def METHOD_NAME(self):
    """
    @brief Boundary node indicator.
    @return: A bool tensor with shape (NN, ) to indicate if a node is\
             on the boundary or not.
    """
    NN = self.number_of_nodes()
    edge = self.edge
    isBdEdge = self.boundary_edge_flag()
    isBdNode = torch.zeros((NN,), dtype=torch.bool, device=self.device)
    isBdNode[edge[isBdEdge, :]] = True
    return isBdNode