折半查找的递归算法
int binary_search(int r[],int k,int low,int high)
{
	int mid;
	if(low<=high)
	{
			mid = (high+low)/2;
			if(r[mid]==k)
				retrun mid;
			else
			  if(r[mid]>k)
					return binary_search(r,k,low,mid-1);
				else
					return binary_search(r,k,mid+1,high);
	}
	else
		return 0;
}