同Binary Tree Level Order Traversal题目一样，也需要一个dict来记录层的信息。dict[1] = [9, 20]。

# 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):
        if level not in Solution.L:
            Solution.L[level] = [root.val]
        else:
            Solution.L[level].append(root.val)
        if root.left:
            self.preOrder(root.left, level + 1)
        if root.right:
            self.preOrder(root.right, level + 1)
    
    def levelOrderBottom(self, root):
        res = []
        if root == None: return res
        Solution.L = {}
        self.preOrder(root, 0)
        for i in sorted(Solution.L.keys(), reverse = True):
            res.append(Solution.L[i])
        return res