E637. Average of Levels in Binary Tree

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def averageOfLevels(self, root: Optional[TreeNode]) -> List[float]:
        levels = []
        d = []
        def helper(node, level):
            try :
                levels[level].append(node.val)
            except:
                levels.append([])
                levels[level].append(node.val)
            if node.left:
                helper(node.left, level + 1)
            if node.right:
                helper(node.right, level + 1)
        helper(root, 0)
        for i in levels:
            d.append(sum(i)/len(i))
        return d