MutableNetwork<Integer, String> directedGraph =
        NetworkBuilder.directed().allowsParallelEdges(true).allowsSelfLoops(true).build();
    directedGraph.addEdge(N1, N3, E13);
    directedGraph.addEdge(N3, N1, E31);
    directedGraph.addEdge(N1, N2, E12);
    directedGraph.addEdge(N1, N2, E12_A);
    directedGraph.addEdge(N1, N1, E11);
    directedGraph.addEdge(N3, N4, E34);

  @Test
  public void hasCycle_emptyGraph() {
    assertThat(hasCycle(directedGraph)).isFalse();
    assertThat(hasCycle(undirectedGraph)).isFalse();
  }

  @Test
  public void hasCycle_isolatedNodes() {
    for (MutableGraph<Integer> graph : graphsToTest) {
      graph.addNode(1);
      graph.addNode(2);
    }
    assertThat(hasCycle(directedGraph)).isFalse();
    assertThat(hasCycle(undirectedGraph)).isFalse();
  }