Given a linked list, reverse the nodes of a linked list k at a time and return its modified list.
If the number of nodes is not a multiple of k then left-out nodes in the end should remain as it is.
You may not alter the values in the nodes, only nodes itself may be changed.
Only constant memory is allowed.
For example,Given this linked list:1->2->3->4->5
For k = 2, you should return:2->1->4->3->5
For k = 3, you should return:3->2->1->4->5

class Solution {
public:
    ListNode* reverseKGroup(ListNode* head, int k) {
        if(k==1||head==NULL||head->next==NULL)
          return head;
        ListNode* first = head;//單個反轉(zhuǎn)模塊的頭指針和尾指針
        ListNode* last = head;
        ListNode* preHead = new ListNode(-1);
        preHead->next = head;
        ListNode* preGroup =preHead;//用于前后連接的指針對
        ListNode* nextGroup = preHead;
        int count = 1;
        while(last!=NULL)
        {
            if(count==k)
             {   
                nextGroup = last->next;
                reverseList(first,last);
                preGroup->next = last;
                preGroup = first;
                first->next = nextGroup;
                first = nextGroup;
                last = nextGroup;
                count = 1;
                continue;
             }
             last = last->next;
             count++;
        }
        return preHead->next;
    }
    void reverseList(ListNode* first,ListNode* last)
    {
        ListNode* pre =  new ListNode(-1);
        pre->next = first;
        while(pre!=last)
        {
            ListNode* next = first->next;
            first->next = pre;
            pre = first;
            first = next;
        }
    }
};