[Leet Code] Binary Tree Level Order Traversal II

`3   / \  9  20    /  \   15   7`
`[  [15,7],  [9,20],  [3]]`
`# 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 = rightclass 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 rootdef 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)`

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