private void printFirstSequence(TreeNode root){         LinkedList<TreeNode> queue = new LinkedList<>();         queue.add(root);         while (!queue.isEmpty()){             TreeNode node = queue.poll();             System.out.println(node.val);             if (node.left!=null){                 queue.add(node.left);                 continue;             }             if (node.right!=null){                 queue.add(node.right);             }         }     }