Parse functionally.

ParseFP.cs

public static int[] Parse(string data)
{
  var result = new List<int>();

  var commands = new Dictionary<char, Func<int, int>>
  {
    {'i', n => ++n },
    {'d', n => --n },
    {'s', n => n * n },
    {'o', n => { result.Add(n); return n; } }
  };

  var value = 0;
  data.Select(c => value = commands[c](value)).ToList();

  return result.ToArray();
}