/*********************************************************************************

* (Improve quick sort) The quick sort algorithm presented in the book selects *

* the first element in the list as the pivot. Revise it by selecting the median *

* among the first, middle, and last elements in the list. *

*********************************************************************************/

public class Exercise_23_04 {

public static void quickSort(int[] list) {

quickSort(list, 0, list.length - 1);

}

public static void quickSort(int[] list, int first, int last) {

if (last > first) {

int pivotIndex = partition(list, first, last);

quickSort(list, first, pivotIndex - 1);

quickSort(list, pivotIndex + 1, last);

}

}

/** Partition the aray list[first..last] */

public static int partition(int[] list, int first, int last) {

int pivot = list[first]; // choose the first element as the pivot

int low = first + 1; // Index for forward search

int high = last; // Index for backward search

while (high > low) {

// Search forward from left

while (low <= high && list[low] <= pivot)

low++;

// Search backward from right

while (low <= high && list[high] > pivot)

high--;

// Swap two elements in the list

if (high > low) {

int temp = list[high];

list[high] = list[low];

list[low] = temp;

}

}

while (high > first && list[high] >= pivot)

high--;

// Swap pivot with list[high]

if (pivot > list[high]) {

list[first] = list[high];

list[high] = pivot;

return high;

}

else {

return first;

}

}

/** A test method */

public static void main(String[] args) {

int[] list = {2, 3, 2, 5, 6, 1, -2, 3, 14, 12};

quickSort(list);

for (int i = 0; i < list.length; i++)

System.out.print(list[i] + " ");

}

}

publicclassExercise_23_04 {

publicstaticvoidquickSort(int[]list) {

quickSort(list,0,list.length -1);

}

publicstaticvoidquickSort(int[]list,intfirst,intlast) {

if (last >first) {

intpivotIndex =partition(list,first,last);

quickSort(list,first,pivotIndex -1);

quickSort(list,pivotIndex +1,last);

}

}

publicstaticintpartition(int[]list,intfirst,intlast) {

intmiddle =list[(list.length -1) /2];

intpivot =median(first,middle,last);

intlow =first +1;// Index for forward search

inthigh =last;// Index for backward search

while (high >low) {

while (low <=high &&list[low] <=pivot)

low++;

while (low <=high &&list[high] >pivot)

high--;

if (high >low) {

inttemp =list[high];

list[high] =list[low];

list[low] =temp;

}

}

while (high >first &&list[high] >=pivot)

high--;

if (pivot >list[high]) {

list[first] =list[high];

list[high] =pivot;

returnhigh;

}

else {

returnfirst;

}

}

publicstaticvoidmain(String[]args) {

int[]list = {2,3,2,5,6,1, -2,3,14,12};

quickSort(list);

for (inti =0;i <list.length;i++)

System.out.print(list[i] +" ");

System.out.println();

}

publicstaticintmedian(intfirst,intmiddle,intlast) {

returnMath.max(Math.min(first,middle),

Math.min(Math.max(first,middle),last));

}

}