/**
* Adds vertex to a graph. If vertex with the same id already exists,
* throws and exception
* @param Vertex $vertex
* @throws \Exception
*/
public function addVertex(Vertex $vertex)
{
$id = $vertex->getId();
if (array_key_exists($id, $this->vertices)) {
throw new \Exception('Vertex with ID="' . $id . '" already exists');
}
$this->vertices[$id] = $vertex;
}