// @Title: K 个一组翻转链表 (Reverse Nodes in k-Group)
// @Author: qisiii
// @Date: 2024-04-13 23:24:17
// @Runtime: 0 ms
// @Memory: 43.4 MB
// @comment: 
// @flag: 
/**
 * 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; }
 * }
 */
class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        if(k==1){
            return head;
        }
        ListNode temp = head;
        int size = 0;
        while (temp != null) {
            size++;
            temp = temp.next;
        }
        if (size < k) {
            return head;
        }
        int kB = k;
        ListNode result = new ListNode(-1, head);
        ListNode pre = result, cur = pre.next, next;
        while (kB > 1) {
            next = cur.next;
            cur.next = next.next;
            next.next = pre.next;
            pre.next=next;
            kB--;
        }
        cur.next = reverseKGroup(cur.next, k);
        return result.next;
    }
}