leetcode Binary Tree Level Order Traversal python

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        res=[]

        if root is None:
            return res
        level=0

        rs=self.levelOrderRe(root,level,res)

        return rs
    def levelOrderRe(self,tree,level,res):
        if tree is None:
            return res

        if len(res) < level+1:
            res.append([])
        res[level].append(tree.val)
        res=self.levelOrderRe(tree.left,level+1,res)
        res=self.levelOrderRe(tree.right,level+1,res)
        return res