Serialize and deserialize binary tree

Solution I: preorder, recursive

public String serialize(TreeNode root) { ArrayList<Integer> result = new ArrayList<Integer>(); serializeHelper(root,result); return result.toString(); } private void serializeHelper(TreeNode root, ArrayList<Integer> result){ if (root == null) { result.add(null); return; } result.add(root.val); serializeHelper(root.left,result); serializeHelper(root.right,result); } // Decodes your encoded data to tree. public TreeNode deserialize(String data) { String[] strArray = data.substring(1,data.length()-1).split(", "); Deque<String> strList = new LinkedList<String>(Arrays.asList(strArray)); return deserializeHelper(strList); } private TreeNode deserializeHelper(Deque<String> strList){ if (strList.size() == 0) return null; String str = strList.pop(); if (str.equals("null")) return null; TreeNode currentRoot = new TreeNode(Integer.parseInt(str)); currentRoot.left = deserializeHelper(strList); currentRoot.right = deserializeHelper(strList); return currentRoot; }