劍指offer37：序列化二叉樹

1.把樹序列爲字符串可以看成遞歸，先序遍歷
2.碰到空指針變爲特殊字符$
3.反序列化時，直接按照‘，’號分隔開

# -*- coding:utf-8 -*-
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class Solution:
    flag = -1
    def Serialize(self, root):
        s = ""
        s = self.recursionSerialize(root, s)
        return s
    def recursionSerialize(self, root, s):
        if root is None:
            s = '$,'
            return s
        s = str(root.val) + ','
        left = self.recursionSerialize(root.left, s)
        right = self.recursionSerialize(root.right, s)
        s += left + right
        return s
    def Deserialize(self, s):
        self.flag += 1
        l = s.split(',')
        if self.flag >= len(s):
            return None
        root = None
        if l[self.flag] != '$':
            root = TreeNode(int(l[self.flag]))
            root.left = self.Deserialize(s)
            root.right = self.Deserialize(s)
        return root