[Leet Code] Binary Tree Level Order Traversal II

Leetcode: https://leetcode.com/problems/binary-tree-level-order-traversal-ii/

Problem:

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]
]

Solution:

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def levelOrderBottom(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        if root:
            elements = [[root.val]]
            stack = [root]
            self.helper(root, stack, elements)
            return reversed(elements)
        return root
def helper(self, root, stack, elements):
        while len(stack) > 0:
            values = []
            length = len(stack)
            for i in range(length):
                node = stack.pop(0)
                if node.left:
                    stack.append(node.left)
                    values.append(node.left.val)
                if node.right:
                    stack.append(node.right)
                    values.append(node.right.val)
            if values != []:
                    elements.append(values)