如何用Python编写深度优先搜索算法？

# 定义图的类class Graph:    def __init__(self, vertices):        self.V = vertices  # 节点数量        self.adj = [[] for _ in range(self.V)]  # 存储节点的邻接节点            # 添加边    def add_edge(self, u, v):        self.adj[u].append(v)            # DFS递归函数    def dfs_util(self, u, visited):        visited[u] = True  # 标记当前节点为已访问                print(u, end=' ')  # 输出当前节点                # 遍历当前节点的所有邻接节点        for i in self.adj[u]:            if not visited[i]:                self.dfs_util(i, visited)                # 对外接口，执行DFS    def dfs(self, u):        visited = [False] * self.V  # 标记所有节点均未访问                self.dfs_util(u, visited)        # 测试代码if __name__ == '__main__':    # 创建一个具有4个节点的图    g = Graph(4)        # 添加图的边    g.add_edge(0, 1)    g.add_edge(0, 2)    g.add_edge(1, 2)    g.add_edge(2, 0)    g.add_edge(2, 3)    g.add_edge(3, 3)        print("深度优先遍历结果：")    g.dfs(2)