折半查找

#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("已有的数是：\n");

  for(i=0;i<20;i++)

    printf("%d ",a[i]);

  printf("\n请输入要查找的数：");

  scanf("%d",&x);

  if((i=find(a,x,0,19))>=0)

    printf("%d是第%d个数\n",x,i+1);

  else

    printf("未找到%d\n",x);

  return 0;

}