LeetCode K个一组翻转链表

题目链接：https://leetcode-cn.com/problems/reverse-nodes-in-k-group/

题目大意

　　略。

分析

　　逆转每一段，然后和上一段与下一段衔接即可，加头结点会比较方便。

代码如下

 /**
  * 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(head == NULL || k == ) return head;

         int len = getLen(head);
         ListNode *start = head, *lastTail = new ListNode(), *newhead = lastTail;
         newhead->next = head;

         for(int i = ; i < len / k; ++i) {
             lastTail->next = reverseGroup(start, k); // 和上一段尾巴接上
             lastTail = start;
             start = lastTail->next;
         }

         return newhead->next;
     }

     // 返回反转链表的头结点,顺便把尾部接到下一段头部
     ListNode* reverseGroup(ListNode* head, int k) {
         ListNode *p1 = head, *p2 = head->next;

         for(int i = ; i < k - ; ++i) {
             head->next = p2->next;
             p2->next = p1;
             p1 = p2;
             p2 = head->next;
         }

         return p1;
     }

     int getLen(ListNode *head, ListNode *end = NULL) {
         int ret = ;
         while(head != end) {
             ++ret;
             head = head->next;
         }
         return ret;
     }
 };