# 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]]:
        from collections import deque # 导入 deque
        result = []
        if root is None:
            return result
        queue = deque()
        queue.append(root)
        while len(queue) > 0:
            level_node_nums = len(queue)
            level_nodes = []
            for _ in range(level_node_nums):
                node = queue.popleft() # popright
                level_nodes.append(node.val) # append 默认从右边插入，等价于 appendright
                if node.left is not None:
                    queue.append(node.left)
                if node.right is not None:
                    queue.append(node.right)
            result.append(level_nodes)
        return result