Answer from cs61c-av (Kenneth Wong 16192104) for Question 4
Reference count method will not work because it is a circular structure, meaning it will always have a pointer to itself. And if there is a pointer to itself, reference count can not claim the object as garbage because the pointer count is more than  0.

Mark and sweep method will work because using depth first traversals, this circular object will never be reached or marked.  Thus it will be considered as garbage and swept away.