Elemnts are inserted into the tree according the value of "index" and the variable height supposed to check if the structure of AVL tree is ok. My problem is that the next element is null, it is not filled. I suppose that problem is in variable height but i am not able to fix it no matter that i spend quite a lot of time on it. so i decided to post it here. I hope that you will help me in finding the mistake. I would be really appreciated.