leetcode-310-最小高度树

题目描述：

 

 

方法：剥洋葱

class Solution:
    def findMinHeightTrees(self, n: int, edges: List[List[int]]) -> List[int]:
        if n==1:return [0]
        e = collections.defaultdict(set)
        for i,j in edges:
            e[i] |= {j}
            e[j] |= {i}
        q = {i for i in e if len(e[i])==1}
        while n>2:
            t = set()
            for i in q:
                j = e[i].pop()
                e[j] -= {i}
                if len(e[j])==1:
                    t |= {j}
                n -= 1
            q = t
        return list(q)