BM2 链表内指定区间反转

代码有点长，但是我比较喜欢这种做法。
注意的点是如果第一个参与了反转，那么返回的就是区间反转之后的头结点，而不是原始头结点。

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    public ListNode reverse(ListNode head,ListNode tail){
         if(head == null || head == tail){
            return head;
         }
         ListNode reverseHead = reverse(head.next,tail);
         head.next.next = head;
         head.next = null;
         return reverseHead;
    }
    public ListNode reverseBetween (ListNode head, int m, int n) {
        if(m == n) return head;
        ListNode first = head,last = head;
        ListNode pre = null;
        for(int i=1;i<m;i++){
            pre = first;
            first = first.next;
        }
        for(int i=1;i<n;i++){
            last = last.next;
        }
        ListNode lastNext = last.next;
        reverse(first,last);
        if(pre != null){
            pre.next = last;
        }
        first.next = lastNext;
        if(m==1) return last;
        return head;
    }
}