leetcode -- Minimum Height Trees -- 关于graph的,重点
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https://leetcode.com/problems/minimum-height-trees/
有trick
对graph不熟悉。再看几遍。
参考http://bookshadow.com/weblog/2015/11/26/leetcode-minimum-height-trees/
基本思路是“逐层删去叶子节点,直到剩下根节点为止”
有点类似于拓扑排序
最终剩下的节点个数可能为1或者2
时间复杂度:O(n),其中n为顶点的个数。
记住这些结论以及这样的code怎么写就行了。就跟拓扑排序一样,逐次去掉leaf node
class Solution(object): def findMinHeightTrees(self, n, edges): """ :type n: int :type edges: List[List[int]] :rtype: List[int] """ children = collections.defaultdict(set) for s, t in edges:#记录edge children[s].add(t) children[t].add(s) vertices = set(children.keys()) while len(vertices) > 2: leaves = [x for x in children if len(children[x]) == 1] for x in leaves: for y in children[x]: children[y].remove(x) del children[x] vertices.remove(x) return list(vertices) if n != 1 else [0]
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