public Map<Integer,List<E>> nodesByLevel(){
            Map<Integer,List<E>> map= new HashMap<>();
            processBstByLevel(root, map, 0);
            return map;
    }

    private void processBstByLevel(Node<E> node, Map<Integer,List<E>> result, int level){
        if (level>height()||node==null) {
            return;
        }
        if (result.get(level)==null) {
            List<E> temp=new ArrayList<>();
            temp.add(node.getElement());
            result.put(level, temp);
        }else{
            result.get(level).add(node.getElement());
        }
            processBstByLevel(node.getLeft(), result, level+1);
            processBstByLevel(node.getRight(), result, level+1);
    }