gistfile1.cs

public static string Reverse(string input)
        {
            var builder = new StringBuilder(input.Length);
            for (var i = input.Length - 1; i >= 0; i--)
            {
                builder.Append(input[i]);
            }
            return builder.ToString();
        }
        
        public static string CountOccurence(string input)
        {
            var bookKeeper = new Dictionary<char, int>();
            foreach (var character in input)
            {
                if (bookKeeper.ContainsKey(character))
                    bookKeeper[character]++;
                else
                    bookKeeper.Add(character, 1);
            }

            var builder = new StringBuilder();
            foreach (var pair in bookKeeper)
            {
                builder.AppendFormat("{0}: {1}, ", pair.Key, pair.Value);
            }
            return builder.ToString();
        }