力扣-25-每K个一组反转链表

java:
public ListNode reverseKGroup(ListNode head, int k) {
    if(head == null ||head.next == null || k == 1)    return head;
    ListNode temp = head;
    for(int i = 0; i < k; i++){
        if(temp == null)    return head;
        temp = temp.next;
    }
    ListNode newHead = reverse(head,temp);
    head.next = reverseKGroup(temp,k);
    
    return newHead;
}

private ListNode reverse(ListNode head,ListNode tail){
    ListNode pre = null;
    ListNode next = null;
    while(head != tail){
        next = head.next;
        head.next = pre;
        pre = head;
        head = next;
    }
    return pre;
}