Google search results: How to find the minimum window that contains all the search keywords?

Question

What is the complexity of the algorithm is that is used to find the smallest snippet that contains all the search key words?

Answer 1

I think the solution proposed by AndreyT assumes no duplicates exists in the keywords/search terms. Also, the current block can get as big as the text itself if text contains lot of duplicate keywords. For example: Text: 'ABBBBBBBBBB' Keyword text: 'AB' Current Block: 'ABBBBBBBBBB'

Anyway, I have implemented in C#, did some basic testing, would be nice to get some feedback on whether it works or not :)

    static string FindMinWindow(string text, string searchTerms)
    {
        Dictionary searchIndex = new Dictionary();
        foreach (var item in searchTerms)
        {
            searchIndex.Add(item, false);
        }

        Queue> currentBlock = new Queue>();
        int noOfMatches = 0;
        int minLength = Int32.MaxValue;
        int startIndex = 0;
        for(int i = 0; i < text.Length; i++)
        {
            char item = text[i];
            if (searchIndex.ContainsKey(item))
            {
                if (!searchIndex[item])
                {
                    noOfMatches++;
                }

                searchIndex[item] = true;
                var newEntry = new Tuple ( item, i );
                currentBlock.Enqueue(newEntry);

                // Normalization step.
                while (currentBlock.Count(o => o.Item1.Equals(currentBlock.First().Item1)) > 1)
                {
                    currentBlock.Dequeue();
                }

                // Figuring out minimum length.
                if (noOfMatches == searchTerms.Length)
                {
                    var length = currentBlock.Last().Item2 - currentBlock.First().Item2 + 1;
                    if (length < minLength)
                    {
                        startIndex = currentBlock.First().Item2;
                        minLength = length;
                    }
                }
            }
        }
        return noOfMatches == searchTerms.Length ? text.Substring(startIndex, minLength) : String.Empty;
    }