serialize binary tree

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
package codec;

/**
 *
 * @author junye.mao
 */
public class Codec {

    public String serialize(TreeNode root) {      //用StringBuilder
        StringBuilder res = ser_help(root, new StringBuilder());
        return res.toString();
    }
    
    public StringBuilder ser_help(TreeNode root, StringBuilder str){
        if(null == root){
            str.append("null,");
            return str;
        }
        str.append(root.val); 
        str.append(",");
        str = ser_help(root.left, str);
        str = ser_help(root.right, str);
        return str;
    }

    public TreeNode deserialize(String data) {
        String[] str_word = data.split(",");
        List<String> list_word = new LinkedList<String>(Arrays.asList(str_word));
        return deser_help(list_word);
    }
    
    public TreeNode deser_help(List<String> li){
        if(li.get(0).equals("null")){
            li.remove(0);
            return null;
        }
        TreeNode res = new TreeNode(Integer.valueOf(li.get(0)));
        li.remove(0);
        res.left = deser_help(li);
        res.right = deser_help(li);
        return res;
    }
}