问生成2-4个字符α-数值

EN

// this generates all strings for characters 0-3 with lengths 2-3
List<string> result = GenerateStrings("0123", 2, 3);

private List<string> GenerateStrings(string allowedChars, int fromLength, int toLength)
{
    List<string> strings = new List<string>();

    int charsNum = allowedChars.Length;

    for (int currentLength = fromLength; currentLength <= toLength; currentLength++)
    {
        // initialize array of indexes to generate individual characters
        int[] indexes = new int[currentLength];
        do
        {
            StringBuilder sb = new StringBuilder(currentLength);
            // generate characters based on current index values
            for (int charPosition = 0; charPosition < currentLength; charPosition++)
            {
                sb.Append(allowedChars[indexes[charPosition]]);
            }
            strings.Add(sb.ToString());
            // until we can increment indexes array (still some character combinations available)
        } while (IncrementIndex(indexes, charsNum - 1, currentLength - 1));
    }

    return strings;
}

/// <summary>
/// Have array of integer indexes such as [0,0,0] with valid values 0-9. 
/// This function increments indexes from right by one with correct overflow handling, 
/// such that value [0,0,3] is incremented to [0,0,4], value [0,2,9] to [0,3,0] etc.
/// </summary>
/// <returns>True if increment was successful, false if maximal value was reached.</returns>
private bool IncrementIndex(int[] indexes, int maxItemValue, int incrementFromPosition)
{
    indexes[incrementFromPosition]++;
    // check if current position overflow allowed range
    if (indexes[incrementFromPosition] > maxItemValue)
    {
        if (incrementFromPosition == 0)
        {
            // we reached left-most position and can't increment more
            return false;
        }

        indexes[incrementFromPosition] = 0;
        // increment next position to the left
        return IncrementIndex(indexes, maxItemValue, incrementFromPosition-1);
    }

    return true;
}

        List<string> newlst = (from p in lst
                               from q in lst
                               select p + q).ToList();