using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        MultiMap<string> m1 = new MultiMap<string>();

        m1.Add("animal", "cat");

        m1.Add("animal", "dog");

        m1.Add("humans", "tom");

        m1.Add("humans", "john");

        foreach(var key in m1.Keys)

        {

            Console.Write("Key = " + key + " => Values = " );

            foreach(var v in m1[key])

            {

                Console.Write(v + " ");

            }

            Console.WriteLine();

        }       

    }

}

​

public class MultiMap<V>

{

    private Dictionary<string, List<V>> _map = new Dictionary<string, List<V>>();

    public void Add(string key, V val)

    {

        List<V> list;

        if(this._map.TryGetValue(key, out list))

        {

            list.Add(val);

        } else {

            list = new List<V>();

            list.Add(val);

            this._map.Add(key, list);

        }

    }

    public IEnumerable<string> Keys

    {

        get { return this._map.Keys; }

    }

    public List<V> this[string key]

    {

        get

        {

                List<V> list;

                if(!this._map.TryGetValue(key, out list)) 

                {

                    list = new List<V>();

                    this._map.Add(key, list);

                }

                return list;

        }

    }

}