也可以用dictionary， 复习怎么initialize 一个空的dict：collections.defaultdict(list)

    1. # Definition for a binary tree node.
    2. # class TreeNode:
    3. #     def __init__(self, val=0, left=None, right=None):
    4. #         self.val = val
    5. #         self.left = left
    6. #         self.right = right
    7. class Solution:
    8.     def findLeaves(self, root: TreeNode): #-> List[List[int]]:
    9.         def helper(root):
    10.             if root == None:
    11.                 return -1
    12.             left = helper(root.left)
    13.             right = helper(root.right)
    14.             depth = max(left, right) + 1
    15.             if depth >= len(result):
    16.                 result.append([])
    17.             result[depth].append(root.val)
    18.             return depth
    19.         result = []
    20.         helper(root)
    21.         return result