Answer from cs61c-aq (Anjali Jagannatha Koppal 16454392) for Question 4
1. the top left cell has no pointers to it, so it can be freed. once it is freed, there are no pointers to the top right and bottom-left cells and they can be freed too. finally the bottom right cell can be freed. thus it is possible to free the structure by reference count method.

2. starting from each of the roots, we can perform a DFS and mark all the cells, so the structure will not be freed by mark and sweep method.