// QuickSort: carries out quicksort
public class QuickSort {

   private void swapRef(double a[], int i, int j) {
      double temp = a[i];
      a[i] = a[j];
      a[j] = temp;
   }

   public void quicksort(double a[]) {
      quicksort(a, 0, a.length - 1);
   }

   private void quicksort(double a[], int low, int high) {
      if (high >low) {
         int pivot = partition(a, low, high);
         quicksort(a, low, pivot-1);
         quicksort(a, pivot+1, high);
       }
   }
   
   private int partition(double a[], int low, int high) {
      int left, right;
      double pivot_item = a[low];
      int pivot = left = low;
      right = high;
      while ( left < right ) {
         while(left < a.length - 1 &&
               a[left] <= pivot_item) // Move left while item < pivot
            left++;
         while(a[right] > pivot_item) // Move right while item > pivot
            right--;
         if (left < right) swapRef(a, left, right);
      }
      // right is final position for the pivot
      a[low] = a[right];
      a[right] = pivot_item;
      return right;
   }
}