二分

	while (l < r)
    {
        int mid = l + r >> 1;	
        if (check(mid))  r = mid;  
        else l = mid + 1;
    }


	while (l < r)
    {
        int mid = l + r + 1 >> 1;	
        if (check(mid))  l = mid;
        else r = mid - 1;
    }