题解 | #链表中的节点每k个一组翻转#

主要思路 分割->反转->合并

k个一组分割加入ArrayList 然后对ArrayList里面的ListNode 进行反转

使用队列合并链表

 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
import java.util.*;
class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        if(head == null){
            return head;
        }
        ListNode curr = head;
        ListNode fast = head;
        ListNode firstNode = head;
        List<ListNode> res = new ArrayList<>();
        int count = 0;
        while(fast != null){
            
            if(count%k == 0){
                curr  = fast;
                firstNode  = curr;
                int index = 0;
                while(index<k-1&&(fast !=null)){
                    curr = curr.next;
                    fast = fast.next;
                    index++;
                    count++;
                }
                if(fast != null){
                  fast = fast.next;
                }
                if(curr !=null){
                    curr.next = null;
                }
                res.add(firstNode);
            }else{
                curr = curr.next;
                fast = fast.next;
            }
            count++;
        }

        ArrayList<ListNode> ans = new ArrayList<>();
        ListNode lastNode  = res.get(res.size()-1);
        int lastCount = 0;
        while(lastNode!=null){
            lastNode = lastNode.next;
            lastCount++;
        }

        for(int i =0;i<res.size();i++){
            ListNode node = null;
            if((i== res.size()-1) &&lastCount<k){
                node = res.get(i);
            }else{
                node =  reverseListNode(res.get(i));
            }
            
            ans.add(node);
        }
        Deque<ListNode> deque = new ArrayDeque();
        for(int i=0;i<ans.size();i++){
            ListNode node = ans.get(i);
            ListNode ptr = node;
            while(ptr != null){
                deque.addLast(ptr);
                ptr  = ptr.next;
            }
        }
        ListNode  guard = new ListNode(0);
        ListNode ptr  = guard;
        while(!deque.isEmpty()){
              ptr.next =  deque.pollFirst();
              ptr = ptr.next;
        }

       return guard.next;
    }

    public ListNode  reverseListNode(ListNode node){
           if(node == null){
               return node;
           }
           ListNode prev = null;
           ListNode curr = node;
           while(curr != null){
               ListNode next = curr.next;
               curr.next = prev;
               prev = curr;
               curr = next;
           }
           return prev;
    }
}