Ayende @ Rahien Interview Question: Fix the Index - http://ayende.com/blog/170402/interview-question-fix-the-index?Key=8f7c7054-ec80-4688-a474-07e404ad0007

Indexer.cs

public class Indexer
{
  private Dictionary<string, List<string>> terms = 
    new Dictionary<string, List<string>>(StringComparer.OrdinalIgnoreCase);
  private Dictionary<string, List<string>> itemsByTerm = 
    new Dictionary<string, List<string>>();

  public void Index(string docId, string text)
  {
    // First check to see if we already have anything for this docId
    if (itemsByTerm.ContainsKey(docId))
    {
      RemoveItemsByTerm(docId);
    }

    var words = text.Split();
    foreach (var term in words)
    {
      List<string> val;
      if (terms.TryGetValue(term, out val) == false)
      {
        val = new List<string>();
        terms[term] = val;
      }
      val.Add(docId);
    }

  List<string> docs;
  if (itemsByTerm.TryGetValue(docId, out docs) == false)
  {
    docs = new List<string>();
    itemsByTerm[docId] = docs;
  }		
  docs.AddRange(words);
}

  public List<string> Query(string term)
  {
    List<string> val;
    terms.TryGetValue(term, out val);
    return val ?? new List<string>();
  }

  public void Delete(string docId)
  {
    RemoveItemsByTerm(docId);
  }

  private void RemoveItemsByTerm(string docId)
  {
    var termsWithItem = itemsByTerm[docId];
    foreach (var term in termsWithItem)
    {
      terms.Remove(term);
    }
    itemsByTerm.Remove(docId);
  }
}