I've struggled for this problem for a while and finally after I change the declaration order of my two ArrayList, I passed the tests. But I am still confused about why the order is so important, as the requirement also mentioned. For people's reference, I originally declared the list divisible by two first and the list divisible by three came second and I can't pass the tests. After I swap the order the declarations, suddenly I made it! Really confused, any hints would be appreciated.