1 int[] a = new int[20] { 2, 3, 6, 7, 12, 18, 19, 21, 25, 28, 30, 33, 37, 39, 42, 45, 47, 49, 50, 51 };
 2             int x = 18, i;
 3             string j, k;
 4             var d = find(a, x, 0, 19);
 5             if (d == -1)
 6             {
 7                 k = String.Format("未找到{0}", x);
 8             }
 9             else
10             {
11                 for (i = 0; i < 20; i++)
12                 {
13                     if ((i = d) > 0)
14                     {
15                         j = String.Format("{0}是第{1}个数\n", x, i + 1);
16                         break;
17                     }
18                 }
19             }

1 int find(int[] a, int x, int n, int m)
2         {
3             int i;
4             if (n > m) return -1;
5             i = (n + m) / 2;
6             if (a[i] == x) return i;
7             if (a[i] > x) return find(a, x, n, i - 1);
8             return find(a, x, i + 1, m);
9         }