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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 class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        if (head == null || k == 1) {
            return head;
        }

        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode prev = dummy;
        ListNode curr = head;

        int count = 0;
        while (curr != null) {
            count++;
            if (count % k == 0) {
                prev = reverse(prev, curr.next);
                curr = prev.next;
            } else {
                curr = curr.next;
            }
        }

        return dummy.next;
    }

    private ListNode reverse(ListNode prev, ListNode next) {
        ListNode last = prev.next;
        ListNode curr = last.next;

        while (curr != next) {
            last.next = curr.next;
            curr.next = prev.next;
            prev.next = curr;
            curr = last.next;
        }

        return last;
    }

    // Helper method to print the linked list
    public static void printList(ListNode head) {
        ListNode curr = head;
        while (curr != null) {
            System.out.print(curr.val + " ");
            curr = curr.next;
        }
        System.out.println();
    }

    public static void main(String[] args) {
        // Example 1
        ListNode head1 = new ListNode(1, new ListNode(2, new ListNode(3, new ListNode(4, new ListNode(5)))));
        Solution solution = new Solution();
        ListNode result1 = solution.reverseKGroup(head1, 2);
        printList(result1); // Output: 2 1 4 3 5

        // Example 2
        ListNode head2 = new ListNode(1, new ListNode(2, new ListNode(3, new ListNode(4, new ListNode(5)))));
        ListNode result2 = solution.reverseKGroup(head2, 3);
        printList(result2); // Output: 3 2 1 4 5
    }
}