class BinaryTree
{
    /* Computes the number of nodes in a tree. */
    static ArrayList<Integer> preorder(Node root)
    {
        // Code here
        ArrayList<Integer> arr=new ArrayList<>();
        
        helper(root,arr);
        
        return arr;
    }
    static void helper(Node root,ArrayList<Integer> arr){
        if(root==null){
            return;
        }
        arr.add(root.data);
        helper(root.left,arr);
        helper(root.right,arr);
    }

}