Merge sort algorithm implemented in Java

Keywords: Java less

The following is a merge sort algorithm implemented in Java, referencing Algorithms Unlocked by Thomas H. Cormen

I read some blogs from other CSDN bloggers, and I think some details can be optimized, such as avoiding overflow due to too long array length, and using mid = low + (high - low) / 2 for mid

For example, instead of creating a (high - low + 1) long array, you can create only the first half of the array[low...mid]

These small details can be optimized to increase the upper bound of the calculation and reduce space use by half.

package sort_and_search;

import java.util.Random;

public class MergeSort {
	public static void mergeSort(int[] array, int low, int high) {
		int mid = low + (high - low) / 2;	// Another algorithm for mid = (high + low)/2 to avoid overflow
		if(low < high) {
			mergeSort(array, low, mid);	  // For array[low...mid]Merge Sort
			mergeSort(array, mid+1, high);    // For array[mid+1...high] merge sort
			mergeArray(array, low, mid, high);// fuse
	public static void mergeArray(int[] array, int low, int mid, int high) {
		int[] temp = new int[mid-low+1];	  //To create a temporary array, just create the first half
		for(int i = 0, j = low; i < temp.length; i++, j++) {
			temp[i] = array[j];		  //Assigning values to temporary arrays
		int i = 0, j = mid+1, m = low;
		while(i < temp.length && j <= high) {     //Merge two ordered arrays
			if(temp[i] <= array[j]) {	  //Less than or equal to is to maintain stability
				array[m] = temp[i];
				i++; m++;
			else {
				array[m] = array[j];
				m++; j++;
		while(i < temp.length)			  //Finally, fill in the remaining elements
			array[m++] = temp[i++];
		/*while(j <= high)
			array[m++] = array[j++];
			You don't really need to do this because array[j]==array[m]


	public static void main(String[] args) {
		Random random = new Random();		
		int []array = new int[50];
		System.out.println("We randomly create an array, the array is below: ");
		for (int i = 0; i < array.length; i++) {
			array[i] = random.nextInt(500);		//Generate Random Numbers up to 500
			System.out.print(array[i]+" ");
		System.out.println("After selection sort, the answer is: ");
		mergeSort(array, 0, array.length-1);
		for (int i : array) {
			System.out.print(i+" ");


Beginner algorithm, limited level, please correct any errors.

Posted by Loran on Fri, 10 Jul 2020 08:37:44 -0700