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
   / \
  9  20
    /  \
   15   7
Output: [3, 14.5, 11]
Explanation:
The average value of nodes on level 0 is 3,  on level 1 is 14.5, and on level 2 is 11. Hence return [3, 14.5, 11].

Note:

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

# 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):
        """
        :type root: TreeNode
        :rtype: List[float]
        """
        if not root:
            return None
        else:
            ans=[]
            
            que=[root]
            
            while que:
                n=len(que)
                sum=0
                for i in range(n):
                    r=que.pop(0)
                    sum+=r.val
                    if r.left is not None:
                        que.append(r.left)
                    if r.right is not None:
                        que.append(r.right)
                ans.append(sum/n)
            
            return ans