92. Reverse Linked List II

/**此算法就是一般的链表的交换，主要是记录交换两者的前驱；
* 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) {
if (head == NULL || head->next == NULL || m == n)//如果链表为空或者只有一个结点，或者m=n都返回head;
return head;
if (m > n)//如果m>n返回空；
return NULL;
int count = 1;
ListNode*s = new ListNode(0);//创建一个头结点，便于交换首结点
s->next = head;
ListNode*p = s;
ListNode *q = NULL, *flag = NULL;
for (int i = 0; i < m - 1; i++)
s = s->next;//记录第m个结点的前驱；
q = s->next;
for (int j = m; j < n; j++)
{
flag = q->next;
q->next = flag->next;
flag->next = s->next;
s->next = flag;
}
return p->next;
}
};