leetcode1120

 1 class Solution:
 2     def __init__(self):
 3         self.maxAvg = 0
 4 
 5     def preOrder(self,node):
 6         if node == None:
 7             return 0,0
 8         leftcount,leftsum = self.preOrder(node.left)
 9         rightcount,rightsum = self.preOrder(node.right)    
10         totalcount = leftcount + rightcount + 1
11         totalsum = leftsum + rightsum + node.val
12         avg = totalsum / totalcount
13         self.maxAvg = max(self.maxAvg,avg)
14         return totalcount,totalsum
15 
16     def maximumAverageSubtree(self, root: TreeNode) -> float:
17         self.preOrder(root)
18         return self.maxAvg

后序遍历，遍历的同时计算节点的数量、节点的和，并求平均值。