void merge(int *arr, int s, int e) {
  int mid = s + (e - s) / 2;
  int len1 = mid - s + 1;
  int len2 = e - mid;
  int *first = new int[len1];
  int *second = new int[len2];

  // copy vales
  int mainArrayIndex = s;
  for (int i = 0; i < len1; i++) {
    first[i] = arr[mainArrayIndex++];
  }
  mainArrayIndex = mid + 1;
  for (int i = 0; i < len2; i++) {
    second[i] = arr[mainArrayIndex++];
  }

  // merge 2 sorted arrays
  int index1 = 0;
  int index2 = 0;
  mainArrayIndex = s;
  while (index1 < len1 && index2 < len2) {
    if (first[index1] < second[index2]) {
      arr[mainArrayIndex++] = first[index1++];
    } else {
      arr[mainArrayIndex++] = second[index2++];
    }
  }
  while (index1 < len1) {
    arr[mainArrayIndex++] = first[index1++];
  }
  while (index2 < len2) {
    arr[mainArrayIndex++] = second[index2++];
  }
}

void mergeSort(int *arr, int s, int e) {
  if (s >= e) {
    return;
  }
  int mid = s + (e - s) / 2;
  // left  part
  mergeSort(arr, s, mid);
  // right part
  mergeSort(arr, mid + 1, e);
  // merge
  merge(arr, s, e);
}