PhoneNumberIterative

public IList<string> LetterCombinations(string digits)
    {
        var result = new List<string>();

            var numberToLetters = new[] { "", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" };

            if (string.IsNullOrEmpty(digits))
                return result;

            if (digits.Any(x => !char.IsDigit(x)))
                return result;

            var queue = new Queue<string>();
            queue.Enqueue("");

            for (var i = 0; i < digits.Length; i++)
            {
                while (queue.Peek().Length == i)
                {
                    var temp = queue.Dequeue();
                    foreach (var letter in numberToLetters[digits[i] - 48])
                    {
                        queue.Enqueue(temp + letter);
                    }
                }
            }
            return queue.ToList();
    }