[leetcode]Binary Tree Level Order Traversal II @ Python

原题地址：http://oj.leetcode.com/problems/binary-tree-level-order-traversal-ii/

题意：

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,#,#,15,7},

    3

   /   9  20

    /     15   7

return its bottom-up level order traversal as:

[

  [15,7]

  [9,20],

  [3],

]

confused
what "{1,#,2,3}" means? >
read more on how binary tree is serialized on OJ.

OJ‘s Binary Tree Serialization:

The serialization of a binary tree follows a level order traversal, where ‘#‘
signifies a path terminator where no node exists below.

Here‘s an example:

   1

  /  2   3

    /

   4

         5

解题思路：由于编程语言为python，所以其实在Binary Tree Level Order
Traversal这道题（http://www.cnblogs.com/zuoyuan/p/3722004.html）的基础上加一句res.reverse()就可以了。

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:

    # @param root, a tree node

    # @return a list of lists of integers

    def preorder(self, root, level, res):

        if root:

            if len(res) < level+1: res.append([])

            res[level].append(root.val)

            self.preorder(root.left, level+1, res)

            self.preorder(root.right, level+1, res)
def levelOrderBottom(self, root):

        res=[]

        self.preorder(root, 0, res)

        res.reverse()

        return res

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