using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        int[] array = {2,7,11,15};

        Console.WriteLine(TwoSum(array, 9));

    }

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

        //creata a hashmap {num, index}

        //{2:0, 7:1, 11:2, 15:3}

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

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

            int num = nums[i]; //item in array

            if (dict.TryGetValue(target-num, out int index)){

                //Console.WriteLine(index);

                return new[]{index, i};

            }

            dict[num] = i;

        }

        return null;

    }

}