public static BinTreeNode BSTToDoubleLinkedList(BinTreeNode head) {
        Queue<BinTreeNode> queue = new LinkedList<BinTreeNode>();
        inOrderToQueue(head, queue);
        head = queue.poll();
        BinTreeNode pre = head;
        head.left = null;
        while (!queue.isEmpty()) {
            BinTreeNode cur = queue.poll();
            pre.right = cur;
            cur.left = pre;
            pre = cur;
        }
        pre.right = null;
        return head;
    }
    public static void inOrderToQueue(BinTreeNode head,Queue<BinTreeNode> queue) {
        if (head == null) {
            return;
        }
        inOrderToQueue(head.left, queue);
        queue.add(head);
        inOrderToQueue(head.right, queue);
    }