static void Main(string[] args)
{
    var numbers = new int[] { 1, 3, 3, 3, 5, 5, 5, 5, 5, 7, 7, 7, 7, 7, 7, 7 };

    var treeList = new List<HuffmanTree>();
    treeList.AddRange(from n in numbers
                     group n by n into g
                     select new HuffmanTree { Value = g.Key, Degree = g.Count() });

    while (treeList.Count>1)
    {
        var sel = from i in treeList
                 orderby i.Degree ascending
                 select i;
        var min = sel.Take(2).ToArray();

        treeList.Add(new HuffmanTree { Left = min[0], Right = min[1], Degree = min[0].Degree + min[1].Degree });

        treeList.Remove(min[0]);
        treeList.Remove(min[1]);
    }
}

class HuffmanTree
{
    public HuffmanTree Left { get; set; }
    public HuffmanTree Right { get; set; }

    public int Value { get; set; }
    public int Degree { get; set; }

    public override string ToString()
    {
        return Value + " " + Degree;
    }
}