LeetCode 筆記 - 210. Course Schedule II

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題目在此 210. Course Schedule II

給定一系列的課程與先修課程，請給出沒有衝突的修課順序

解題思維

如果你沒解過第一題可以先看一下 LeetCode 筆記 - 207. Course Schedule

這題本質上一樣是在判斷是否有 cycle 存在，詳見 Cycle detection

那我們可以使用 Depth First Search 來處理這個問題
搜尋方向是由課程往先修課程前進，而最頂的先修課程必定會先結束 Depth First Search

所以就按照結束的順序，將課程收集起來即可

程式碼

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class Solution:
    def findOrder(self, numCourses: int, prerequisites: List[List[int]]) -> List[int]:
        
        node = {i:[] for i in range(numCourses)}
        for p in prerequisites:
            node[p[0]].append(p[1])
        
        visited = collections.OrderedDict()
        cycle = set()
        def dfs(index):
            if index in cycle:
                # in dfs path
                return False
            if index in visited.keys():
                # visited before, so doesn't need dfs any more
                # print(index)
                return True
            
            # put this course in dfs
            cycle.add(index)
            for i in node[index]:
                if not dfs(i):
                    return False
            cycle.remove(index)
            # dfs is done and all course is ok!
            visited.update({index:0})
            return True
        
        for i in range(numCourses):
            if not dfs(i):
                return []
        return visited

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