python DFS 判断有向图是否有环

DFS 判断有向图是否存在环
vis ：DFS 遍历的结果
trace：也是遍历的结果，需要对结果进行分析，找到重复值时，判断重复值的索引位置，依次进行遍历，输出结果

##
graph = {
    "a": ["b", "c"],
    "b": ["a", "d"],
    "c": ["a", "d"],
    # "d": [ "e"],
    "d": ["c", "e"],  # add loop
    "e": ["d"]
}

# print(graph["a"])

vis = []
trace = []


def dfs(v):
    if v in vis:
        if v in trace:
            v_index = trace.index(v)
            print("有环：")
            for i in range(v_index, len(trace)):
                print(trace[i] + ' ', end='')
            print(v)
            print("n")
            return
        return

    vis.append(v)
    trace.append(v)
    for vs in graph[v]:
        dfs(vs)
    trace.pop()

    # print(vis)
    # return vis


dfs("a")
# print(dfs("a"))

# print(vis)

结果展示

有环：
a b a


有环：
a b d c a


有环：
d c d


有环：
d e d