leetcode-第5周双周赛-1136平行课程

方法一：

class Solution(object):
    def minimumSemesters(self, N, relations):
        """
        :type N: int
        :type relations: List[List[int]]
        :rtype: int
        """
        dic = {}
        dic2 = {}
        for i in range(1, N+1):
            dic[i] = 0
            dic2[i] = []
        for r in relations:
            dic[r[1]]+=1
            dic2[r[0]].append(r[1])
        ans = 0
        keys = dic.keys()
        while keys:
            arr = []
            for k in keys:
                if dic[k] == 0:
                    arr.append(k)
            if not arr: return -1
            for k in arr:
                if k in dic2:
                    for w in dic2[k]:
                        dic[w] -= 1
                del dic[k]
            ans += 1
            keys = dic.keys()
        return ans