方法一：；来自大佬

public ListNode reverseKGroup(ListNode head, int k) {
    ListNode dummy = new ListNode(0);
    dummy.next = head;

    ListNode pre = dummy;
    ListNode end = dummy;

    while (end.next != null) {
        for (int i = 0; i < k && end != null; i++) end = end.next;
        if (end == null) break;
        ListNode start = pre.next;
        ListNode next = end.next;
        end.next = null;
        pre.next = reverse(start);
        start.next = next;
        pre = start;

        end = pre;
    }
    return dummy.next;
}

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

作者：reals
链接：https://leetcode-cn.com/problems/reverse-nodes-in-k-group/solution/tu-jie-kge-yi-zu-fan-zhuan-lian-biao-by-user7208t/
来源：力扣（LeetCode）
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前几天都在gy，在外面都没时间写拉，补回来