Binary Tree Inorder Traversal

public class Solution {
    public ArrayList<Integer> inorderTraversal(TreeNode root) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        ArrayList<Integer> result = new ArrayList<Integer>();
        traversal(root, result);
        return result;
    }
    private void traversal(TreeNode root, ArrayList<Integer> list)
    {
        if(root == null)
            return;
        traversal(root.left, list);
        list.add(root.val);
        traversal(root.right, list);
    }
}