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

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

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+# sorting-algorithms/dart/merge-sort
+
+Created by [@Divlo](https://github.com/Divlo) on 29 June 2021.
+
+| Algorithm                                                   | Best Case   | Average Case | Worst Case  |
+| ----------------------------------------------------------- | ----------- | ------------ | ----------- |
+| [Merge sort](https://wikipedia.org/wiki/Merge_sort)         | O(n log(n)) | O(n log(n))  | O(n log(n)) |

challenges/sorting-algorithms/solutions/dart/merge-sort/solution.dart

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+import 'dart:io';
+
+void main() {
+  int length = int.parse(readLineSync());
+  List<int> numbers = [];
+  for (int indexInput = 0; indexInput < length; indexInput++) {
+    numbers.add(int.parse(readLineSync()));
+  }
+  List<int> sortedNumbers = mergeSort(numbers);
+  for (int index = 0; index < length; index++) {
+    print(sortedNumbers[index]);
+  }
+}
+
+List<List<int>> divideList(List<int> numbers) {
+  var middle = (numbers.length / 2).round();
+  var left = numbers.sublist(0, middle);
+  var right = numbers.sublist(middle, numbers.length);
+  return [left, right];
+}
+
+List<int> merge(List<int> numbers1, List<int> numbers2) {
+  var indexNumbers1 = 0;
+  var indexNumbers2 = 0;
+  var result = <int>[];
+  while (indexNumbers1 < numbers1.length && indexNumbers2 < numbers2.length) {
+    if (numbers1[indexNumbers1] < numbers2[indexNumbers2]) {
+      result.add(numbers1[indexNumbers1]);
+      indexNumbers1 += 1;
+    } else {
+      result.add(numbers2[indexNumbers2]);
+      indexNumbers2 += 1;
+    }
+  }
+  for (var index = indexNumbers1; index < numbers1.length; index++) {
+    result.add(numbers1[index]);
+  }
+  for (var index = indexNumbers2; index < numbers2.length; index++) {
+    result.add(numbers2[index]);
+  }
+  return result;
+}
+
+List<int> mergeSort(Iterable<int> numbersInput) {
+  var numbers = [...numbersInput];
+  if (numbers.length <= 1) {
+    return numbers;
+  }
+  var dividedList = divideList(numbers);
+  var left = mergeSort(dividedList[0]);
+  var right = mergeSort(dividedList[1]);
+  return merge(left, right);
+}
+
+String readLineSync() {
+  String? string = stdin.readLineSync();
+  return string == null ? '' : string;
+}