List<string> result = new List<string>();
    public List<string> generateGeneralizedAbbreviation(string word)
    {
        // Begin process
        generate(word, 0, new List<char>());
        // coalesce contiguous spaces to count of spaces
        processResult();
        return result;
    }

    private void processResult()
    {
        for (int i = 0; i < result.Count; i++)
        {
            result[i] = processResult(result[i]);
        }
    }

    private string processResult(string result)
    {
        Stack<char> chars = new Stack<char>();
        for (int i = 0; i < result.Length; i++)
        {
            if (result[i] == ' ')
            {
                // if top is digit: pop, increment and put back otherwise push 1
                if (chars.TryPeek(out char top) && char.IsDigit(top))
                {
                    chars.Push((char)(chars.Pop() + 1));
                }
                else
                {
                    chars.Push('1');
                }
            }
            else
            {
                chars.Push(result[i]);
            }
        }
        string s = string.Empty;
        while (chars.TryPop(out char item))
        {
            s = $"{item}{s}";
        }
        return s;
    }

    private void generate(string word, int index, List<char> list)
    {
        // Base case: index out of bound, add to result
        if (index == word.Length)
        {
            result.Add(string.Concat(list));
            return;
        }
        // Take character at index and recursively generate with next character
        list.Add(word[index]);
        generate(word, index + 1, list);
        // Backtrack: undo taking character
        list.RemoveAt(list.Count - 1);
        // Take space and recursively generate with next character
        list.Add(' ');
        generate(word, index + 1, list);
        // Backtrack: undo taking space
        list.RemoveAt(list.Count - 1);
    }