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3. A directed acyclic graph (DAG) is a directed graph without cycles (the underlying graph may have cycles, so it is not atree). Show that a DAG has a labeling of its nodes (that is alabeling 1, n of its nodes) such that every arc goes from a lower-numbered node to a higher-numbered node. (You need to find a way to do it, and also show that is always possible)

Fig: 1