二分查找

int bsearch(int a[], int n, int x)
{
	int s = 0, e = n - 1;
	int mid;
	while(s <= e)
	{
		mid = (s + e) / 2;
		if(a[mid] == x)
			return mid;
		else if(a[mid] > x)
			e = mid - 1;
		else
			s = mid + 1;
	}
	return -1;
}