请实现两个函数，分别用来序列化和反序列化二叉树

这里采用先序遍历的方法来做序列化和反序列化。序列化比较简单，反序列化要设置一个index来记录点的位置：

# -*- coding:utf-8 -*-
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class Solution:
    # 先序
    def Serialize(self, root):
        # write code here
        if not root:
            return '#,'
        return str(root.val) + ',' + self.Serialize(root.left) + self.Serialize(root.right)

    def Deserialize(self, s):
        # write code here
        l = s.split(',')
        self.index = 0
        return self.helper(l)

    def helper(self, l):
        c = l[self.index]
        if c == '#':
            self.index += 1
            return None
        root = TreeNode(int(c))
        self.index += 1
        root.left = self.helper(l)
        root.right = self.helper(l)
        return root