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The diameter of a graph is the largest of all shortest-path distances in it. How can we find a tree of maximum diameter within an undirected unweighted graph?

Note that the tree does not have to be a spanning tree.

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  • $\begingroup$ It makes no difference whether the tree is spanning or not. If there is a tree of diameter $d$, there is a spanning tree of diameter $d$ or greater so the question "Is there a tree of diameter $d$?" has the same complexity as "Is there a spanning tree of diameter $d$?" $\endgroup$ – David Richerby Nov 6 '14 at 12:46
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This (probably) can't be done efficiently since a maximum-diameter spanning tree could be a Hamiltonian path.

More specifically, an $n$-vertex graph has a tree of diameter $n-1$ as a subgraph if, and only if, it has a Hamiltonian path (the tree is the Hamiltonian path). Therefore, the problem of finding a maximum-diameter subtree is NP-hard, since you can reduce Hamiltonian path to it by finding the maximum-diameter subtree and checking whether or not it's a path of length $n-1$.

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  • $\begingroup$ The problem includes the Hamiltonian path problem, and therefore is NP complete. $\endgroup$ – Bangye Nov 6 '14 at 12:40
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    $\begingroup$ @Bangye Careful. $\Sigma^*$ includes Hamiltonian path, too. $\endgroup$ – Raphael Nov 6 '14 at 12:58

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