207. 课程表 - 力扣（LeetCode）

大佬图示

可以正确执行

会形成Loop

代码

# encoding = utf-8
# 开发者：Alen
# 开发时间： 21:13 
# "Stay hungry，stay foolish."

class Solution(object):
    def canFinish(self, numCourses, prerequisites):
        """
        :type numCourses: int
        :type prerequisites: List[List[int]]
        :rtype: bool
        """
        preMap = [[] for _ in range(numCourses)]

        for crs, pre in prerequisites:
            preMap[crs].append(pre)

        # preMap = [
        #     [],      # 课程 0 没有先修课
        #     [0],     # 课程 1 需要先修 0
        #     [0],     # 课程 2 需要先修 0
        #     [1]      # 课程 3 需要先修 1
        # ]

        # visitSet = all courses along the curr DFS path
        visitSet = set()

        def dfs(crs):
            if crs in visitSet:
                return False
            if preMap[crs] == []:
                return True
            visitSet.add(crs)

            for pre in preMap[crs]:
                if not dfs(pre):
                    return False
            visitSet.remove(crs)
            preMap[crs] = []
            return True

        for crs in range(numCourses):
            if not dfs(crs):
                return False
        return True

结果

解题步骤：www.bilibili.com