Get Longest Prefix from a set of strings with and without prefix sharing

longestPrefix.cs

// Enumerate all possible m-size combinations of [0, 1, ..., n-1] array
// in lexicographic order (first [0, 1, 2, ..., m-1]).
private static IEnumerable<int[]> GetCombinations(int m, int n)
{
    int[] result = new int[m];
    Stack<int> stack = new Stack<int>(m);
    stack.Push(0);
    while (stack.Count > 0)
    {
        int index = stack.Count - 1;
        int value = stack.Pop();
        while (value < n)
        {
            result[index++] = value++;
            stack.Push(value);
            if (index != m) continue;
            yield return (int[])result.Clone(); // thanks to @xanatos
                                                //yield return result;
            break;
        }
    }
}

private static IEnumerable<T[]> GetCombinations<T>(T[] array, int m)
{
    if (array.Length < m)
        throw new ArgumentException("Array length can't be less than number of selected elements");
    if (m < 1)
        throw new ArgumentException("Number of selected elements can't be less than 1");
    T[] result = new T[m];
    foreach (int[] j in GetCombinations(m, array.Length))
    {
        for (int i = 0; i < m; i++)
        {
            result[i] = array[j[i]];
        }
        yield return result;
    }
}

private static string LongestCommonPrefix(params string[] strs)
{
    if (strs.Length == 0) return String.Empty;
    Array.Sort(strs);

    var first = strs[0];
    var last = strs[strs.Length - 1];
    var strbuilder = new StringBuilder();
    for (int i = 0; i < first.Length; i++)
    {
        if (first[i] != last[i])
        {
            break;
        }
        strbuilder.Append(first[i]);
    }

    return strbuilder.ToString();
}


var occurences =
  GetCombinations(arr, 2)
  .Select(x => LongestCommonPrefix(x[0], x[1]))
  .Where(x => !string.IsNullOrWhiteSpace(x))
  .GroupBy(x => x)
  .Select(x =>
  {
      var obj = new
      {
          Prefix = x.Key,
          CountByCombinations = x.Count(),
          Occurences = arr.Where(y => y.StartsWith(x.Key)).Count()
      };

      Console.WriteLine($"{obj.Prefix} (CountByCombinations: {obj.CountByCombinations}, Occurences: {obj.Occurences})");
      return obj;
  }).ToList();