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) {
        if(head ==null || head.next == null){
            return head;
        }
        
        ListNode tmp = new ListNode(-1);
        tmp.next = head;
        ListNode pre1 = tmp;
        ListNode next1 = pre1.next;
        ListNode pre2 = find(pre1, k);
        if(pre2 == null){
            return head;
        }
        
        while(pre2 != null){
            ListNode next2 = pre2.next;
            pre2.next = null;
            ReverseList(next1);
            pre1.next = pre2;
            next1.next = next2;
            pre1 = next1;
            next1 = pre1.next;
            pre2 = find(pre1, k);
            if(pre2 == null){
                break;
            }
        }
        
        return tmp.next;
        
    }
    
    public static ListNode find(ListNode node, int k){
        ListNode pre2 = node;
        for(int i=0;i<k;i++){
            pre2 = pre2 == null?null:pre2.next;
        }
        return pre2;
    }
    
    
    public static ListNode ReverseList(ListNode head) {
        if(head==null||head.next==null)
            return head;
        
        ListNode pre = head;        
        ListNode cur = head.next;
        pre.next=null;
        while(cur!=null){
            ListNode next = cur.next;
            cur.next = pre;
            pre=cur;
            cur=next;
        }
        return pre;
    }
}