Given a linked list, reverse the nodes of a linked list k at a time and return its modified list.

k is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of k then left-out nodes in the end should remain as it is.

Example:

Given this linked list: 1->2->3->4->5

For k = 2, you should return: 2->1->4->3->5

For k = 3, you should return: 3->2->1->4->5

Note:

• Only constant extra memory is allowed.
• You may not alter the values in the list‘s nodes, only nodes itself may be changed.

M1: recursive

cur指向head，用cnt計數，先走k步。當走了k步時（即cnt == k），遞歸，然後反轉這一部分的鏈表

time: O(n), space: O(n)

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        if(head == null || head.next == null) {
            
 
return head;
        }
        int cnt = 0;
        ListNode cur = head;
        while(cur != null && cnt < k) {
            cur = cur.next;
            cnt++;
        }
        
        if(cnt == k) {
            cur = reverseKGroup(cur, k);
            while(cnt-- > 0) {
                ListNode next = head.next;
                head.next = cur;
                cur = head;
                head = next;
            }
            head = cur;
        }
        return head;
    }
}

25. Reverse Nodes in k-Group - Hard