呃呃，没想到还会有这种若只东西

写一下：

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