二分板子

檢查左端點

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int l = 1, r = n;
while (l < r)
{
    int mid = (l + r) / 2;
    if(check(mid)) r = mid;
    else l = mid + 1;
}

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int l = 1, r = n;
while (l < r)
{
    int mid = (l + r + 1) / 2;
    if(check(mid)) l = mid;
    else r = mid - 1;
}