Answer from cs61c-cf (Aspandiar Dahmubed 16812056) for Question 4
Not using the reference count method since its kind of circular and the reference count wouldn't work as well. 
Mark and sweep would work better since going down through all the roots, it would be able to ascertain which structures aren't really used and accordingly mark them to be sweeped. Actually, on second thought I am not sure one could do that since this structure links itself together in a weird circular fashion where sweeping one region might affect the rest.