getGraphDiameter(木の直径を求める)

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auto res = getGraphDiameter(g,s);

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struct getGraphDiameter_return {
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  int u, v, cost;
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};
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getGraphDiameter_return getGraphDiameter(DirectedGraph &_g) {
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  auto u = getFarthestVertex(_g, 0);
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  auto v = getFarthestVertex(_g, u.v);
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  return {u.v, v.v, v.cost};
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}