Generate a Java program to implement Merge Sort algorithm to efficiently sort an array of elements in ascending order.

public class Solution {

  public int[] mergeSort(int[] arr) {
    // Write your solution here.
    if(arr == null || arr.length <= 1){
      return arr;
    }
    int[] helper = new int[arr.length];
    mergeSort(arr, helper, 0, arr.length - 1);
    return arr;
  }
  
  private void mergeSort(int[] arr, int[] helper, int left, int right){
    if(left >= right){
      return;
    }
    int mid = left + (right - left) / 2;
    mergeSort(arr, helper, left, mid);
    mergeSort(arr, helper, mid + 1, right);
    merge(arr, helper, left, mid, right);
  }
  
  private void merge(int[] arr, int[] helper, int left, int mid, int right){
    for(int i = left; i <= right; i++){
      helper[i] = arr[i];
    }
    int leftIndex = left;
    int