每日一题20201124*（637. 二叉树的层平均值）

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class Solution:
    def averageOfLevels(self, root: TreeNode) -> List[float]:
        if root is None:
            return []
        ans = []
        queue = [root]
        while len(queue) > 0:
            size = len(queue)
            # 记录这一层的总数
            total = 0
            for _ in range(size):
                current = queue.pop(0)
                total += current.val
                if current.left is not None:
                    queue.append(current.left)
                if current.right is not None:
                    queue.append(current.right)
            # ans数组添加进去平均值，不用担心size为0的问题，因为size肯定>0 
            # 因为queue后面的出队和入队已经不影响size了
            ans.append(total / size)
        return ans