LeetCode 1120. Maximum Average Subtree

原题链接在这里：https://leetcode.com/problems/maximum-average-subtree/
题目：
Given the root of a binary tree, return the maximum average value of a subtree of that tree. Answers within 10-5 of the actual answer will be accepted.
A subtree of a tree is any node of that tree plus all its descendants.
The average value of a tree is the sum of its values, divided by the number of nodes.
Example 1:

1. Input: root = [5,6,1]
2. Output: 6.00000
3. Explanation:
4. For the node with value = 5 we have an average of (5 + 6 + 1) / 3 = 4.
5. For the node with value = 6 we have an average of 6 / 1 = 6.
6. For the node with value = 1 we have an average of 1 / 1 = 1.
7. So the answer is 6 which is the maximum.

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Example 2:

1. Input: root = [0,null,1]
2. Output: 1.00000

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Constraints:
<ul ><li >The number of nodes in the tree is in the range [1, 104].<li >0