从上到下打印出二叉树的每个节点，同一层的节点按照从左到右的顺序打印。

    例如:
    给定二叉树: [3,9,20,null,null,15,7],
    3
    / \
    9 20
    / \
    15 7
    返回：
    [3,9,20,15,7]

    提示：
    节点总数 <= 1000

    1. # Definition for a binary tree node.
    2. # class TreeNode:
    3. #     def __init__(self, x):
    4. #         self.val = x
    5. #         self.left = None
    6. #         self.right = None
    8. class Solution:
    9.     def levelOrder(self, root: TreeNode) -> List[int]:
    10.         if root is None:
    11.             return []
    12.         queue = []
    13.         queue.append(root)
    14.         res = []
    15.         while queue:
    16.             root = queue.pop(0)
    17.             res.append(root.val)
    18.             if root.left is not None:
    19.                 queue.append(root.left)
    20.             if root.right is not None:
    21.                 queue.append(root.right)
    22.         return res