144. Binary Tree Preorder Traversal

 1     public List<Integer> preorderTraversal(TreeNode root) {
 2         List<Integer> res = new ArrayList<Integer>();
 3         if(root == null) {
 4             return res;
 5         }
 6         dfs(root, res);
 7         return res;
 8     }
 9     
10     private void dfs(TreeNode root, List<Integer> res) {
11         if(root == null) {
12             return;
13         }
14         res.add(root.val);
15         dfs(root.left, res);
16         dfs(root.right, res);
17     }