using System;

using System.Collections.Generic;

​

namespace TwoSum

{

    public class Program

    {

        public static Tuple<int, int> FindTwoSum(IList<int> list, int sum)

        {

            var l = list as List<int>;

            if (l == null || l.Count < 2)

                return null;

​

            Dictionary<int, int> dict = new Dictionary<int, int>();

​

            for (int i = 0; i < l.Count; i++)

            {

                if(!dict.ContainsKey(l[i]))

                    dict[l[i]] = i;

            }

​

            for (int i = 0; i < l.Count; i++)

            {

                int diff = sum - l[i];

                if (dict.ContainsKey(diff))

                {

                    int index = dict[diff];

                    return new Tuple<int, int>(i, index);

                }

            }

​

            return null;

        }

​

        public static void Main(string[] args)

        {

            Tuple<int, int> indices = FindTwoSum(new List<int>() { 10, 12, 0, 1,1,3,3, 5, 7, 9 }, 8);

            if(indices == null)

            {

                Console.WriteLine("not found");

                return;

            }

            Console.WriteLine(indices.Item1 + " " + indices.Item2);

        }

    }

}

​