207

class Solution:
    def canFinish(self, numCourses: int, prerequisites: List[List[int]]) -> bool:

        g = defaultdict(list)
        visited = defaultdict(int)

        for v in range(numCourses):
            g[v] = []
        for a, b in prerequisites:
            g[b].append(a)


        def dfs(v):
            visited[v] = 1

            for n in g[v]:
                if visited[n] == 1 or (visited[n] == 0 and not dfs(n)):
                    return False

            visited[v] = 2
            return True

        for v in g:
            if visited[v] == 0 and not dfs(v):
                return False

        return True