297. Serialize and Deserialize Binary Tree

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Codec {
    
    public List<String> list = new ArrayList<String>();

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        return helper(root, "");
    }
    
    public String helper(TreeNode root, String s) {
        if (root == null) {
            s += "null,";
            list.add("null");
        } else {
            s = s + root.val + ",";
            list.add(String.valueOf(root.val));
            helper(root.left, s);
            helper(root.right, s);
        }
        
        return s;
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        return helpert();
    }
    
    public TreeNode helpert() {
        if (list.get(0) == "null") {
            list.remove(0);
            return null;
        }
        
        TreeNode root = new TreeNode(Integer.valueOf(list.get(0)));
        list.remove(0);
        root.left = helpert();
        root.right = helpert();
        
        return root;
    }
}

// Your Codec object will be instantiated and called as such:
// Codec ser = new Codec();
// Codec deser = new Codec();
// TreeNode ans = deser.deserialize(ser.serialize(root));

index out of bounds??

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Codec {
    public String rserialize(TreeNode root, String str) {
    // Recursive serialization.
    if (root == null) {
      str += "null,";
    } else {
      str += str.valueOf(root.val) + ",";
      str = rserialize(root.left, str);
      str = rserialize(root.right, str);
    }
    return str;
  }

  // Encodes a tree to a single string.
  public String serialize(TreeNode root) {
    return rserialize(root, "");
  }
  public TreeNode rdeserialize(List<String> l) {
    // Recursive deserialization.
    if (l.get(0).equals("null")) {
      l.remove(0);
      return null;
    }

    TreeNode root = new TreeNode(Integer.valueOf(l.get(0)));
    l.remove(0);
    root.left = rdeserialize(l);
    root.right = rdeserialize(l);

    return root;
  }

  // Decodes your encoded data to tree.
  public TreeNode deserialize(String data) {
    String[] data_array = data.split(",");
    List<String> data_list = new LinkedList<String>(Arrays.asList(data_array));
    return rdeserialize(data_list);
  }
}
// Your Codec object will be instantiated and called as such:
// Codec ser = new Codec();
// Codec deser = new Codec();
// TreeNode ans = deser.deserialize(ser.serialize(root));

Runtime: 484 ms, faster than 5.22% of Java online submissions for Serialize and Deserialize Binary Tree.
Memory Usage: 219.2 MB, less than 5.13% of Java online submissions for Serialize and Deserialize Binary Tree.

내거랑 비슷해보이는데.. 왜 안되는지 모를 ㅠㅠ