//区间[l,r]被划分为[l, mid]和[mid+1, r]
int bsearch_1(int l, int r)
{
    while(l < r)
    {
        int mid = l + r >> 1;
        if(check(mid)) r = mid;
        else l = mid + 1;
    }
    return l;
}

//区间[l,r]被划分为[l, mid-1]和[mid, r]
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;
}