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 == 1) return head;
        
        int len = getLen(head);
        ListNode *start = head, *lastTail = new ListNode(0), *newhead = lastTail;
        newhead->next = head;
        
        for(int i = 0; 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 = 0; i < k - 1; ++i) {
            head->next = p2->next;
            p2->next = p1;
            p1 = p2;
            p2 = head->next;
        }
        
        return p1;
    }
    
    int getLen(ListNode *head, ListNode *end = NULL) {
        int ret = 0;
        while(head != end) {
            ++ret;
            head = head->next;
        }
        return ret;
    }
};