merge sort

merge sort 
https://www.youtube.com/watch?v=iMT7gTPpaqw


// time : nlogn 
// space: n 
// n : the number of elements in the array

public class MergeSort{
  public int[] mergeSort(int[] array){
    if(array == null){
      return array;
    }
    
    int[] helper = new int[array.length];
    mergeSort(array, helper, 0, array.length - 1);
    return array;
  }
  
  private void mergeSort(int[] array, int[] helper, int left, int right){
    if(left >= right){
      return;
    }
    
    int mid = left + (right - left) / 2;
    mergeSort(array, helper, left, mid);
    mergeSort(array, helper, mid + 1, right);
    merge(array, helper, left, mid, right);
  }
  
  private void merge(int[] array, int[] helper, int mid, int left, int right){
    // copy the content to helper array and we will merge from the helper array
    for(int i = 0; i <= right; i++){
      helper[i] = array[i];
    }
    
    int leftIndex = left;
    int rightIndex = mid + 1;
    while(leftIndex <= mid && rightIndex <= right){
      if(helper[leftIndex] <= helper[rightIndex]){
        array[left++] = helper[leftIndex++];
      }else{
        array[left++] = helper[rightIndex++];
      }
    }
    
    // if we still have some elements at left side, we need to copy them 
    while(leftIndex <= mid){
      array[left++] = helper[leftIndex++];
    }
    // if there are some elements at right side, we dont need to copy them because 
    // they are already in their position 
  }
}




// testing 
public static void main(String[] args){
  MergeSort solution = new MergeSort();
  
  
  int[] array = null;
  array = solution.mergeSort(array);
  System.out.println(Arrays.toString(array));
  
  
  array = new int[0];
  array = solution.mergeSort(array);
  System.out.println(Arrays.toString(array));
  
  
  array = new int[] {4,3,5,7,1};
  array = solution.mergeSort(array);
  System.out.println(Arrays.toString(array));
}