每K个节点翻转

    ListNode* ReverseList(ListNode* head, ListNode* tail)
    {
        ListNode* pre = NULL;
        ListNode* next = NULL;
        while (head != tail)
        {
            next = head->next;
            head->next = pre;
            pre = head;
            head = next;
        }

        return pre;
    }

    ListNode* reverseKGroup(ListNode* head, int k)
    {
        if (head == NULL || head->next == NULL) 
        {
            return head;
        }

        ListNode* tail = head;
        for (int i = 0; i < k; i++) 
        {
            //剩余数量小于k的话，则不需要反转。
            if (tail == NULL)
            {
                return head;
            }
            tail = tail->next;
        }

        // 反转前 k 个元素
        ListNode* newHead = ReverseList(head, tail);
        //下一轮的开始的地方就是tail
        head->next = reverseKGroup(tail, k);

        return newHead;
    }

struct ListNode
{
    int val;
    struct ListNode *next;
    ListNode(int x):val(x), next(NULL) 
    {
    }
};