注意的点：

1、队列注意用popleft
2、注意用len(queue)控制层数

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:from collections import dequeq = deque([])if not root: return []q.append(root)res = []while len(q) != 0:l = len(q)level = []for _ in range(l):t = q.popleft()level.append(t.val)if t.left:  q.append(t.left)if t.right: q.append(t.right)res.append(level)return res