如何从城市顶点构建无向图

class Graph:
    def __init__(self):
        self.vertices = {}  # 用字典表示邻接表

    def add_vertex(self, city):
        self.vertices[city] = []

    def add_edge(self, city1, city2):
        if city1 in self.vertices and city2 in self.vertices:
            self.vertices[city1].append(city2)
            self.vertices[city2].append(city1)

    def get_neighbors(self, city):
        if city in self.vertices:
            return self.vertices[city]
        else:
            return []

# 创建一个无向图对象
graph = Graph()

# 添加城市顶点
graph.add_vertex("北京")
graph.add_vertex("上海")
graph.add_vertex("广州")
graph.add_vertex("深圳")

# 添加城市之间的边
graph.add_edge("北京", "上海")
graph.add_edge("北京", "广州")
graph.add_edge("上海", "深圳")
graph.add_edge("广州", "深圳")

# 获取某个城市的邻居城市
neighbors = graph.get_neighbors("北京")
print(neighbors)  # 输出：['上海', '广州']