networksToTest = ImmutableList.of(mutableDirectedNetwork, mutableUndirectedNetwork);
    directedNetwork = mutableDirectedNetwork;
    undirectedNetwork = mutableUndirectedNetwork;
  }

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

  @Test
  public void hasCycle_isolatedNodes() {