public static int singleNumber(int[] nums) {
      int once = 0;     int twice = 0;     for (int i = 0; i < nums.length; i++) {
          twice |= once & nums[i];         once ^= nums[i];         int not_three = ~(once & twice);         once = not_three & once;         twice = not_three & twice;     }     return once; }