/**

 * Definition for a binary tree node.

 * public class TreeNode {

 *     int val;

 *     TreeNode left;

 *     TreeNode right;

 *     TreeNode(int x) { val = x; }

 * }

 */

class Solution {

    List<Integer> res = new ArrayList<Integer>();

    public List<Integer> inorderTraversal(TreeNode root) {

        //inorder traveral : left root right

        inorder( root);

        return res;

    }

    void inorder(TreeNode node){

        if(node==null) return;

        inorder(node.left);

        res.add(node.val);

        inorder(node.right);

    }

}