Arrays.sort use Quick Sort (Worst case complexity of O(n^2)) when passing an array of primitive values, and Merge Sort (Worst case complexity of O(n log(n))) when using an array of object references.

I assume the problem has got some anti-tests against inefficient sorting.

Java sorts primitive types by quick sort which is O(N^2) in the worst case, while it sorts Objects in O(nlogn) using merge sort I think.

Learn to read the API documentation (for your own good).