参考代码：

class TreeNode:
    def __init__(self, val=None, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right


class Solution:
    def levelOrder(self, root):
        self.result = []
        if not root:
            return self.result
        q = [root]
        while q:
            new_q = []
            self.result.append([n.val for n in q])
            for node in q:
                if node.left:
                    new_q.append(node.left)
                if node.right:
                    new_q.append(node.right)
            q = new_q
        return list(reversed(self.result))