add solution for Two Sum task

Author: nMykhailenko

C#/LeetCodeResolves/LeetCodeResolves/LeetCodeResolves.csproj

@@ -3,7 +3,7 @@
   <PropertyGroup>
     <OutputType>Exe</OutputType>
     <TargetFramework>netcoreapp3.1</TargetFramework>
-    <StartupObject>LeetCodeResolves.LongestPalindrome.Program</StartupObject>
+    <StartupObject>LeetCodeResolves.TwoSum.Program</StartupObject>
   </PropertyGroup>
 
 </Project>

C#/LeetCodeResolves/LeetCodeResolves/TwoSum/Program.cs

@@ -0,0 +1,36 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace LeetCodeResolves.TwoSum
+{
+    class Program
+    {
+        static void Main(string[] args)
+        {
+            var nums = new int[] { -3, 4, 3, 90};
+            var target = 0;
+            var result = TwoSum(nums, target);
+
+            Console.WriteLine($"The result is: {result[0]}, {result[1]}.");
+            Console.ReadLine();
+        }
+
+        static int[] TwoSum(int[] nums, int target)
+        {
+            var dictionary = new Dictionary<int, int>();
+            for(var i = 0; i < nums.Length; i++)
+            {
+                var complement = target - nums[i];
+                if (dictionary.ContainsKey(complement))
+                {
+                    return new int[] { i, dictionary[complement] };
+                }
+
+                dictionary.Add(nums[i], i);
+            }
+
+            throw new InvalidOperationException();
+        }
+    }
+}