binary tree inorder traversal

 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class  {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        inOrder(res, root);
        return res;
    }
    
    public void inOrder(List<Integer> res, TreeNode node) {
        if(node != null) {
            inOrder(res,node.left);
            res.add(node.val);
            inOrder(res,node.right);
        }
    }
}