leetcode刷題筆記-graph

310. Minimum Height Trees

思路: 最短的MHT的root是最長的路徑的中間節點。所以最多隻會有2個root。計算所有點的入度，每次去掉入度爲1的節點。對應的鄰居就要入度-1.再次重複步驟去掉入度爲1的節點，直到剩下小於2個點。 

class Solution(object):
    def findMinHeightTrees(self, n, edges):
       
        if n == 1: return [0]
        neighbors = collections.defaultdict(list)
        degress = collections.defaultdict(int)
        
        for u,v in edges:
            neighbors[u].append(v)
            neighbors[v].append(u)
            degress[u] += 1
            degress[v] += 1
        
        level = []
        for i in xrange(n):  # 找到入度爲1的
            if degress[i] == 1:
                level.append(i)
        unvisited = set(range(n))
        while len(unvisited) > 2:  # 這裏是unvisited >2 不是len(level)
            nextLevel = []
            for u in level:
                unvisited.remove(u)
                for neighbor in neighbors[u]:
                    if neighbor in unvisited:
                        degress[neighbor] -= 1
                        if degress[neighbor] == 1:
                            nextLevel.append(neighbor)
            level = nextLevel
        return level