what is the worst-case running time for finding two given vertices are adjacent in Adjacency matrix implmentation of a graph

What is the worst-case running time for finding two given vertices are adjacent in Adjacency matrix implmentation of a graph? Is that not O(1) as I know the indices of those vertices in the matrix so that I can pick the value in constant time开发者_开发知识库? I read it as O(n^2) in a book. Someone please explain it how to get to this measure.

Thanks

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An adjacency matrix occupies O(n^2) memory, which may be where you're confused. But yes lookup given two vertices is O(1), that's the advantage of an adjacency matrix.

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yes. it is O(1) for the reasons stated by you.