Ayende.com Interview question: fix the index

gistfile1.txt

public class Indexer
{
    private Dictionary<string, List<string>> terms = 
        new Dictionary<string, List<string>>(StringComparer.OrdinalIgnoreCase);
    private Dictionary<string,  List<List<string>>> docIDs = 
        new Dictionary<string, List<List<string>>>();
        
    public void Index(string docId, string text)
    {
    
        List<List<string>> idList;
        if(docIDs.TryGetValue(docId, out idList) == true)
        {
            foreach(List<string> ids in idList)
            {
                ids.Remove(docId);
            }
        }
        idList = new List<List<string>>();
        docIDs[docId] = idList;
        
        var words = text.Split();
        foreach (string term in words)
        {
            List<string> val;
            if (terms.TryGetValue(term, out val) == false)
            {
                val = new List<string>();
                terms[term] = val;
            }
            val.Add(docId);
            idList.Add(val);
        }
    }
 
    public List<string> Query(string term)
    {
        List<string> val;
        terms.TryGetValue(term, out val);
        return val ?? new List<string>();
    }
}