Solved Timus online challenge 1102 (http://acm.timus.ru/problem.aspx?space=1&num=1102)

StrangeDialogue.cs

using System.Collections.Generic;
using System.IO;
using System.Text;

namespace ConsoleApplication13
{
    class Program
    {
        /// <summary>
        /// I state my assumptions here for completeness.  All assumptions are made to facilitate concise
        /// code writing, but not taking away from the validity of the solution to the problem.
        /// 
        /// This Program takes input from the text file "input.txt", and outputs the results to "output.txt"
        /// as per the specification at http://acm.timus.ru/problem.aspx?space=1&num=1102.  I further assume
        /// that input.txt is located in the same directory from which the program is run.
        /// 
        /// The specification is slightly vague, in that determining valid "dialogues" is left to interpretation.
        /// For this solution, I leave semantic analysis of language completely alone, and focus only on vocabulary.
        /// If the sentence in question can be split, completely, into specified by the vocabulary of either person,
        /// Then I assume that this is a valid dialogue.  This also assumes that the correct person is speaking each
        /// word, in that "One" never says "in", "input", or "One" and "Puton" never says "out", "output", or "Puton".
        /// Also, I assume that an empty dialogue is a valid dialogue for the purposes of this solution.
        /// Lastly, I assume well-formed input.  For instance, I will not perform ToLower() conversions on the string,
        /// since the specification explicitly states that all input characters are "small Latin letters".
        /// </summary>
        static void Main()
        {
            var inputReader = new InputFileStreamReader();
            var outputWriter = new OutputFileStreamWriter();
            var dialogueChecker = new DialogueValidityChecker();

            var numInputs = int.Parse(inputReader.ReadNextLineFromFile());
            for (var idx = 0; idx < numInputs; idx++)
            {
                var potentialDialogue = inputReader.ReadNextLineFromFile();
                var valid = dialogueChecker.IsValidDialogue(potentialDialogue);
                outputWriter.WriteNextLineToFile(valid);
            }
        }
    }

    class DialogueValidityChecker
    {
        private readonly List<string> _vocabulary;

        /// <summary>
        /// Hard coded dialogue from problem description
        /// </summary>
        public DialogueValidityChecker()
        {
            _vocabulary = new List<string>
            {
                "out", "output", "puton",
                "in","input", "one"
            };
        }

        public bool IsValidDialogue(string potentialDialogue)
        {
            // see if the start of the string is a valid dialogue... empty string is true (look in Main() assumptions)
            if (string.IsNullOrEmpty(potentialDialogue)) return true;

            foreach (var word in _vocabulary)
            {
                if (potentialDialogue.StartsWith(word))
                {
                    if (IsValidDialogue(potentialDialogue.Substring(word.Length)))
                    {
                        return true;
                    }
                }
            }

            return false;
        }
    }

    /// <summary>
    /// Uses Enumerator instead of Array, for memory efficiency
    /// </summary>
    class InputFileStreamReader
    {
        private readonly IEnumerator<string> _fileEnumerator;

        public InputFileStreamReader()
        {
            var fileLines = File.ReadLines("input.txt"); // filename assumption in Main() summary
            _fileEnumerator = fileLines.GetEnumerator();
        }

        public string ReadNextLineFromFile()
        {
            _fileEnumerator.MoveNext();
            return _fileEnumerator.Current;
        }
    }
    class OutputFileStreamWriter
    {
        private readonly FileStream _fileHandle;
        private int _writeBytePosition;

        public OutputFileStreamWriter()
        {
            _fileHandle = File.OpenWrite("output.txt"); // filename assumption in Main() summary
            _writeBytePosition = 0;
        }

        public void WriteNextLineToFile(bool validDialogue)
        {
            var stringToWrite = validDialogue ? "YES\n" : "NO\n";
            var bytesToWrite = Encoding.UTF8.GetBytes(stringToWrite.ToCharArray());
            _fileHandle.Write(bytesToWrite, _writeBytePosition, bytesToWrite.Length);
            _writeBytePosition += 0;//bytesToWrite.Length;
        }
    }
}