序列化二叉树

请实现两个函数，分别用来序列化和反序列化二叉树
Java:
 1 /*
 2 public class TreeNode {
 3     int val = 0;
 4     TreeNode left = null;
 5     TreeNode right = null;
 6 
 7     public TreeNode(int val) {
 8         this.val = val;
 9 
10     }
11 
12 }
13 */
14 public class Solution {
15     private String s ;
16     
17     String Serialize(TreeNode root) {
18         if (root == null){
19             return "#" ;
20         }
21         return root.val + " " + Serialize(root.left) + " " + Serialize(root.right) ;
22     }
23     TreeNode Deserialize(String str) {
24         s = str ;
25         return Deserialize() ;
26     }
27     
28     TreeNode Deserialize() {
29         if (s.length() == 0)
30             return null ;
31         int index = s.indexOf(' ') ;
32         String node = index == -1 ? s : s.substring(0,index) ;
33         s = index == -1 ? "" : s.substring(index+1) ;
34         if (node.equals("#"))
35             return null ;
36         int val = Integer.valueOf(node) ;
37         TreeNode t = new TreeNode(val) ;
38         t.left = Deserialize() ;
39         t.right = Deserialize() ;
40         return t ;
41     }
42 }