Answer from cs61c-ew (Joo-Rak Son 16103505) for Question 4
It can be reclaimed by reference count method, because it is not a circular structure. Firstly, the upper-left corner object has zero reference, so it will be removed, and then the other two adjacent to it, and lastly the object at lower-right hand corner will be removed.

Mark and sweep will also reclaim this structure, because starting from the root, the marker will never come across this structure (since no external pointer points to it).