» » » Merge sort algorithm in java with example program

  • Merge sort one of the recursive sorting algorithm.
  • First divide the list of unsorted elements in two two parts. left half and right half.

  • Divide the left part elements in to sub lists until it become single element.
  • Divide right half of array or list of elements in to sub lists until it becomes one element in list.
  • After dividing all elements in two parts in to single entity. 
  • Merge the elements into two by comparing lesser element will be first. apply same at right half
  • Merge all the elements in the left part until it become single sorted list
  • Now we have two sorted parts.
  • Means two parts sorted in ascending order and smaller element will be in first position.
  • Compare fist elements of two parts , lesser one should be takes first place in new sorted list
  • New sorted list or array having only one element now.
  • Compare two lists elements and place in sorting order and merge it. 
  • Finally we have all elements sorted.
  • Compared to remaining algorithms like selection sort, insertion sort and bubble sort  merge sort works faster.
  • Lets see what will be the time complexity of merge sort algorithm.

Time complexity of merge sort algorithm:


1.Best case  time complexity:      O(n log (n))
2.Average case time complexity: O(n log (n))
3.Worst case time complexity:    O(n log (n))
4.Worst case space complexity:  O(n)


Merge sort in java with explanation diagram

Implement merge sort in java


Program#1: Java program to implement merge sort algorithm data structure

  1. package com.instanceofjava.mergesortalgorithm;

  2. public class MergeSortAlgorithm {

  3.    private int[] resarray;
  4.    private int[] tempMergArray;
  5.    private int length;
  6.  
  7.  public static void main(String a[]){
  8.         
  9.  int[] inputArr ={6,42,2,32,15,8,23,4};
  10.         System.out.println("Before sorting");

  11.        for(int i:inputArr){
  12.           System.out.print(i);
  13.            System.out.print(" ");
  14. }

  15.  MergeSortAlgorithm mergesortalg = new MergeSortAlgorithm();

  16.        mergesortalg.sort(inputArr);
  17.        System.out.println();

  18.        System.out.println("After sorting");
  19.        for(int i:inputArr){
  20.            System.out.print(i);
  21.            System.out.print(" ");
  22.        }
  23.  }
  24.     
  25. public void sort(int inputArray[]) {

  26.        this.resarray = inputArray;
  27.        this.length = inputArray.length;
  28.        this.tempMergArray = new int[length];
  29.        doMergeSort(0, length - 1);

  30. }
  31.  
  32. private void doMergeSort(int lowerIndex, int higherIndex) {
  33.         
  34.     if (lowerIndex < higherIndex) {

  35.    int middle = lowerIndex + (higherIndex - lowerIndex) / 2;

  36.            //to sort left half of the array
  37.    doMergeSort(lowerIndex, middle);

  38.            // to sort right half of the array
  39.    doMergeSort(middle + 1, higherIndex);

  40.            //merge two halfs
  41.     mergehalfs(lowerIndex, middle, higherIndex);
  42. }
  43. }
  44.  
  45. private void mergehalfs(int lowerIndex, int middle, int higherIndex) {
  46.  
  47.        for (int i = lowerIndex; i <= higherIndex; i++) {
  48.         tempMergArray[i] = resarray[i];
  49.        }

  50.        int i = lowerIndex;
  51.        int j = middle + 1;
  52.        int k = lowerIndex;

  53.   while (i <= middle && j <= higherIndex) {
  54.            if (tempMergArray[i] <= tempMergArray[j]) {
  55.             resarray[k] = tempMergArray[i];
  56.                i++;
  57.            } else {
  58.             resarray[k] = tempMergArray[j];
  59.                j++;
  60.            }
  61.            k++;
  62.       }

  63.      while (i <= middle) {
  64.         resarray[k] = tempMergArray[i];
  65.            k++;
  66.            i++;
  67.        }
  68.    }

  69. }

Output:

  1. Before sorting
  2. 6  42  2  32  15  8  23  4
  3. After sorting
  4. 2  4  6  8  15  23  32  42 

Instance Of Java

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