#include<stdio.h>
int find(int a[],int x,int n,int m)
{int i;
if(n>m)return -1; i=(n+m)/2; if(a[i]==x)return i; if(a[i]>x)return find(a,x,n,i-1); return find(a,x,i+1,m);}
int main()
{
int a[20]={2,3,6,7,12,18,19,21,25,28,30,33,37,39,42,45,47,49,50,51};
int x,i;
printf("已有的數是：\n");
for(i=0;i<20;i++)
printf("%d ",a[i]);
printf("\n請輸入要查找的數：");
scanf("%d",&x);
if((i=find(a,x,0,19))>=0)
printf("%d是第%d個數\n",x,i+1);
else printf("未找到%d\n",x);
return 0;}