Longest Increasing Subsequence(LIS) with C#

lis.cs

public static long LowerBound(long[] a, long key)
{
    long l = 0;
    long r = a.Length;
    long mid;
    while (l < r)
    {
        mid = ((r - l) / 2) + l;
        if (a[mid] <= key)
            l = mid + 1;
        else
            r = mid;
    }
    return l;
}

public static long LIS(long[] a)
{
    var dp = Enumerable.Range(0, a.Length).Select(x => long.MaxValue).ToArray();
    for (int i = 0; i < a.Length; i++)
    {
        dp[LowerBound(dp, a[i])] = a[i];
    }
    return LowerBound(dp, long.MaxValue-1);
}