Divide and conquer

Merge sort

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Merge sort is a divide and conquers and comparison-based sorting algorithm. It divide the array into subarray and conquer recursively sorting two subarrays. It combines the results into a final array.


Divide the problem into small subproblems.


Solve the subproblem using recursive function.


Combine the sub solution into solution.

Merge sort can be implemented Top-down and Bottom-up. Top down merge sort split the list into sub list and sort the sub list. Finally, it merges sorted sublist into a single sorted list. Bottom up merge sort treats array of sub list and iteratively merges sub-lists back and forth between two buffers.

Merge sort uses for linked list sorting, external sorting on the tab. It is stable sort compare then other sorting algorithm.

Write a program to take the un-sorted array and sort the array using Merge sort.

source Code

package com.dsacode.Algorithm.sorting;
import java.util.Arrays;
public class Mergesort {
    private static void merge(int arr[], int l, int m, int r) {
        int i, j, k;
        int n1 = m - l + 1;
        int n2 =  r - m;
        int[] L=new int[n1];
        int[] R=new int[n2];
        for(i = 0; i < n1; i++)
            L[i] = arr[l + i];
        for(j = 0; j < n2; j++)
            R[j] = arr[m + 1+ j];
        i = 0;
        j = 0;
        k = l;
        while (i < n1 && j < n2) {
            if (L[i] <= R[j]) {
                arr[k] = L[i];
            } else {
                arr[k] = R[j];
        while (i < n1)  {
            arr[k] = L[i];
        while (j < n2)  {
            arr[k] = R[j];
    public static void mergeSort(int arr[], int l, int r) {
        if (l < r) {
            int m = l+(r-l)/2;
            mergeSort(arr, l, m);
            mergeSort(arr, m+1, r);
            merge(arr, l, m, r);
    public static void main(String[] args) {
        int arr[] = {12, 11, 13, 5, 6, 7};
        System.out.println("unsorted array before sorting : " + Arrays.toString(arr));
        mergeSort(arr, 0, arr.length - 1);
        System.out.println("Sorted array After Mergesort sorting : " + Arrays.toString(arr));


Unsorted array before sorting: [12, 11, 13, 5, 6, 7]
Sorted array After Merge sort sorting: [5, 6, 7, 11, 12, 13]

Algorithm Explanation

Take unsorted array as input. Find the middle point to divide the array into two halves.
Sort the first half of elements using recursively.
Sort the second half of elements using recursively.
Merge the first half sorted array and second half sorted array.
Return the sorted array.

Time Complexity

Best CaseAverage CaseWorst Case
O(n log(n))O(n log(n))O(n log(n))


  1. http://algs4.cs.princeton.edu/22mergesort/
  2. http://en.wikipedia.org/wiki/Merge_sort
  3. http://www.ee.ryerson.ca/~courses/coe428/sorting/mergesort.html
  4. http://www.personal.kent.edu/~rmuhamma/Algorithms/MyAlgorithms/Sorting/mergeSort.htm

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