请实现两个函数，分别用来序列化和反序列化二叉树

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

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

    }

}
*/
public class Solution {
    //序列化：把对象序列装换成字节序列
    String Serialize(TreeNode root) {
        
        if(root==null){
            return "";
        }
        StringBuffer sb = new StringBuffer();
        serializeRes(root,sb);
        return sb.toString();
  }
    public static void serializeRes(TreeNode root,StringBuffer sb){
        if(root == null){
            sb.append("$,");
            return ;
        }
        sb.append(root.val);
        sb.append(',');
        serializeRes(root.left,sb);
        serializeRes(root.right,sb);
        
    }
    //反序列化：把字节序列恢复为对象的过程
    TreeNode Deserialize(String str) {
        if(str.length() == 0){
            return null;
        }
        String[] string = str.split(",");
        return deserializeMethod(string);
    }
    int index = -1;
    TreeNode deserializeMethod(String[] string){
        index++;
        if(!(string[index].equals("$"))){
            TreeNode node = new TreeNode(0);
            node.val = Integer.parseInt(string[index]);
            node.left = deserializeMethod(string);
            node.right = deserializeMethod(string);
            return node;
        }
        return null;
    }
}