public class Solution {
    public int singleNumber(int[] nums) {
        int one = 0;
        int two = 0;
        int mask = 0;
        for(int num : nums){
            two ^= num & one;
            //two |= num & one;
            one ^= num;
            mask = ~ (one & two);
            one &= mask;
            two &= mask;
        }

        return one;
    }
}

/*

00 => 01 => 10 => 00

    [7, 7, 7, 3]

one  0 0 0
two  0 0 0

first 7:
one  1 1 1
two  0 0 0

second 7:

one  0 0 0
two  1 1 1

third 7:

one  0 0 0
two  0 0 0

first 3:

one  0 1 1
two  0 0 0

return 3


extend to five times

000 => 001 => 010 => 011 => 100 => 000
    [7, 7, 7, 7, 7, 3]

    three ^= num & one & two;
    two ^= num & one;
    one ^= num;

    mask = ~ (one & ~two & three); //when == 101, go back to 000
    one &= mask;
    two &= mask;
    three &= mask;

*/