DFS，在无向图中找到圆

Question

This is my approach to deal with this problem. But it seems this doesn't work as I expected. What's wrong?

def dfs(graph, start, visited = None): '''find if there is a circle in the graph, if there is ,return True''' if visited == None: visited = set() visited.add(start) for next in graph[start]: if next in visited: return True else: dfs(graph, next, visited) return False

Answer 1

Since it is a Undirected graph, you have to take care of parent of current node, suppose you go from node 1 to node 2, then while traversing the edges of node 2 you will again find node 1 and it has already been visited so the answer will return true, ie cycle exists, but it might not be the case, so you need to add another parameter to check if adjacent node is not the parent of current node. Here the parent node refers to the immediate predecessor of current node in the DFS Traversal.