题目：

题解：

/* 定义保存两个地址的结构体* 用来保存反转后结果的头节点和尾节点*/
typedef struct {struct ListNode* head; struct ListNode* tail; 
} TwoAddress;
// 反转中间链表
TwoAddress* reverse(struct ListNode* head){struct ListNode* prev = NULL;struct ListNode* curr = head;TwoAddress* ans = (TwoAddress*)malloc(sizeof(TwoAddress));ans->tail = head;while(curr){struct ListNode* temp = curr->next;curr->next = prev;prev = curr;curr = temp;}ans->head = prev;return ans;
}
struct ListNode* reverseKGroup(struct ListNode* head, int k) {struct ListNode* dummyHead = (struct ListNode*)malloc(sizeof(struct ListNode));dummyHead->next = head;// 每一组的查找头节点,做标志用,本身不查找,并且用来连接两组之间的头节点和尾节点struct ListNode* prev = dummyHead; // 翻转头节点struct ListNode* curr = head;while(curr){// 尾节点,查找本组的末尾,将其next设置为NULLstruct ListNode* tail = prev;for(int i=0;i<k;i++){tail = tail->next;if(!tail) return dummyHead->next;}// 保存下一组的头节点struct ListNode* next = tail->next;tail->next = NULL;  //断开TwoAddress* temp = reverse(curr);  // 翻转这组prev->next = temp->head;  // 拼接temp->tail->next = next;prev = temp->tail;curr = temp->tail->next;}return dummyHead->next;
}