int partition(int a[], int start, int end) {

  int pivot = a[end];

  int pIndex = start;

  for (int i = start; i < end; i++) {
    if (a[i] <= pivot) {
      swap(a[i], a[pIndex]);
      pIndex++;
    }
  }

  swap(a[pIndex], a[end]);

  return pIndex;
}

void quicksort(int a[], int start, int end) {

  if (start >= end) {
    return;
  }

  int pivot = partition(a, start, end);

  quicksort(a, start, pivot - 1);

  quicksort(a, pivot + 1, end);
}