/**
* 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 (k == 1) return head;
if (!head) return NULL;
ListNode* endNode = head;
for (int i = 0; i < k; ++i) {
if (endNode == NULL) return head;
endNode = endNode->next;
}
ListNode* curNode = head;
ListNode* pre = reverseKGroup(endNode, k);
while (curNode != endNode) {
ListNode* next = curNode->next;
curNode->next = pre;
pre = curNode;
if (next == endNode) break;
curNode = next;
}
return curNode;
}
};