297. Serialize and Deserialize Binary Tree

public class Codec {

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
   if (root == null) return "[]";
        
        StringBuilder sb = new StringBuilder();
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        
        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            if (node == null) {
                sb.append("null,");
            } else {
                sb.append(node.val).append(",");
                queue.add(node.left);
                queue.add(node.right);
            }
        }
        
        sb.deleteCharAt(sb.length() - 1); // Remove the trailing comma
        return "[" + sb.toString() + "]";
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if (data.equals("[]")) return null;
        
        String[] nodes = data.substring(1, data.length() - 1).split(",");
        Queue<TreeNode> queue = new LinkedList<>();
        
        TreeNode root = new TreeNode(Integer.parseInt(nodes[0]));
        queue.add(root);
        
        int i = 1;
        while (!queue.isEmpty() && i < nodes.length) {
            TreeNode parent = queue.poll();
            
            if (!nodes[i].equals("null")) {
                TreeNode left = new TreeNode(Integer.parseInt(nodes[i]));
                parent.left = left;
                queue.add(left);
            }
            i++;
            
            if (i < nodes.length && !nodes[i].equals("null")) {
                TreeNode right = new TreeNode(Integer.parseInt(nodes[i]));
                parent.right = right;
                queue.add(right);
            }
            i++;
        }
        
        return root;
    }
}