637. 二叉树的层平均值

https://leetcode-cn.com/problems/average-of-levels-in-binary-tree/

解法一：层序

层序遍历，每次求每层平均值输出

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
import queue
class Solution:
    def averageOfLevels(self, root: TreeNode) -> List[float]:        
        if not root:    #边界
            return []
        res = []
        q = queue.Queue()
        q.put(root)
        while not q.empty():
            count = q.qsize()  #该层大小
            sum = 0
            for _ in range(count):
                node = q.get()
                sum += node.val
                if node.left:
                    q.put(node.left)
                if node.right:
                    q.put(node.right)
            res.append(sum/count)
        return res