feat(solutions): add sorting-algorithms/cs/merge-sort

challenges/sorting-algorithms/solutions/cs/merge-sort/README.md

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+# sorting-algorithms/cs/merge-sort
+
+Created by [@Divlo](https://github.com/Divlo) on 11 September 2021.

challenges/sorting-algorithms/solutions/cs/merge-sort/Solution.cs

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+using System;
+using System.Collections.Generic;
+
+namespace Solution
+{
+    class Program
+    {
+        static void Main()
+        {
+            string line = Console.ReadLine();
+            List<int> numbers = new List<int>();
+            while ((line = Console.ReadLine()) != null)
+            {
+                numbers.Add(int.Parse(line));
+            }
+            int[] result = MergeSort(numbers.ToArray());
+            foreach (int number in result)
+            {
+                Console.WriteLine(number);
+            }
+        }
+
+        public static int[] MergeSort(int[] array)
+        {
+            if (array.Length <= 1)
+            {
+                return array;
+            }
+            int middle = array.Length / 2;
+            int[] left = new int[middle];
+            int[] right = new int[array.Length - middle];
+            for (int index = 0; index < middle; index++)
+            {
+                left[index] = array[index];
+            }
+            for (int index = middle; index < array.Length; index++)
+            {
+                right[index - middle] = array[index];
+            }
+            left = MergeSort(left);
+            right = MergeSort(right);
+            return Merge(left, right);
+        }
+
+        public static int[] Merge(int[] left, int[] right)
+        {
+            int[] result = new int[left.Length + right.Length];
+            int leftIndex = 0;
+            int rightIndex = 0;
+            for (int index = 0; index < result.Length; index++)
+            {
+                if (leftIndex >= left.Length)
+                {
+                    result[index] = right[rightIndex];
+                    rightIndex++;
+                }
+                else if (rightIndex >= right.Length)
+                {
+                    result[index] = left[leftIndex];
+                    leftIndex++;
+                }
+                else if (left[leftIndex] < right[rightIndex])
+                {
+                    result[index] = left[leftIndex];
+                    leftIndex++;
+                }
+                else
+                {
+                    result[index] = right[rightIndex];
+                    rightIndex++;
+                }
+            }
+            return result;
+        }
+    }
+}