C# 斐波那契数列 第n项数字/前n项的和

 1  static void Main(string[] args)
 2         {
 3             int a = Convert.ToInt32(Console.ReadLine());
 4 
 5             //求第n位数字是多少
 6             Console.WriteLine(F1(a));
 7 
 8             //求前n项的和
 9             Console.WriteLine(sum(a));
10             Console.ReadKey();
11 
12 
13         }
14 
15         /// <summary>
16         /// 求第n位的数是几
17         /// </summary>
18         /// <param name="a"></param>
19         /// <returns></returns>
20         private static int F1(int a)
21         {
22             if (a == 1 || a == 2)
23                 return 1;
24             else
25                 return F1(a - 1) + F1(a - 2);
26         }
27 
28         /// <summary>
29         /// 求前n项的和
30         /// </summary>
31         /// <param name="num"></param>
32         /// <returns></returns>
33         private static int sum(int num)
34         {
35             int sum_new=0;
36             for (int i = 1; i <= num; i++)
37             {
38                 sum_new =sum_new+ F1(i);
39             }
40             return sum_new;
41         }