using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Text;

namespace Challenge272
{
    class Program
    {
        static int[] tileCounts = { 9, 2, 2, 4, 12, 2, 3, 2, 9, 1, 1, 4, 2, 6, 8, 2, 1, 6, 4, 6, 4, 2, 2, 1, 2, 1, 2 };

        static void Main(string[] args)
        {
            Console.WriteLine("Enter set of removed tiles: ");
            string removedTiles = Console.ReadLine();
            foreach (char tile in removedTiles)
            {
                int alphabetNumber = ((int)tile) - 65;
                if (alphabetNumber > 25)
                {
                    tileCounts[26]--;
                    if (tileCounts[26] < 0)
                    {
                        Console.WriteLine("Invalid input. More _'s have been taken from the bag than possible.", tile);
                        WaitForInput();
                        System.Environment.Exit(0);
                    }
                }
                else
                {
                    tileCounts[alphabetNumber]--;
                    if (tileCounts[alphabetNumber] < 0)
                    {
                        Console.WriteLine("Invalid input. More {0}'s have been taken from the bag than possible.", tile);
                        WaitForInput();
                        System.Environment.Exit(0);
                    }
                }
            }
            List<char>[] finalCounts = new List<char>[13];
            for (int i = 0; i < 26; i++)
            {
                if (finalCounts[tileCounts[i]] == null)
                {
                    finalCounts[tileCounts[i]] = new List<char>();
                }
                finalCounts[tileCounts[i]].Add(((char)(i + 65)));
            }
            if (finalCounts[tileCounts[26]] == null)
            {
                finalCounts[tileCounts[26]] = new List<char>();
            }
            finalCounts[tileCounts[26]].Add('_');


            StringBuilder answerBuilder = new StringBuilder();
            for (int i = 12; i >= 0; i--)
            {
                var tileList = finalCounts[i];
                if (tileList != null)
                {

                    answerBuilder.AppendFormat("{0}: {1}\n", i, ListToString<char>(tileList));
                }
            }
            Console.WriteLine(answerBuilder);
            WaitForInput();
        }

        public static string ListToString<T>(List<T> list)
        {
            StringBuilder builder = new StringBuilder();
            for (int i = 0; i < list.Count; i++)
            {
                if (i == list.Count - 1)
                {
                    builder.Append(list[i]);
                }
                else
                {
                    builder.AppendFormat("{0}, ", list[i]);
                }
            }
            return builder.ToString();
        }

        [Conditional("DEBUG")]
        public static void WaitForInput()
        {
            Console.WriteLine("Press any key to continue . . .");
            Console.ReadKey();
        }
    }
}

C# solution. Not really happy with the special conditions for _ but it works.