Answer from cs61c-cp (Raymond Chou 16078389) for Question 4
ref. count: yes, because it's not a circular structure (and not also doubly linked) Thus, eventually all the nodes will eventually have a ref count of 0 (nothing pointing to it), and be collected.

mark and sweep: yes, since doing a depth search it will recognize that the top left is the root, and marks which nodes can be reached. If a node is freed, it is unmarked and will get collected during the sweep phase.