public ListNode swapPairs(ListNode head) {
        if(head == null || head.next == null) return head;

        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        ListNode prev = dummy;
        ListNode cur = head;
        ListNode next = cur.next;
        dummy = next;
        while(next != null){
            cur.next = next.next;
            prev.next = next;
            next.next = cur;
            prev = cur;
            cur = cur.next;
            if(cur == null || cur.next == null) break;
            next = cur.next;
        }
        return dummy;
    }