course_schedule1.py

class Solution:
    def canFinish(self, numCourses: int, prerequisites: List[List[int]]) -> bool:
        reqs = defaultdict(list)
        for course, req in prerequisites:
            reqs[course].append(req)
        visited = set()
            
        def detect_cycle(i, visiting):
            visiting.add(i)
            for j in reqs[i]:
                if j in visited:
                    continue
                if j in visiting: 
                    return True
                if detect_cycle(j, visiting):
                    return True
            visiting.remove(i)
            visited.add(i)
            return False


        for i in range(numCourses):  
            if detect_cycle(i, set()): 
                return False
        return True