144. Binary Tree Preorder Traversal

 public static List<Integer> preorderTraversal(TreeNode root) {        List<Integer> list = new ArrayList<>();        if(root == null) {            return list;        }else {            preorder(list, root);            return list;        }    }    public static void preorder(List<Integer> list, TreeNode root) {        if(root == null) {            return ;        }else {            list.add(root.val);            preorder(list,root.left);            preorder(list, root.right);        }    }