Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

return [1,2,3].

 1 /**
 2  * Definition for binary tree
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     List<Integer> result = new ArrayList<Integer>();
12     public List<Integer> preorderTraversal(TreeNode root) {
13         if(null == root)
14             return result;
15         else{
16             result.add(new Integer(root.val));
17             preorderTraversal(root.left);
18             preorderTraversal(root.right);
19             return result;
20         }
21     }
22 }