LeetCode Online Judge 题目C# 练习 - Add two number

You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)

Output: 7 -> 0 -> 8

 1     class LinkedListNode
 2     {
 3         public object Value;
 4         public LinkedListNode Next;
 5 
 6         public LinkedListNode()
 7         {
 8             this.Value = null;
 9             this.Next = null;
10         }
11 
12         public LinkedListNode(int value)
13         {
14             this.Value = value;
15             this.Next = null;
16         }
17     }

 1         public static LinkedListNode AddTwoNumber(LinkedListNode n1, LinkedListNode n2)
 2         {
 3             LinkedListNode ret = new LinkedListNode();
 4             LinkedListNode cur = ret;
 5             int carry = 0;
 6             int sum;
 7 
 8             while (n1 != null || n2 != null)
 9             {
10                 sum = (n1 == null ? 0 : (int)n1.Value) + (n2 == null ? 0 : (int)n2.Value) + carry;
11                 carry = sum / 10;
12                 cur.Value = sum % 10;
13 
14                 if(n1 != null)
15                     n1 = n1.Next;
16                 if(n2 != null)
17                     n2 = n2.Next;
18                 
19                 cur.Next = new LinkedListNode();
20                 cur = cur.Next;
21             }
22 
23             if (carry == 1) // Don't forget the carry
24                 cur.Next = new LinkedListNode(1);
25 
26             return ret;
27         }

代码分析:

　　因为.NET提供的LinkedListNode<T>是generic的， 所以我自己写了一个LinkedListNode,里面的value用boxing Object，暂时只写了一个constractor，输入整型。

开始以为 2 -> 4 -> 3 表示243， 觉得这样麻烦了，还要反转链表，相加之后再反转。原来2 -> 4 -> 3 表示342，这样就不用反转了。

　　注意两点，当一个链表到头了而另外一个链表还有NODE，相加要补0。 还有注意最后一个进位。

补充一点:

　　我在面试的时候被问到ArrayList和List<T>有什么区别,答案是ArrayList里面的元素都是box成一个object，每次插入和读取都要boxing和unboxing,性能上自然不如泛型List.