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:

/*
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }

}
*/
public class Solution {
    private String s ;
    
    String Serialize(TreeNode root) {
        if (root == null){
            return "#" ;
        }
        return root.val + " " + Serialize(root.left) + " " + Serialize(root.right) ;
    }
    TreeNode Deserialize(String str) {
        s = str ;
        return Deserialize() ;
    }
    
    TreeNode Deserialize() {
        if (s.length() == 0)
            return null ;
        int index = s.indexOf(' ') ;
        String node = index == -1 ? s : s.substring(0,index) ;
        s = index == -1 ? "" : s.substring(index+1) ;
        if (node.equals("#"))
            return null ;
        int val = Integer.valueOf(node) ;
        TreeNode t = new TreeNode(val) ;
        t.left = Deserialize() ;
        t.right = Deserialize() ;
        return t ;
    }
}