組み合わせとか順列とか

SetUtil.cs

public static class SetUtil
{
	/// <summary>組み合わせ</summary>
	public static IEnumerable<IEnumerable<T>> Combination<T>(this IEnumerable<T> items, int r)
	{
		if (r == 0)
		{
			yield return Enumerable.Empty<T>();
			yield break;
		}

		int i = 1;
		foreach (var item in items)
			foreach (var c in Combination(items.Skip(i++), r - 1))
				yield return c.Before(item);
	}

	/// <summary>順列</summary>
	public static IEnumerable<IEnumerable<T>> Permutation<T>(this IEnumerable<T> items, int? k = null)
	{
		if (k == null) k = items.Count();
		if (k == 0)
		{
			yield return Enumerable.Empty<T>();
			yield break;
		}

		int i = 1;
		foreach (var item in items)
			foreach (var p in Permutation(items.Where((_, index) => i++ != index), k - 1))
				yield return p.Before(item);
	}

	private static IEnumerable<T> Before<T>(this IEnumerable<T> items, T first)
	{
		yield return first;
		foreach (var item in items) yield return item;
	}

	/// <summary>複数のリストから一つずつ取り出した組み合わせを作る</summary>
	public static IEnumerable<List<T>> GetCombinationSets<T>(List<List<T>> items)
	{
		var counter = new int[items.Count];
		do
		{
			yield return items.Zip(counter, (item, index) => item.ElementAt(index)).ToList();
		} while (CountUp(items, counter));
	}

	private static bool CountUp<T>(List<List<T>> items, int[] counter)
	{
		int index = counter.Length - 1;
		counter[index]++;
		while (counter[index] >= items.ElementAt(index).Count)
		{
			counter[index] = 0;
			index--;
			if (index < 0) return false;
			counter[index]++;
		}

		return index >= 0;
	}
}