力扣297. 二叉树的序列化与反序列化

public class Codec {

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        StringBuilder res=ser_help(root,new StringBuilder());
        return res;  
    }
    public StringBuilder res(TreeNode root,StringBuilder str){
        if(root==null){
            str.append("null,");
            return str;
        }
        //先序遍历
        str.append(root.val);
        str.append(",");
        str=ser_help(root.left,str);
        str=ser_help(right.left,str);
        return str;
    }

    // Decodes your encoded data to tree.
    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;
    }
}

力扣题目：297. 二叉树的序列化与反序列化