swap nodes in pairs

public ListNode (ListNode head) {
    if(head == null || head.next == null)
        return head;
    ListNode first = head, second = head.next;
    first.next = swapPairs(second.next);
    second.next = first;
    return second;
}

public ListNode (ListNode head) {
    if(head == null || head.next == null)
        return head;
    ListNode dummy = new ListNode(-1);
    ListNode first = dummy, second = head;
    while (second != null && second.next != null) {
        ListNode temp = second.next.next;
        second.next.next = second;
        
        first.next = second.next;
        second.next = temp;
        first = second;
        second = temp;
    }
    return dummy.next;
}