Leetcode** 210. Course Schedule II

Example 1:
Input: numCourses = 2, prerequisites = [[1,0]]
Output: [0,1]
Explanation: There are a total of 2 courses to take. To take course 1 you should have finished course 0. 
So the correct course order is [0,1].

Example 2:
Input: numCourses = 4, prerequisites = [[1,0],[2,0],[3,1],[3,2]]
Output: [0,2,1,3]
Explanation: There are a total of 4 courses to take. To take course 3 you should have finished both courses 1 and 2. 
Both courses 1 and 2 should be taken after you finished course 0.
So one correct course order is [0,1,2,3]. Another correct ordering is [0,2,1,3].

Example 3:
Input: numCourses = 1, prerequisites = []
Output: [0]

class Solution(object):
    def findOrder(self, numCourses, prerequisites):
        """
        :type numCourses: int
        :type prerequisites: List[List[int]]
        :rtype: List[int]
        """
        self.pre = collections.defaultdict(list)
        for p in prerequisites:
            self.pre[p[0]].append(p[1])
        self.visited = [0]*numCourses
        self.path = []
        for i in range(numCourses):
            if not self.dfs(i):
                return []
        return self.path
    
    def dfs(self, i):
        if self.visited[i] == 1: return True
        if self.visited[i] == 2: return False
        self.visited[i] = 2
        for b in self.pre[i]:
            if not self.dfs(b): return False
        self.visited[i] = 1
        self.path.append(i)
        return True