MergeSort.java

/**
 * Java implementation of merge sort
 *
 * @author Carlos Abraham Hernandez 
 * @email  abraham@abranhe.com
 */

import java.util.Arrays;

public class MergeSort {

    // Merge the two half into a sorted data.
    public void merge(int arr[], int left, int middle, int right) {

        int n1 = middle - left + 1;
        int n2 = right - middle;

        int L[] = new int[n1];
        int R[] = new int[n2];

        for (int i = 0; i < n1; ++i)
            L[i] = arr[left + i];
        for (int j = 0; j < n2; ++j)
            R[j] = arr[middle + 1 + j];

        int i = 0, j = 0;

        int k = left;
        while (i < n1 && j < n2) {
            if (L[i] <= R[j]) {
                arr[k] = L[i];
                i++;
            } else {
                arr[k] = R[j];
                j++;
            }
            k++;
        }

        while (i < n1) {
            arr[k] = L[i];
            i++;
            k++;
        }

        /* Copy remaining elements of R[] if any */
        while (j < n2) {
            arr[k] = R[j];
            j++;
            k++;
        }
    }

    // Main function that sorts arr[l..r] using
    void sort(int arr[], int left, int right) {
        if (left < right) {
            // Find the middle point
            int middle = (left + right) / 2;

            // Sort first and second halves
            sort(arr, left, middle);
            sort(arr, middle + 1, right);

            // Merge the sorted halves
            merge(arr, left, middle, right);
        }
    }

    // Default values
    void sort(int arr[]) {
        int l = 0; // First index
        int r = arr.length - 1; // Last index
        sort(arr, l, r);
    }

    // Test Functionality
    public static void main(String args[]) {

        int[] arr = new int[20];

        for (int i = 0; i < arr.length; i++) {
            arr[i] = (int) (Math.random() * 999 + 1);
        }

        System.out.println("Unordered List: \n" + Arrays.toString(arr));

        MergeSort m = new MergeSort();
        m.sort(arr);

        System.out.println("Ordered  List:  \n" + Arrays.toString(arr));
    }
}