LeetCode (Python): Binary Tree Level Order Traversal II

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],
]

Solution:

# 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 levelOrderBottom(self, root):
        solution = []
        thisLevel = []
        if root != None:
            thisLevel.append(root)
        while len(thisLevel)>0:
            levelNumbers = []
            nextLevel = []
            for node in thisLevel:
                levelNumbers.append(node.val)
                if node.left != None:
                    nextLevel.append(node.left)
                if node.right != None:
                    nextLevel.append(node.right)
            solution.insert(0,levelNumbers)
            thisLevel = nextLevel
        return solution