21. 合并两个有序链表

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

    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;
        }
    }