链表的倒转，
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* reverseKGroup(ListNode* head, int k){
if(!head || !head->next || k <= 1)
return head;
ListNode* pFirst = head;
ListNode* pBefore = NULL;
ListNode* pKth = pFirst;
while(1) {
int cnt = k -1;
while(cnt && pKth){
pKth = pKth->next;
cnt--;
}
if(cnt > 0 || pKth == NULL) break;
reverse(pBefore,pFirst,pKth);
if(pBefore == NULL)
head = pFirst;
pBefore = pKth;
pFirst = pKth->next;
pKth = pFirst;
}
return head;
}
void reverse(ListNode* pBefore, ListNode*& pFirst, ListNode*& pKth){
ListNode* p = pFirst;
while(p != pKth) {
ListNode* tmp = p->next;
p->next = pKth->next;
pKth->next = p;
p = tmp;
}
ListNode* tmp = pFirst;
pFirst = pKth;
pKth = tmp;
if(pBefore)
pBefore->next = pFirst;
}
};