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

又是一道链表的题，关于链表的题好像都是关于指针的，题目都不难吧，主要是需要细心，因为不难，所以代码没有自己写，这里还可以考虑之前做过的链表翻转m至n那道题，然后调用函数就解决了······················

下面的代码的思路是用子函数挨个处理每个K段。

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public:    ListNode* reverse(ListNode* head){        ListNode* prev = nullptr;        ListNode* nowNode = head;        while(nowNode){            ListNode* next = nowNode -> next;            nowNode -> next =  prev;            prev = nowNode;            nowNode = next;        }        return prev;    }    ListNode *reverseKGroup(ListNode *head, int k) {        if(head == nullptr || head -> next == nullptr || k < 2) return head;                int cnt = 1;        ListNode* nowHead = head;        ListNode* nowNode = head;        while(nowNode && cnt < k){            cnt ++;            nowNode = nowNode -> next;        }        if(nowNode && cnt == k){            ListNode* tail = reverseKGroup(nowNode -> next , k);            nowNode -> next = nullptr;            head = reverse(head);            nowHead -> next = tail;        }        return head;    }};