python - python 中的 dfs 不工作请指导我有关错误

Question

def dfs(graph, start, visited=None):

    if visited is None:
        visited = set()
    if start in visited:
        return
    visited.add(start)
    for a in graph[start]:
        if a not in visited:
            dfs(graph, a, visited)
    return visited
graph={'A':['B'],'B':['C']}

b=dfs(graph,'A')

print(b)

Answer 1

graph[start] 将提高 KeyError如果 start 不在字典中。您可以将其替换为 dict.get返回默认值(空序列):

def dfs(graph, start, visited=None):
    if visited is None:
        visited = set()
    if start in visited:
        return
    visited.add(start)
    for a in graph.get(start, []):  # <---
        if a not in visited:
            dfs(graph, a, visited)
    return visited