1 ≤ m ≤ n ≤ length of list.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode reverseBetween(ListNode head, int m, int n) {
        ListNode dummyHead=new ListNode(0);
		dummyHead.next=head;
		ListNode p=dummyHead;
		for(int i=1;i<m;i++){
			p=p.next;
		}
		p.next=reverse(p.next,n-m+1);
		return dummyHead.next;
    }
	private ListNode reverse(ListNode head,int count){
		ListNode pre=null;
		ListNode next=null;
		ListNode cur=head;
		for(int i=0;i<count;i++){
			next=cur.next;
			cur.next=pre;
			pre=cur;
			cur=next;
		}
		head.next=next;
		return pre;
	}
}

LeetCode Solutions : Reverse Linked List II