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As a graph is, by definition, a set of vertices and edges, checking if a graph is empty is equivalent to check if the edges set is empty, constant time. This result should not vary between data structure representations. What about removing that weird example ? Or at least, make it true. Because for the moment, it is meaningless (at least to me). — Preceding unsigned comment added by 18.104.22.168 (talk) 19:00, 22 January 2012 (UTC)
You didn't read the first sentence of the artickle, did you? Specifically the part about only having access to the graph via questions of the form "Is there an edge between vertex u and vertex v?" —David Eppstein (talk) 19:09, 22 January 2012 (UTC)