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[List[int]]:
    10.         if not root: return []
    11.         queue = [root]
    12.         ans = []
    13.         while queue:
    14.             tmp_ans = []
    15.             for node in queue[:]:
    16.                 if node.left:
    17.                     queue.append(node.left)
    18.                 if node.right:
    19.                     queue.append(node.right)
    20.                 tmp_ans.append(queue.pop(0).val)
    21.             ans.append(tmp_ans)
    23.         return ans