"Lowest unique number" challenge
I threw your code through a profiler and it told me that ToList was responsible for quite a bit of your execution time. So why is that? 

You have 2 ToList calls in your code::

var nonRepeated = groups.Where(x =&gt; x.freq == 1).ToList();
if (nonRepeated.Count() != 0)
{
    var singles = nonRepeated.Select(x =&gt; x.number).ToList();
    var smallest = singles.Min();


So what happens here? You start by iterating through groups to find those with freq equal to 1. Count is optimized to lists, so that one should be constant time. Then you iterate through nonRepeated to select number and then finally you iterate through it to select the minimum value. That's a lot of iterations. Let's try to cut that down:

var nonRepeated = groups.Where(x =&gt; x.freq == 1)
                        .Min(x =&gt; x.number);
if (nonRepeated != null)
{
    var playerNumber = Array.IndexOf(numbers, nonRepeated);


By comparing the execution time of this and the other one (on my system) it seems we have achieved a small improvement. Not enough to brag about though. Since we don't have any ToList calls anymore, I profiled the code once again. This time it is Min that takes up the time (not really surprising - It is forcing the GroupBy, Select and Where to be evaluated). So our bottleneck is (still) the LINQ queries. Can we speed it up? 


  Hint: LINQ usually have a slight overhead compared to doing things 'manually'.


Let's try to do these things ourselves, instead of relying on LINQ. Let's replace that GroupBy and Select stuff:

var values = new Dictionary&lt;string, int&gt;();
foreach (var number in numbers)
{
    if (values.ContainsKey(number)) //No curly-brackets to keep it short.
        values[number]++;
    else
        values.Add(number, 1);
}


A benchmark of the code told me that was a clear improvement. Nice! But the profiler still says that Where and Min are expensive. Let's do those manually too.

string minValue = null; 
foreach (var value in values)
{
    if (value.Value == 1)
    {
        if (minValue == null || String.Compare(value.Key, minValue, StringComparison.InvariantCulture) == -1)
            minValue = value.Key;
    }
}

if (minValue != null)
    ...


Once again, it seems we got an overall improvement in performance. Interestingly now the bottlenecks seem to be WriteLine and OpenText. We could try to experiment with other ways of reading from file/writing to console, but I don't really feel like doing that right now (and I don't think the 'easy' methods like File.ReadAllLines and File.ReadLines is much faster than your way, if at all). You could try to play with StringBuilder and only write to console once, but in my experience it is rarely an improvement when the method is only executed once. 

But maybe you can do some other things to squeeze just a little bit more out of your method. 

First of all, is the IsNullOrEmpty really necessary? It's a safety mechanism and in real life we would probably want to have it. But this is a coding exercise and we are all about performance right now. 

Another thing you might want to consider is that strings are expensive. As it is, you are currently working with all your numbers as strings. Split creates a bunch of new strings and every time we compare values we do a string comparison. It might be worth to convert your values to int or char instead.


  HINT: If you try to convert to int values, it can be done by using char ASCII values. You know that the values will always be in the range [1-9]. The ASCII value for '1' is 49, '2' is 50, etc. So if you do value[0] - 48, you can convert you strings to ints with very little overhead.


When you want to optimize something it is always a good idea to measure and profile it and see just where the time is spent. Sometimes you can get big improvements with very few changes.. But you can also make things worse with very few changes. 



After the changes mentioned in this post (except the string to int conversion) is applied, the code will look something like this:

using (StreamReader reader = File.OpenText(args[0]))
{
    while (!reader.EndOfStream)
    {
        var linevalue = reader.ReadLine();
        var numbers = linevalue.Split(' ');

        var values = new Dictionary&lt;string, int&gt;();
        string minValue = null;
        foreach (var number in numbers)
        {
            if (values.ContainsKey(number))
                values[number]++;
            else
                values.Add(number, 1);
        }

        foreach (var value in values)
        {
            if (value.Value == 1)
            {
                if (minValue == null || String.Compare(value.Key, minValue, StringComparison.InvariantCulture) == -1)
                    minValue = value.Key;
            }
        }

        if (minValue != null)
        {
            var playerNumber = Array.IndexOf(numbers, minValue);
            Console.WriteLine(playerNumber + 1);
        }
        else
        {
            Console.WriteLine("0");
        }
    }
}


As you can see, you have 2 loops within the while now. It might be possible to merge them into one by doing something clever. 

I tried the code on CodeEval and got a 181ms execution time. That is a 74 ms improvement. To motivate you a bit, I know it can be done (at least a little) faster. ;)