merge_sort.dart

void mergeSorting(List list, int leftIndex, int middleIndex, int rightIndex) {
  int leftSize = middleIndex - leftIndex + 1;
  int rightSize = rightIndex - middleIndex;

  List leftList = new List(leftSize);
  List rightList = new List(rightSize);

  for (int i = 0; i < leftSize; i++) leftList[i] = list[leftIndex + i];
  for (int j = 0; j < rightSize; j++) rightList[j] = list[middleIndex + j + 1];

  int i = 0;
  int j = 0;
  int k = leftIndex;

  while (i < leftSize && j < rightSize) {
    if (leftList[i] <= rightList[j]) {
      list[k] = leftList[i];
      i++;
    } else {
      list[k] = rightList[j];
      j++;
    }
    k++;
  }

  while (i < leftSize) {
    list[k] = leftList[i];
    i++;
    k++;
  }

  while (j < rightSize) {
    list[k] = rightList[j];
    j++;
    k++;
  }
}

void mergeSort(List list, int leftIndex, int rightIndex) {
  if (leftIndex < rightIndex) {
    int middleIndex = (rightIndex + leftIndex) ~/2;

    mergeSort(list, leftIndex, middleIndex);
    mergeSort(list, middleIndex + 1, rightIndex);

    mergeSorting(list, leftIndex, middleIndex, rightIndex);   //merging here!
  }
}