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]
]

Python

 
# 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 __init__(self):
        self.temp_result = dict()
        self.result = list()
        self.level = 0
    def levelOrderBottom(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        if root:
            self.return_node([root])
            for i in range(0,len(self.temp_result)):
                self.result.append(self.temp_result[i])

            return self.result[::-1]

        else:
            return []


    def return_node(self, root_list):
        if root_list:
            temp_list = []
            for i in root_list:
                temp_list.append(i.val)
            self.temp_result[self.level] = temp_list
            self.level += 1
            new_root_list = list()
            for i in root_list:
                if i:
                    if i.left:
                        new_root_list.append(i.left)
                    if i.right:
                        new_root_list.append(i.right)
            print(new_root_list)
            return self.return_node(new_root_list)
        return

        

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