反转从位置 m 到 n 的链表。请使用一趟扫描完成反转。

说明:
1 ≤ m ≤ n ≤ 链表长度。

示例:

输入: 1->2->3->4->5->NULL, m = 2, n = 4

输出: 1->4->3->2->5->NULL

 /**

  * Definition for singly-linked list.

  * struct ListNode {

  *     int val;

  *     ListNode *next;

  *     ListNode(int x) : val(x), next(NULL) {}

  * };

  */

 class Solution {

 public:

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

         ListNode *left = head, *first, *pre;

         int step = m;

         while(step -  > ){

             left = left->next;

             step--;

         }

         if(m > ) first = pre = left->next;

         else first = pre = head;

         step = n - m;

         if(step > ){

             ListNode *now = pre->next;

             ListNode *next = now->next;

             while(step-- > ){

                 now->next = pre;

                 pre = now;

                 now = next;

                 if(next) next = now->next;

             }

             if(m > ) left->next = pre;

             first->next = now;

         }

         if(m > ) return head;

         else return pre;

     }

 };