leetcode 102. Binary Tree Level Order Traversal

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).
For example:
Given binary tree [3,9,20,null,null,15,7],

  3
   / \
  9  20
    /  \
   15   7

return its level order traversal as:

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

这是一个层次遍历，可以用list模拟队列，不能用queue库，因为queue库似乎没有pop，而list有pop。
python代码：

# 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]]:
        q = []
        q.append(root)
        nums = []
        if not root:
            return []
        while len(q):
            length = len(q)
            num = []
            for i in range(length):
                T = q[0]
                q.pop(0)
                num.append(T.val)
                if T.left:
                    q.append(T.left)
                if T.right:
                    q.append(T.right)
            nums.append(num)
        return nums