LeetCode - 102-- binary tree hierarchy traversal (python)

Given a binary tree, the node returns its value hierarchical traversal. (Ie, layer by layer, from left to right to access all nodes).

For example:
Given a binary tree: [3,9,20, null, null, 15,7],

    3
    / \
  920
  / \
157
to return to its level through the results:

[
[3],
[9,20],
[15,7]
]

 1 class Solution:
 2     def levelOrder(self, root: TreeNode) -> List[List[int]]:
 3         levels= []
 4         if not root:
 5             return levels
 6         def helper(node ,level):
 7             if len(levels)==level:
 8                 levels.append([])
 9             levels[level].append(node.val)
10             if node.left:
11                 helper(node.left,level+1)
12             if node.right:
13                 helper(node.right,level+1)
14         helper(root,0)
15         return levels

Source: https: //leetcode-cn.com/problems/binary-tree-level-order-traversal/solution/er-cha-shu-de-ceng-ci-bian-li-by-leetcode/