Answer from cs61c-ed (Cory Benavides 14101530) for Question 4
It can be erased by reference counting since the upper left node has no references to it.  After it is removed, the other nodes will end up with no references, and they will likewise be reclaimed.

I can be reclaimed by the mark & sweep method.  After traversing and marking all the nodes from the roots, this structure will remain unmarked so it will be swept away.