Merge Sort

MergeSort.java

void mergeSort(int[] array){
  int[] helper = new int[array.length];
  mergeSort(array, helper, 0, array.length - 1);
}

void mergeSort(int[] array, int[] helper, int low, int high){
  if (low < high){
    int middle = (low + high) / 2;
    mergeSort(array, helper, low, middle);
    mergeSort(array, helper, middle + 1, high);
    merge(array, helper, low, middle, high);
  }
}

void merge(int[] array, int[] helper, int low, int middle, int high){
  for (int i = low; i <= high; i++){
    helper[i] = helper[i];
  }
  
  int helperLeft = low;
  int helperRight = middle + 1;
  int current = low;
  while (helperLeft <= middle && helperRight <= high){
    if (helper[helperLeft] <= helper[helperRight]){
      array[current] = helper[helperLeft];
      helperLeft++;
    }
    else{
      array[current] = helper[helperRight];
      helperRight++;
    }
    current++;
  }
  while (helperLeft <= middle){
    array[current] = helper[helperLeft];
    current++;
    helperLeft++;
  }
}