1 /**
 2  * Definition for a binary tree node.
 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         preorderTraversal2(root);
14         return result;
15     }
16     
17     public void preorderTraversal2(TreeNode root){
18         if (root == null)
19             return;
20         result.add(root.val);
21         preorderTraversal(root.left);
22         preorderTraversal(root.right);
23     }
24 }