using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        int[] nums = new int[] { 2, 7, 7, 8 };

        var result = TwoSum(nums, 15);

        Console.WriteLine(string.Join(" ", result));

    }

    public static int[] TwoSum(int[] nums, int target)

    {

        var numsDictionary = new Dictionary<int, int>();

​

        var complement = 0;

        for (var i = 0; i < nums.Length; i++)

        {

            complement = target - nums[i];// 5 = 7 -2

            var index = 0;

            if (complement > 0 && numsDictionary.TryGetValue(complement, out index))

            {

                return new int[] { index, i };

            }

​

            if (numsDictionary.ContainsKey(nums[i]) == false)

            {

                numsDictionary.Add(nums[i], i);

            }

        }

​

        return null;

    }

}