using System;
using System.Globalization;
using System.Linq;
using System.Collections.Generic;

public class Test
{
	public static void Main()
	{
		var words = new List<string>{"golf", "hip", "hop", "hotel", "grass", "world", "wee"};
	    IEnumerable<string> lastWordOfConsecutiveFirstCharGroups = words
	        .GroupAdjacent(str => str[0])
	        .Select(g => g.Last());
	    Console.WriteLine(string.Join(",",lastWordOfConsecutiveFirstCharGroups.ToArray()));
	
	    words=new List<string>{"apples", "armies", "black", "beer", "bastion", "cat", "cart", "able", "art", "bark"};
	    lastWordOfConsecutiveFirstCharGroups = words
	       .GroupAdjacent(str => str[0])
	       .Select(g => g.Last());
	    Console.WriteLine(string.Join(",",lastWordOfConsecutiveFirstCharGroups.ToArray()));
	}
}

public static class Extensions
{
    public static IEnumerable<IGrouping<TKey, TSource>> GroupAdjacent<TSource, TKey>(
        this IEnumerable<TSource> source,
        Func<TSource, TKey> keySelector)
    {
        TKey last = default(TKey);
        bool haveLast = false;
        List<TSource> list = new List<TSource>();
        foreach (TSource s in source)
        {
            TKey k = keySelector(s);
            if (haveLast)
            {
                if (!k.Equals(last))
                {
                    yield return new GroupOfAdjacent<TSource, TKey>(list, last);
                    list = new List<TSource>();
                    list.Add(s);
                    last = k;
                }
                else
                {
                    list.Add(s);
                    last = k;
                }
            }
            else
            {
                list.Add(s);
                last = k;
                haveLast = true;
            }
        }
        if (haveLast)
            yield return new GroupOfAdjacent<TSource, TKey>(list, last);
    }

    public class GroupOfAdjacent<TSource, TKey> : IEnumerable<TSource>, IGrouping<TKey, TSource>
    {
        public TKey Key { get; set; }
        private List<TSource> GroupList { get; set; }
        System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
        {
            return ((System.Collections.Generic.IEnumerable<TSource>)this).GetEnumerator();
        }
        System.Collections.Generic.IEnumerator<TSource> System.Collections.Generic.IEnumerable<TSource>.GetEnumerator()
        {
            foreach (var s in GroupList)
                yield return s;
        }
        public GroupOfAdjacent(List<TSource> source, TKey key)
        {
            GroupList = source;
            Key = key;
        }
    }
}