leetcode:课程表

思路：
1.记住已经可以选的课
2.然后用map记录先修课，用set做减法找到满足全部先修课的课加入可以选的课

代码：

class Solution:
    def canFinish(self, numCourses: int, prerequisites: List[List[int]]) -> bool:
        total = set([i for i in range(numCourses)])
        # 暂时不能学的课
        preCourses = set([item[0] for item in prerequisites])
        # 目前可以学的课
        now_acquire = total - preCourses
        if len(now_acquire) == 0:
            return False

        print(now_acquire)
        # 一门课程可能有多个先修课
        preClassDict = defaultdict(set)
        for item in prerequisites:
            preClassDict[item[0]].add(item[1])
        
        while True:
            after_acquire = copy.deepcopy(now_acquire)
            for myclass in preClassDict:
                if len(preClassDict[myclass]) != 0:
                    preClassDict[myclass] -= now_acquire
                    if len(preClassDict[myclass]) == 0:
                        after_acquire.add(myclass)
            if len(after_acquire) == numCourses:
                return True
            if len(after_acquire) == len(now_acquire):
                return False
            else:
                now_acquire = copy.deepcopy(after_acquire)
        return False

总结：
set的使用