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]
]

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


# BFS
class Solution():
    def levelOrder(self, root):
        res = []
        if root:
            level = [root]
            while len(level):
                res.append([node.val for node in level])
                tmp = []
                for node in level:
                    if node.left:
                        tmp.append(node.left)
                    if node.right:
                        tmp.append(node.right)
                level = tmp
        return res