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Perform simple graph traversal either DFS or BFS(you can do it even in parallel)

Garbage collector needs to iterate (may be DFS or BFS) and then build a graph for objects which are pointing to each other (may be using shared_ptr) using shared_ptr (original objects ) and weak_ptr (for objects which are pointing to original object). So if original objects goes away it can be deleted, because other objects which are pointing to it will be weak_ptr (which don;t increment reference count).

check this out
en.wikipedia.org/wiki/Garbage_collection_(computer_science)#Na.C3.AFve_mark-and-sweep