题目：

题目分析：

题目解析转载：

代码实现：

package BMP3;

import java.util.List;

class ListNode {
    int val;
    ListNode next = null;
    public ListNode(int val) {
        this.val = val;
    }
}
public class BM3 {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param head ListNode类
     * @param k int整型
     * @return ListNode类
     */
    public ListNode reverseKGroup (ListNode head, int k) {
        // write code here
        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        ListNode pre = dummy;
        ListNode end = dummy;
        while (end.next != null) {
            for (int i = 0 ; i < k && end != null ; i++) {
                end = end.next ;
            }
            if (end == null) {
                break;
            }
            ListNode next = end.next;
            end.next = null;
            ListNode start = pre.next;
            pre.next = null;
            pre.next = reverseList(start);
            start.next = next;
            pre = start;
            end = start;
        }
        return dummy.next;
    }
    private ListNode reverseList(ListNode head) {
        if (head == null || head.next == null) return head;
        ListNode cur = reverseList(head.next);
        head.next.next = head;
        head.next = null;
        return cur;
    }
}