C#

List<string> dictionary = new List<string>()

// get the next sentence to process (you have to write 
// the GetNextSentence function)
string sentence = GetNextSentence();
// split the sentence into words
string[] words = sentence.ToLower().Split(' ');
// for each word
for (int i = 0; i < words.Length; i++)
{
    // if it's not already in the dictionary using the 
    // List.Contains method
    if (!dictionary.Contains(words[i])
    {
        // add it to the dictionary
        dictionary.Add(words[i]);
    }
}