Binary Tree Inorder Traversal

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