Merge sort Program in Java

Merge Sort is one of the most popular sorting algorithm that is based on the principle of Divide and Conquer Algorithm. 

class MergeSort {

  // Merge two subarrays L and M into arr

  void merge(int arr[], int p, int q, int r) {

    // Create L ← A[p..q] and M ← A[q+1..r]

    int n1 = q - p + 1;

    int n2 = r - q;

    int L[] = new int[n1];

    int M[] = new int[n2];

    for (int i = 0; i < n1; i++)

      L[i] = arr[p + i];

    for (int j = 0; j < n2; j++)

      M[j] = arr[q + 1 + j];

    // Maintain current index of sub-arrays and main array

    int i, j, k;

    i = 0;

    j = 0;

    k = p;

    // Until we reach either end of either L or M, pick larger among

    // elements L and M and place them in the correct position at A[p..r]

    while (i < n1 && j < n2) {

      if (L[i] <= M[j]) {

        arr[k] = L[i];

        i++;

      } else {

        arr[k] = M[j];

        j++;

      }

      k++;

    }

    // When we run out of elements in either L or M,

    // pick up the remaining elements and put in A[p..r]

    while (i < n1) {

      arr[k] = L[i];

      i++;

      k++;

    }

    while (j < n2) {

      arr[k] = M[j];

      j++;

      k++;

    }

  }

  // Divide the array into two subarrays, sort them and merge them

  void mergeSort(int arr[], int l, int r) {

    if (l < r) {

      // m is the point where the array is divided into two subarrays

      int m = (l + r) / 2;

      mergeSort(arr, l, m);

      mergeSort(arr, m + 1, r);

      // Merge the sorted subarrays

      merge(arr, l, m, r);

    }

  }

  // Print the array

  static void printArray(int arr[]) {

    int n = arr.length;

    for (int i = 0; i < n; ++i)

      System.out.print(arr[i] + " ");

    System.out.println();

  }

  // Driver program

  public static void main(String args[]) {

    int arr[] = { 6, 5, 12, 10, 9, 1 };

    MergeSort ob = new MergeSort();

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

    System.out.println("Sorted array:");

    printArray(arr);

  }

}