前言：有点忘记是怎么判断一个图中是否是有环

如果是一个无向图，其实可以直接dfs，加上一个vis数组来一起判断

如果是有向图呢，

class Solution:
    def canFinish(self, numCourses: int, prerequisites: List[List[int]]) -> bool:
        n = numCourses
        ed = [[] for _ in range(n)]
        for x in prerequisites:
            u,v = x
            ed[v].append(u)

        cnt = [0]*(n)
        flag = False
        def dfs(x):
            nonlocal flag
            cnt[x] = 1
            if flag:
                return
            for v in ed[x]:
                if cnt[v] == 0:
                    dfs(v)
                if cnt[v] == 1:
                    flag = True
                    return
            cnt[x] = 2
        
        for i in range(n):
            if cnt[i] == 0:
                dfs(i)
        return not flag