@Test
public void testRoot() {

	Node node1 = new Node(1, 1);
	Node node2 = new Node(1, 2);
	Set<Node> nodes = new HashSet<>(Arrays.asList(node1, node2));
	Map<Node, Collection<Node>> nodeMap = GraphUtil.getNodeMap(
		nodes, Collections.emptyMap());
	Assert.assertEquals(nodeMap.toString(), 1, nodeMap.size());
	Assert.assertEquals(nodes, nodeMap.get(new Node(0, 0)));
}