Java Data Structure How to - Implement a Quick sort with median-of-three partitioning

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Question

We would like to know how to implement a Quick sort with median-of-three partitioning.

Answer

public class MainClass {
  public static void main(String[] args) {
    int[] intArray = { 1, 9, 2, 8, 3, 7, 4, 6, 5 };
    for (int i : intArray) {
      System.out.println(i);//from  w  w  w.  j  a  v  a2  s .  c o  m
    }
    quickSort(intArray);
    for (int i : intArray) {
      System.out.println(i);
    }
  }

  public static void quickSort(int[] intArray) {
    recQuickSort(intArray, 0, intArray.length - 1);
  }

  public static void recQuickSort(int[] intArray, int left, int right) {
    int size = right - left + 1;
    if (size <= 3)
      manualSort(intArray, left, right);
    else {
      double median = medianOf3(intArray, left, right);
      int partition = partitionIt(intArray, left, right, median);
      recQuickSort(intArray, left, partition - 1);
      recQuickSort(intArray, partition + 1, right);
    }
  }

  public static int medianOf3(int[] intArray, int left, int right) {
    int center = (left + right) / 2;

    if (intArray[left] > intArray[center])
      swap(intArray, left, center);

    if (intArray[left] > intArray[right])
      swap(intArray, left, right);

    if (intArray[center] > intArray[right])
      swap(intArray, center, right);

    swap(intArray, center, right - 1);
    return intArray[right - 1];
  }

  public static void swap(int[] intArray, int dex1, int dex2) {
    int temp = intArray[dex1];
    intArray[dex1] = intArray[dex2];
    intArray[dex2] = temp;
  }

  public static int partitionIt(int[] intArray, int left, int right, double pivot) {
    int leftPtr = left;
    int rightPtr = right - 1;

    while (true) {
      while (intArray[++leftPtr] < pivot)
        ;
      while (intArray[--rightPtr] > pivot)
        ;
      if (leftPtr >= rightPtr)
        break;
      else
        swap(intArray, leftPtr, rightPtr);
    }
    swap(intArray, leftPtr, right - 1);
    return leftPtr;
  }

  public static void manualSort(int[] intArray, int left, int right) {
    int size = right - left + 1;
    if (size <= 1)
      return;
    if (size == 2) {
      if (intArray[left] > intArray[right])
        swap(intArray, left, right);
      return;
    } else {
      if (intArray[left] > intArray[right - 1])
        swap(intArray, left, right - 1);
      if (intArray[left] > intArray[right])
        swap(intArray, left, right);
      if (intArray[right - 1] > intArray[right])
        swap(intArray, right - 1, right);
    }
  }

}

The code above generates the following result.

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