merge sort

public void mergeSort(int initial[], int start, int end){
  if(start < end){
    int mid = (start + end) / 2;
    mergeSort(initial, start, mid);
    mergeSort(initial, mid+1, end);
    merge(initial, start, mid, end);
  }
}

public int[] merge(int initial[], int start, int mid, int end){
  int helper[] = new int[initial.length];
  int left = start;
  int right = mid + 1;
  int current = left;
  for(int i=start; i<end; i++){
    helper[i] = initial[i];
  }
  while(left <= mid && right <=end){
    //merge until one list is empty
    if(helper[left] < helper[right]){
        initial[current] = helper[left]; left++;
    }
    else{ initial[current] = helper[right]; right++;}
    current++;
  } 
  //one list is over, enter the remaining numbers from the other list
  if(left == mid){while(right <= end){initial[right] = helper[right]; right++;}}
  else if(right == end) {while(left <= mid){initial[left] = helper[left]; left++;}}
}