102. 二叉树的层次遍历

https://leetcode-cn.com/problems/binary-tree-level-order-traversal/

给定一个二叉树，返回其按层次遍历的节点值。 （即逐层地，从左到右访问所有节点）。

例如: 给定二叉树: [3,9,20,null,null,15,7],

  3
 / \
9  20
  /  \
 15   7

返回其层次遍历结果：

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

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备注

• 时间复杂度：O(n)

• 空间复杂度：O(n)

---

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        q, res = [root], []
        while any(q):
            res.append([c.val for c in q])
            q = [n for node in q for n in (node.left, node.right) if n]
        return res