剑指 Offer 32 - II. 从上到下打印二叉树 II

从上到下按层打印二叉树，同一层的节点按从左到右的顺序打印，每一层打印到一行。
例如:
给定二叉树: [3,9,20,null,null,15,7],

3<br />   / \<br />  9  20<br />    /  \<br />   15   7<br />返回其层次遍历结果：

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

提示：
节点总数 <= 1000

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

class Solution:
    def levelOrder(self, root: TreeNode) -> List[List[int]]:
        if root is None:
            return []
        queue = []
        queue.append(root)
        res = []
        path = []
        temp = []
        while queue:
            path = []
            temp = []
            while queue != []:
                temp.append(queue.pop(0))
            while temp != []:
                root = temp.pop(0)
                path.append(root.val)
                if root.left is not None:
                    queue.append(root.left)
                if root.right is not None:
                    queue.append(root.right)
            res.append(path)
        return res