Algorithm-Implementation | Personal implementation of algorithms | Learning library

As already said, a first quick thing to fix is your typo : the even case should read T temp = aTemp.get(i);

The crux of the matter is that you did not grasp how / when the list/array is modified in place, versus when it is copied over as a result to accumulate.

Without even looking at what your method is doing, we can see that the array version manipulates the array in place, except when its size is one in which case it is copied to the list of results.

In other words, in the array version, it always is the same array that has its items swapped, however deep the recursion goes. Only when we want to remember a certain permutation of the items do we take a copy of it, to make sure the permutation is frozen (a.clone()), meaning we can still swap items after that without risking any modification of the already accumulated permutations.

The list version, on the other hand, copies its input each time it starts. In other words, at each recursion stage a local copy of the original list is used to swap items. When the recursion unrolls, the current arrangment of items of this copy is lost.

So if you remove the cloning of the list where it is, only keeping it inside the if size == 1) case, you should be OK.

In the end, it's not that there is a subtelty in the list case versus the ararys, just that you moved some "cloning" logic without analyzing the impact.

With that in place, the output becomes :