LeetCode 筆記 - 207. Course Schedule

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題目在此 207. Course Schedule

給定一系列的課程與先修課程，請判斷是否有衝突

解題思維

這題本質上是在判斷是否有 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，一路上把節點放進 set 裡，一但看到先修課程已經在目前的路徑上
就表示 cycle 存在

程式碼

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class Solution:
    def canFinish(self, numCourses: int, prerequisites: List[List[int]]) -> bool:
        
        node = [[] for _ in range(numCourses)]
        for p in prerequisites:
            node[p[0]].append(p[1])
        
        visited = set()
        
        def dfs(index, cycle):
            if index in cycle:
                # cycle detected
                return False
            if index in visited:
                # visited before, so doesn't need dfs any more
                return True
            
            # put this course in dfs path
            current_cycle = cycle + [index]
            
            for i in node[index]:
                if not dfs(i, current_cycle):
                    return False
            # dfs is done and all course is ok!
            visited.add(index)
            
            return True
        
        for i in range(numCourses):
            if not dfs(i, []):
                return False
        return True

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