Given a non-empty binary tree, return the average value of the nodes on each level in the form of an array.

Example 1:

Input: 3 / \ 920 / \ 157Output: [3, 14.5, 11] Explanation: Theaveragevalueofnodesonlevel0is3, onlevel1is14.5, andonlevel2is11.Hencereturn [3, 14.5, 11].

Note:

The range of node's value is in the range of 32-bit signed integer.

按层序从上到下遍历一颗树，计算每一层的平均值。

• 用一个队列即可实现。
• 第 102 题和第 107 题都是按层序遍历的。