;
            threes = ones & twos;
            ones &= ~threes;
            twos &= ~threes;
        }
        return ones;
    }
}

The truth is that I cannot understand this logic easily. I used those two links for details.