public ListNode reverseBetween(ListNode head, int m, int n) {

        //链表长度

        int length = getLength(head);

        int t = n - m;

        //判断链表是否合法，并且m和n是否合法

        if(length<n || m>=n || head == null || head.next == null) return head;

        //定义头结点的前驱结点

        ListNode dummyHead = new ListNode(-1);

        dummyHead.next = head;

        //定义临时结点，指向dummyHead

        ListNode cur = dummyHead;

        //让cur指向m位置结点的前驱结点

        while(m>1){

            cur = cur.next;

            m--;

        }

        //逆转链表n-m次

        ListNode f = cur.next;

        ListNode s = f.next;

        while(s!=null && t>0){

            f.next = s.next;

            s.next = cur.next;

            cur.next = s;

            s = f.next;

            t--;

        }

        //返回

        return dummyHead.next;

    }

    //求链表长度

    public static int getLength(ListNode head){

        int count = 0;

        while(head!=null){

            head = head.next;

            count++;

        }

        return count;

    }