leetcode102 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]
]

"""
class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

class Solution:
    def levelOrder(self, root):
        if root ==  None:    #树为空，直接返回[]
            return []
        queue = [root]
        cur = []
        res = []
        while queue:
            cur = [x.val if x else None for x in queue] #存当前层的value
            newqueue = []                   #newqueue用来遍历下一层使用
            for x in queue:
                if x.left:                  #bug 如果不判断，结果包含[None]
                    newqueue.append(x.left)
                if x.right:
                    newqueue.append(x.right)
            queue = newqueue
            res.append(cur)                 #将当前[]append到最后结果
        return res
"""
本题与leetcode101相似，用BFS可解
"""