LinkedList - 25. Reverse Nodes in k-Group

/**

 * 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 count = 0;
        ListNode cur = head;
        while (cur != null && count != k) {
            cur = cur.next;
            count++;
        }
        
        if (count == k) {
            cur = reverseKGroup(cur, k);
            // resever list
            while (count-- > 0) {
                ListNode tmp = head.next;
                head.next = cur;
                cur = head;
                head = tmp;
            }
            head = cur;
        }
        
        return head;
    }
}