MergeSortExample.java

public class MergeSortExample {

    // Main function to sort the array using Merge Sort
    public static void mergeSort(int[] arr, int left, int right) {
        if (left < right) {
            // Find the middle point
            int mid = (left + right) / 2;

            // Sort first and second halves
            mergeSort(arr, left, mid);
            mergeSort(arr, mid + 1, right);

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

    // Merge two subarrays of arr[]
    public static void merge(int[] arr, int left, int mid, int right) {
        // Sizes of two subarrays
        int n1 = mid - left + 1;
        int n2 = right - mid;

        // Temp arrays
        int[] leftArray = new int[n1];
        int[] rightArray = new int[n2];

        // Copy data to temp arrays
        for (int i = 0; i < n1; ++i)
            leftArray[i] = arr[left + i];
        for (int j = 0; j < n2; ++j)
            rightArray[j] = arr[mid + 1 + j];

        // Merge the temp arrays

        int i = 0, j = 0; // Initial indexes
        int k = left;     // Initial index of merged subarray

        while (i < n1 && j < n2) {
            if (leftArray[i] <= rightArray[j]) {
                arr[k] = leftArray[i];
                i++;
            } else {
                arr[k] = rightArray[j];
                j++;
            }
            k++;
        }

        // Copy remaining elements of leftArray[]
        while (i < n1) {
            arr[k] = leftArray[i];
            i++;
            k++;
        }

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

    // Utility function to print the array
    public static void printArray(int[] arr) {
        for (int value : arr)
            System.out.print(value + " ");
        System.out.println();
    }

    // Driver code
    public static void main(String[] args) {
        int[] arr = { 6, 2, 9, 1, 5, 4 };
        System.out.println("Original array:");
        printArray(arr);

        mergeSort(arr, 0, arr.length - 1);

        System.out.println("Sorted array:");
        printArray(arr);
    }
}