0144 Binary Tree Preorder Traversal

Question

My Solution:

class{
  public List<Integer> preorderTraversal(TreeNode root) {
          List<Integer> list = new ArrayList<>();
          in(root, list);
          return list;
      }

      private void in(TreeNode root, List<Integer> list){
          if(root == null)
              return;
          list.add(root.val);
          in(root.left, list);
          in(root.right, list);
      }
}