/**
 * @Author:田宇寒.
 * @Date:Created in 14:12 2021年6月7日
 * @Description:K个一组翻转链表
 * @ModifiedBy:
 * @Version: 1.0
 */
public class code25 {

 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;
 }
 }

 public ListNode reverseKGroup(ListNode head, int k) {
 /**
 * create by: 田宇寒
 * description: 时间复杂度:O(n) 空间复杂度O(1)
 * create time: 10:29 2021年6月9日
 * @Param: head
 * @Param: k
 * @return code25.ListNode
 */
 ListNode hair = new ListNode(0);
 hair.next = head;
 ListNode pre = hair;

 while (head != null) {
 ListNode tail = pre;
 for (int i = 0; i < k; i++) {
 tail = tail.next;
 if (tail == null) {
 return hair.next;
 }
 }
 ListNode next = tail.next;
 ListNode[] listNodes = reverseNodeList(head, tail);
 head = listNodes[0];
 tail = listNodes[1];
 pre.next = head;
 tail.next = next;
 pre = tail;
 head = tail.next;
 }
 return hair.next;
 }

 private ListNode[] reverseNodeList(ListNode head, ListNode tail) {
 ListNode prev = tail.next;
 ListNode p = head;
 while (prev != tail) {
 ListNode nex = p.next;
 p.next = prev;
 prev = p;
 p = nex;
 }
 return new ListNode[] {tail, head};
 }
}