题解 | #链表中的节点每k个一组翻转#

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 * }
 */

public class Solution {
    /**
     *
     * @param head ListNode类
     * @param k int整型
     * @return ListNode类
     */
    public ListNode reverseKGroup (ListNode head, int k) {
        // write code here

        if(head == null || head.next == null || k < 2){
            return head;
        }
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        int len = 0;  //统计链表中结点个数
        ListNode prestart = dummy, start = head;
        while (head != null) {
            head = head.next;
            len++;
        }
        ListNode temp = null;  //中间结点 放外面减少内存
        for (int i = 0 ; i < len / k ; i++ ) { //进行分段
            for (int j = 1 ; j < k   ; j++ ) { //段内排序
                temp = start.next; 
                start.next = temp.next;
                temp.next = prestart.next;
                prestart.next = temp;
            }
            //此时start已经由第1位变为第三位 进入下一段
            prestart = start;
            start = start.next;
        }


        return dummy.next;
    }
}