Creating a power set of a Sequence

Question

I am trying to create a program that is a base for creating possible combinations of a sequence, string or a number. This is some sort of encryption / decryption program. I

Answer 1

Very late to the game, but why not the approach below? It seems significantly simpler than the suggestions posted here:

    /*
    Description for a sample set {1, 2, 2, 3}:
    Step 1 - Start with {}:
    {}
    Step 2 - "Expand" previous set by adding 1:
    {}
    ---
    {1}
    Step 3 - Expand previous set by adding the first 2:
    {}
    {1}
    ---
    {2}
    {1,2}
    Step 4 - Expand previous set by adding the second 2:
    {}
    {1}
    {2}
    {1,2}
    ---
    {2}
    {1,2}
    {2,2}
    {1,2,2}
    Step 5 - Expand previous set by adding 3:
    {}
    {1}
    {2}
    {1,2}
    {2}
    {1,2}
    {2,2}
    {1,2,2}
    ---
    {3}
    {1,3}
    {2,3}
    {1,2,3}
    {2,3}
    {1,2,3}
    {2,2,3}
    {1,2,2,3}
    Total elements = 16 (i.e. 2^4), as expected.
    */

    private static void PowerSet(IList nums, ref IList> output)
    {
        // ToDo: validate args
        output.Add(new List());
        ExpandSet(nums, 0, ref output);
    }

    private static void ExpandSet(IList nums, int pos, ref IList> output)
    {
        if (pos == nums.Count)
        {
            return;
        }

        List tmp;
        int item = nums[pos];

        for (int i = 0, n = output.Count; i < n; i++)
        {
            tmp = new List();
            tmp.AddRange(output[i]);
            tmp.Add(item);
            output.Add(tmp);
        }

        ExpandSet(nums, pos + 1, ref output);
    }