Reverse Linked List II

/**
 * 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<m-1){
            pre=pre->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<n-m+1){
            post=post->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<l){
                  post = cur->next;
                  cur->next = pre;
                  pre = cur;
                  cur = post;
                  i++;
              }
              return pre;
    }
};