/** * 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 * res=head;        if(m==n) return res;                int i=1;        ListNode * pre=NULL;   //记录待反转链表的前驱，注意，带反转链表从头开始的情况，没有前驱        ListNode * left;   //记录待反转链表的开始        if(m>1) {            pre=head;            while(i
      
       next;            i++;            }         left = pre->next;        }         else left = head;                i=0;        ListNode *post=left;   //记录待反转链表的后继，注意赋初值为left，而不是head，pre之类的        while(i
       
        next;            i++;        }                ListNode *right=reverselist(left,n-m+1); //记录待反转链表的结束                if(pre) pre->next =right;        else res = right;                while(right->next){            right=right->next;        }                right->next=post;        return res;    }        //把head开始长度为l的拉链，反转      ListNode* reverselist(ListNode* head,int l)     {              ListNode * pre=NULL;              ListNode * cur=head;              ListNode * post=NULL;              int i=0;                      while(i
        
         next;                  cur->next = pre;                  pre = cur;                  cur = post;                  i++;              }              return pre;    }};