Answer from cs61c-ev (Roland Carlos 16583196) for Question 4
Yes, this structure can be reclaimed with reference count. When an object has nothing pointing to it, it cannot be accessed and those reference count will delete it (the object will have a reference count of zero). The top-left object has nothing pointing to it and will thus be reclaimed.

No, this structure cannot be reclaimed with mark and sweep. Starting from the top-left object, you can follow the pointers all the way that they will "mark" all four objects, keeping them safe from reclaiming.