24. Swap Nodes in Pairs

 public static ListNode swapPairs(ListNode head) {        ListNode pre = null, cur =null;        if(head == null){            return head;        }else {            pre = head;            cur = head.next;        }         while(cur!=null) {            int tmp = pre.val;            pre.val = cur.val;            cur.val = tmp;            pre = cur.next;            if(pre != null) {                cur = pre.next;            }else{                cur = null;            }        }        return head;    }