public void sortByFour()
    {
        int zero = -1;
        int one = 0;
        int two = _arr.length;
        int three = _arr.length;

        while (one < two)
        {
            switch (_arr[one] % 4)
            {
                case 1: { one++; break; }
                case 0: { zero++; swap(zero,one); one++; break; }
                case 2: { two--; swap(one,two); break; }
                default: { two--; three--; swap(two,three); swap(three,one); break; }
            }
        }
    }

    private void swap(int a, int b)
    {
        int tmp = _arr[a];
        _arr[a] = _arr[b];
        _arr[b] = tmp;
    }