private static HashMap<Integer, List<Integer>> treeLevels = new HashMap<>();
    private static void addLevels(Node node, int level){
        if(node == null){
            return;
        }
        
        List<Integer> levels;
        
        if(treeLevels.containsKey(level)){
            levels = treeLevels.get(level);
        }else{
            levels = new ArrayList<>();
        }
        
        levels.add(node.data);
        treeLevels.put(level, levels);
        addLevels(node.left, level + 1);
        addLevels(node.right, level + 1);
        
    }
	public static void levelOrder(Node root) {
      addLevels(root, 0);
      for(List<Integer> list: treeLevels.values()){
        for(int val: list){
            System.out.printf("%d ", val);
        }
      }
      
    }