Tak třeba C#:

public IEnumerable<string> GetIds(char[] chars, int length)
{
    foreach(var ch in chars)
    {
        if (length == 1)
	{
	    yield return ch.ToString();
	    continue;
	}
	var subs = GetIds(chars, length - 1);
	foreach(var sub in subs)
	{
	    yield return ch + sub;
	}
    }
}