利用python找出偏序集中极大元、极小元、最大元和最小元

import networkx as nx
import matplotlib.pyplot as plt
elements = [1, 2, 3, 4, 6, 9, 12, 18]
relations = [(a, b) for a in elements for b in elements if a != b and b % a == 0]
G = nx.DiGraph()
G.add_nodes_from(elements)
G.add_edges_from(relations)
maximals = []
for node in G.nodes():
   if len(list(nx.descendants(G, node))) == 0:
       maximals.append(node)
print("极大元为：", maximals)
minimals = []
for node in G.nodes():
   if len(list(nx.ancestors(G, node))) == 0:
       minimals.append(node)
print("极小元为：", minimals)
if len(maximals) != 0:
   print("没有最大元")
else:
   print("最大元为：", max(maximals))
if len(minimals) == 0:
   print("没有最小元")
else:
   print("最小元为：", min(minimals))