合併兩個有效鏈表

 public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(-1);
        ListNode pre = dummy;
        while (l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                pre.next = l1;
                l1 = l1.next;
            }else {
                pre.next = l2;
                l2 = l2.next;
            }
            pre = pre.next;
            
        }
        pre.next = l1 == null ? l2: l1;
        return dummy.next;
    }