graph.putEdge(2, 1); // no-op in undirected case
    }
    assertThat(hasCycle(directedGraph)).isTrue();
    assertThat(hasCycle(undirectedGraph)).isFalse();
  }

  @Test
  public void hasCycle_threeAcyclicEdges() {
    for (MutableGraph<Integer> graph : graphsToTest) {
      graph.putEdge(1, 2);
      graph.putEdge(2, 3);
      graph.putEdge(1, 3);
    }
    assertThat(hasCycle(directedGraph)).isFalse();