题解 | #链表中的节点每k个一组翻转#直线思维（时间90%）（内存 98%）

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){
            return head;
        }
        //第一步 分割链表
        ArrayList<ListNode> list = new ArrayList<>();
        while(head != null){
            ListNode node = head;
            boolean isHaveKSize = true;
            for(int i = 0;i< k - 1;i++){
                if(node.next != null){
                   node = node.next;
                } else {
                    //不够k个
                    isHaveKSize = false;
                    break;
                }
            }
            if (isHaveKSize){
                ListNode next = node.next;
                node.next = null;
                list.add(head);
                head = next;
            } else {
                break;
            }

        }

        //第二步 分别翻转
        for(int i = 0;i<list.size();i++){
            ListNode reverseNode = reverse(list.remove(i));
            list.add(i,reverseNode);
        }
        //第三步 拼接
        if(list.size() > 0){
            ListNode first = list.get(0);
            ListNode tail = findTail(first);
            for(int i = 1;i<list.size();i++){
                ListNode node = list.get(i);
                tail.next = node;
                tail = findTail(node);
            }
            //不是k的倍数 保持原样
            if(head != null){
                tail.next = head;
            }
            return first;
        } else {
            return head;
        }
    }

    public static ListNode findTail(ListNode node){
        if(node == null || node.next == null){
            return node;
        }
        ListNode tail = node;
        while(tail.next != null){
            tail = tail.next;
        }
        return tail;
    }

    public static ListNode reverse(ListNode head){
        if(head == null || head.next == null){
            return head;
        }
        ListNode last = null;
        while(head != null){
            ListNode next = head.next;
            head.next = last;

            last = head;
            head = next;
        }
        return last;
    }
}