private Vertex getMinimum(Set<Vertex> vertexes) {
    Vertex minimum = null;
    for (Vertex vertex : vertexes) {
      if (minimum == null) {
        minimum = vertex;
      } else {
        if (getShortestDistance(vertex) < getShortestDistance(minimum)) {
          minimum = vertex;
        }
      }
    }
    return minimum;
  }