Answer from cs61c-ac (U alias Vitoria Lok 16597715) for Question 4
This cannot be reclaimed using the reference count method because even if nothing pointed to the top left node, there will be pointers pointed the the last node and it will not be removed.

This can be reclaimed using the mark and sweep method because it uses depth-first traversal. When nothing points to the first node, it is simply not marked and the whole structure is later deleted.