public static void main(String[] args){
        TreeNode root = new TreeNode(1);
        TreeNode node1 = new TreeNode(2);
        TreeNode node2 = new TreeNode(3);
        TreeNode node3 = new TreeNode(4);
        root.left = node1;
        root.right = node2;
        node2.left = node3;
        List<List<Integer>> result = new Binary_Tree_Level_Order_Traversal().levelOrder(null);
        for(int i = 0 ; i < result.size();i++){
            System.out.println(Arrays.toString(result.get(i).toArray()));
        }
        System.out.println("Hello World!");
    }