Mergesort in Java

Mergesort

 
Mergesort is a divide and conquer algorithm. The sorting elements are stored in a collection. This collection is divided into two collections and these are again sorted via mergesort. Once the two collections are sorted then the result is combined
Mergesort will take the middle of the collection and takes then the two collection for the next iteration of mergesort. In the merging part mergesort runs through the both collections and selects the lowest of the both to insert it into a new collection.
In comparison to quicksort the divide part is simple in mergesort while the merging take is complex. In addition quicksort can work "inline", e.g. it does not have to create a copy of the collection while mergesort requires a copy

Program

Mergesort .java :-  
public class Mergesort {
  private int[] numbers;
  private int[] helper;

  private int number;

  public void sort(int[] values) {
    this.numbers = values;
    number = values.length;
    this.helper = new int[number];
    mergesort(0, number - 1);
  }

  private void mergesort(int low, int high) {
    // Check if low is smaller then high, if not then the array is sorted
    if (low < high) {
      // Get the index of the element which is in the middle
      int middle = (low + high) / 2;
      // Sort the left side of the array
      mergesort(low, middle);
      // Sort the right side of the array
      mergesort(middle + 1, high);
      // Combine them both
      merge(low, middle, high);
    }
  }

  private void merge(int low, int middle, int high) {

    // Copy both parts into the helper array
    for (int i = low; i <= high; i++) {
      helper[i] = numbers[i];
    }

    int i = low;
    int j = middle + 1;
    int k = low;
    // Copy the smallest values from either the left or the right side back
    // to the original array
    while (i <= middle && j <= high) {
      if (helper[i] <= helper[j]) {
        numbers[k] = helper[i];
        i++;
      } else {
        numbers[k] = helper[j];
        j++;
      }
      k++;
    }
    // Copy the rest of the left side of the array into the target array
    while (i <= middle) { numbers[k] = helper[i]; k++; i++; } }}


Test.java:-
import java.util.Random;

class Test
{

public static void main(String args[])
{
     int numbers = new int[SIZE];
    Random generator = new Random();
    for (int i = 0; i < numbers.length; i++) {
      numbers[i] = generator.nextInt(MAX);
   }
Mergesort sorter = new Mergesort();
sorter.sort(numbers);


}
}

Complexity Analysis

The following describes the runtime complexity of mergesort.
Mergesort sorts in worst case in O(n log n) time. Due to the required copying of the collection Mergesort is in the average case slower then Quicksort.