* EndpointPair}s, we keep track of the nodes that we have visited. When processing endpoint
   * pairs, we skip if the "other node" is in the visited set, as shown below:
   *
   * <pre>
   * Nodes = {N1, N2, N3, N4}
   *    N2           __
   *   /  \         |  |
   * N1----N3      N4__|
   *
   * Visited Nodes = {}
   * EndpointPair [N1, N2] - return