leetcode107 Binary Tree Level Order Traversal II

"""
层次遍历
"""
"""
Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).
For example:
Given binary tree [3,9,20,null,null,15,7],
    3
   / 
  9  20
    /  
   15   7
return its bottom-up level order traversal as:
[
  [15,7],
  [9,20],
  [3]
]
"""

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

class Solution:
    def levelOrderBottom(self, root):
        if root == None:
            return []
        queue = []
        res = []
        queue.append(root)
        while queue:
            n = len(queue)  #!!!记录每一层有几个结点
            newqueue = []   #存储当前层
            for _ in range(n):
                x = queue.pop(0)  #pop()弹出最后一个元素, pop(0)弹出第一个元素
                newqueue.append(x.val) #bug newqueue.append(x),这里只存值就可以了
                if x.left:
                    queue.append(x.left)
                if x.right:
                    queue.append(x.right)
               #bug wrong answer   res.append(newqueue)
            res.insert(0,newqueue)  #插入到尾部
            #aList = [123, 'xyz', 'zara', 'abc']
            #aList.insert( 3, 2009) (索引,数值)
            #[123, 'xyz', 'zara', 2009, 'abc']
        return res