牛课题霸每日一题NC50题解Java描述

链接：https://www.nowcoder.com/practice/b49c3dc907814e9bbfa8437c251b028e?tpId=190&&tqId=35192&rp=1&ru=/ta/job-code-high-rd&qru=/ta/job-code-high-rd/question-ranking

思路： 递归翻转。
代码：

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){ return null; }
        ListNode dummyHead = new ListNode(0), prev = dummyHead, 
            curr = head, next = head.next, He = head;
        dummyHead.next = head;
        int count = 0;
        while(head != null && count < k){
            ++count;
            head = head.next;
        }
        if(count != k){ return dummyHead.next; }
        while(curr != head){
            curr.next = prev;
            prev = curr;
            curr = next;
            if(curr != null){ next = curr.next; }
        }
        dummyHead.next = prev;
        He.next = reverseKGroup(head, k);
        return dummyHead.next;
    }
}