/**

 * Definition for ListNode

 * public class ListNode {

 *     int val;

 *     ListNode next;

 *     ListNode(int x) {

 *         val = x;

 *         next = null;

 *     }

 * }

 */

public class Solution {

    /**

     * @param ListNode head is the head of the linked list

     * @oaram m and n

     * @return: The head of the reversed ListNode

     */

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

        // write your code

        if( m>=n || head == null){

            return head;

        }

        ListNode dummy = new ListNode(0);

        dummy.next = head;

        head = dummy;

        // 找到旋转链表的第一个节点

        for(int i=1;i<m;i++){

            if( head ==null)

                return null;

            head = head.next;

        }

        // 第m个节点的前一个节点

        ListNode premNode = head;

        // 第m个节点

        ListNode mNode = head.next;

        ListNode nNode = mNode;

        ListNode postnNode = mNode.next;

        for(int i=m;i< n;i++){

            if(postnNode == null){

                return null;

            }

            // 旋转

            ListNode tmp = postnNode.next;

            postnNode.next = nNode;

            nNode = postnNode;

            postnNode = tmp;

        }

        mNode.next = postnNode;

        premNode.next = nNode;

        return dummy.next;

    }

}