int bs(int n)
{
  int l = -1, r = n;
  while (l + 1 != r)
  {
    int m = l + r >> 1;
    if (xx)
    {
      l = m;
    }
    else
    {
      r = m;
    }
  }
  return l / r;
}

https://www.bilibili.com/video/BV1d54y1q7k7