using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        const int Take = 3;

        var pepperoni = new List<string>()

        {"pepperoni"};

        var meat = new List<string>()

        {"pepperoni", "beef"};

        var boring = new List<string>()

        {"cheese"};

        var pizzas = new List<Pizza>()

        {new Pizza{Name = "Pepperoni Feast", Toppings = new[]{"pepperoni"}}, new Pizza{Name = "Meat Feast", Toppings = meat}, new Pizza{Name = "Boring", Toppings = boring}, new Pizza{Name = "Pepperoni Feast", Toppings = new List<string>{"pepperoni"}}};

        var groupedPizzas = pizzas.GroupBy(e => e.Toppings, new ToppingComparer()).Select(e => new

        {

        Toppings = e.Key, Count = e.Count()}

​

        ).OrderByDescending(e => e.Count).Take(20);

        foreach (var pizza in groupedPizzas)

        {

            Console.WriteLine(string.Format("{0}: {1}", string.Join(", ", pizza.Toppings), pizza.Count));

        }

    }

}

​

public class ToppingComparer : IEqualityComparer<IEnumerable<string>>

{

    public bool Equals(IEnumerable<string> x, IEnumerable<string> y)

    {

        return x.SequenceEqual(y);

    }

​

    public int GetHashCode(IEnumerable<string> obj)

    {

        var hash = 0;

        foreach (var word in obj)

        {

            hash += word.GetHashCode();

        }

​

        return hash;

    }

}

​

public class Pizza

{

    public string Name

    {

        get;

        set;

    }

​

    public IEnumerable<string> Toppings

    {

        get;

        set;

    }

}