Leetcode207

numCourses->总的课程数目

Prerequisited->pair in a list denoting have to finish b to study a

 

class Solution:

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

  

        if numCourses <=1 : return True

        # 入度数组，一开始全部为 0

        in_degrees = [0 for _ in range(numCourses)]

        # 邻接表

        adj = [set() for _ in range(numCourses)]

        # 想要学习课程 0 ，你需要先完成课程 1 ，我们用一个匹配来表示他们: [0,1]

        # [0,1] 表示 1 在先，0 在后

        # 注意：邻接表存放的是后继 successor 结点的集合

        for second, first in prerequisites:

            in_degrees[second] += 1

            adj[first].add(second)

        # print("in_degrees", in_degrees)

        # 首先遍历一遍，把所有入度为 0 的结点加入队列

        res = []

        queue = []

        for i in range(numCourses):

            if in_degrees[i] == 0:

                queue.append(i)

        counter = 0

        while queue:

            top = queue.pop(0)

            counter += 1

            for successor in adj[top]:

                in_degrees[successor] -= 1

                if in_degrees[successor] == 0:

                    queue.append(successor)

        return counter == numCourses