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GeezerVison is showing github.com — Algorithm-Implementations/QuickSortList.java at master · devForTheFuture/Algorithm-Implementations

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package algorithms;
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Random;
/**
* Quick sort algorithm (simple) based on pseudo code on Wikipedia "Quick Sort" article.
*
* Complexity worst case O(N^2), best case O(N log N)
*
* @see http://en.wikipedia.org/wiki/Quicksort#Simple_version
* @author djitz
*/
public class QuickSortList
{
public static void main(String[] args)
{
//Generate an integer array of length 5
List<Integer> input = generateRandomNumbers(5);
//Before sort
System.out.println(input);
//After sort
System.out.println(quicksort(input));
}
/**
* This method sort the input ArrayList using quick sort algorithm.
* @param input the ArrayList of integers.
* @return sorted ArrayList of integers.
*/
private static List<Integer> quicksort(List<Integer> input)
{
if(input.size() <= 1)
{
return input;
}
int middle = input.size() / 2;
int pivot = input.get(middle);
LinkedList<Integer> less = new LinkedList<Integer>();
LinkedList<Integer> greater = new LinkedList<Integer>();
for (int i = 0; i < input.size(); i++)
{
if(input.get(i) <= pivot)
{
if(i == middle)
{
continue;
}
less.add(input.get(i));
}
else
{
greater.add(input.get(i));
}
}
return concatenateLists(quicksort(less), pivot, quicksort(greater));
}
/**
* Join the less array, pivot integer, and greater array
* to single array.
* @param less integer ArrayList with values less than pivot.
* @param pivot the pivot integer.
* @param greater integer ArrayList with values greater than pivot.
* @return the integer ArrayList after join.
*/
private static List<Integer> concatenateLists(List<Integer> less, int pivot, List<Integer> greater)
{
List<Integer> list = new ArrayList<Integer>();
for (Integer i : less)
{
list.add(i);
}
list.add(pivot);
for (Integer i : greater)
{
list.add(i);
}
return list;
}
/**
* This method generate a ArrayList with length n containing random integers .
* @param n the length of the ArrayList to generate.
* @return ArrayList of random integers with length n.
*/
private static List<Integer> generateRandomNumbers(int n)
{
List<Integer> list = new ArrayList<Integer>();
Random random = new Random();
for (int i = 0; i < n; i++)
{
list.add(random.nextInt(n * 10));
}
return list;
}
}
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