class ListNode {
    int val;
    ListNode next;
    ListNode(int x) { val = x; }
}

public class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        // 虚拟头节点
        ListNode dummy = new ListNode(0);
        dummy.next = head;

        // prev 是当前组的前一个节点
        ListNode prev = dummy;

        while (true) {
            // 检查是否还有 k 个节点
            ListNode curr = prev.next;
            int count = 0;
            while (count < k && curr != null) {
                curr = curr.next;
                count++;
            }

            if (count < k) break; // 不足 k 个，停止

            // 反转当前组：从 prev.next 到 curr - 1
            ListNode first = prev.next;
            ListNode last = prev.next;
            for (int i = 0; i < k - 1; i++) {
                last = last.next;
            }

            // 反转区间 [first, last]
            ListNode nextGroup = last.next;
            ListNode prevNode = prev;
            ListNode currNode = first;

            // 反转 k 个节点
            for (int i = 0; i < k; i++) {
                ListNode next = currNode.next;
                currNode.next = prevNode;
                prevNode = currNode;
                currNode = next;
            }

            // 连接反转后的组
            prev.next = prevNode;
            first.next = nextGroup;

            // 更新 prev 为当前组的最后一个节点（即原来的第一个节点）
            prev = first;
        }

        return dummy.next;
    }

    // 测试代码
    public static void main(String[] args) {
        // 构建链表 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7
        ListNode node1 = new ListNode(1);
        node1.next = new ListNode(2);
        node1.next.next = new ListNode(3);
        node1.next.next.next = new ListNode(4);
        node1.next.next.next.next = new ListNode(5);
        node1.next.next.next.next.next = new ListNode(6);
        node1.next.next.next.next.next.next = new ListNode(7);

        Solution solution = new Solution();
        ListNode result = solution.reverseKGroup(node1, 3);

        // 打印结果
        while (result != null) {
            System.out.print(result.val + " ");
            result = result.next;
        }
    }
}