Answer from cs61c-ei (Jing Chen 16669246) for Question 4
The structure can be reclaimed using the reference count method but cannot be using the mark and sweep method. 

Reference counting garbage collection keeps track of how many copies there are of a pointer to a particular object. When this number is 0, the object can no longer be reached from any root and can be deleted. Because this structure is circular, it will always have pointers to itself, even when they cannot be reached from the root. Thus, this will not be deleted as garbage when it should be. 

In the mark and sweep method, objects are marked by traversal starting at the root to see whether or not they can be reached. If not, they cannot be reached, they are considered as garbage. In this case, because this structure is not connected to the root in any way, it will be cleaned up by the mark and sweep method of garbage collection.