Problem Binary Tree Preorder Traversal

 1     public List<Integer> preorderTraversal(TreeNode root) {
 2         if (root == null) return new LinkedList<Integer>();
 3         List<Integer> list = new LinkedList<Integer>();
 4         if (root.left == null && root.right == null) {
 5             List<Integer> leaf = new LinkedList<Integer>();
 6             leaf.add(root.val);
 7             return leaf;
 8         } 
 9         list.add(root.val);
10         if (root.left != null) list.addAll(preorderTraversal(root.left));
11         if (root.right != null) list.addAll(preorderTraversal(root.right));
12         
13         return list;
14     }