判断一个链表是否是回文结构

private ListNode sList(ListNode slow){
        ListNode pre = new ListNode(1);
        pre.next = null;
        while(slow!=null){
            ListNode temp = slow.next;
            slow.next = pre.next;
            pre.next = slow;
            slow = temp;
        }
        return pre.next;
    }
    //将链表的后半部分进行反转
    public boolean isPail (ListNode head) {
        // write code here
        ListNode slow = head;
        ListNode fast = head;
        ListNode pre =new ListNode(0);
        pre.next = head;
        while(fast!=null && fast.next!=null){
            fast = fast.next.next;
            slow = slow.next;
            pre = pre.next;
        }

        if(fast!=null){
            pre = pre.next;
            slow = slow.next;
        }

        slow = sList(slow);
        while(slow!=null){
            if(head.val!=slow.val) return false;
            head = head.next;
            slow = slow.next;
        }
        slow = sList(slow);//将反转的链表还原
        pre.next = slow;
        return true;
    }