二分查找算法

l + r >> 1 和 l + r + 1 >> 1的区别

l + r >> 1是向下取整，l + r + 1 >> 1是向上取整

while (l < r)
    {
        int mid = l + r >> 1;
        if (check(mid)) r = mid;    // check()判断mid是否满足性质
        else l = mid + 1;
    }
    return l;

r = mid，是向下取整的，所以一定比r小，
l = mid + 1，一定比l大

---

int bsearch_2(int l, int r)
{
    while (l < r)
    {
        int mid = l + r + 1 >> 1;
        if (check(mid)) l = mid;
        else r = mid - 1;
    }
    return l;
}