O(nlogn)
整数

bool check(int mid); //检查mid是否符合要求

int binary_search1(int l, int r) // 区间[l, r]被划分成[l, mid]和[mid + 1, r]
{
    while (l < r)
    {
        int mid = (l + r) >> 1;
        if (check(mid)) //答案还可以更小
        {
            r = mid;
        }
        else
        {
            l = mid + 1;
        }
    }
    return l;
}

int binary_search2(int l, int r) // 区间[l, r]被划分成[l, mid - 1]和[mid, r]
{
    while (l < r)
    {
        int mid = (l + r + 1) >> 1;
        if (check(mid)) //答案还可以更大
        {
            l = mid;
        }
        else
        {
            r = mid - 1;
        }
    }
    return l;
}

浮点数

bool check(double mid);

double binary_search3(double l, double r)
{
    while (r - l > eps)
    {
        double mid = (l + r) / 2;
        if (check(mid))
        {
            r = mid;
        }
        else
        {
            l = mid ;
        }
    }
    return l;
}