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();
}