合并两个有序链表

    public ListNode MergeTwoLists(ListNode l1, ListNode l2)
    {
    
        if (l1 == null)
            return l2;
        else if (l2 == null)
            return l1;
        else if (l1.val < l2.val)
        {
    
            l1.next = MergeTwoLists(l1.next, l2);
            return l1;
        }
        else
        {
    
            l2.next = MergeTwoLists(l1, l2.next);
            return l2;
        }
    }