Write a C ++ program implementing an optimization for the Merge Sort algorithm that aims to improve the performance for small subarrays. When the size of a subarray falls below a certain threshold, use a more efficient sorting algorithm like Insertion Sort instead of recursively applying Merge Sort.

#include <iostream>
#include <vector>

#define THRESHOLD 10

using namespace std;

void printArray(vector<int> A){
  for (int i : A)
    cout << i << " ";
  cout << endl;
}

vector<int> merge(vector<int> left, vector<int> right){
  int i = 0;
  int j = 0;
  vector<int> res;

  while (i < left.size() && j < right.size()){
    if (left[i] < right[j]){
      res.push_back(left[i]);
      i++;
    }else{
      res.push_back(right[j]);
      j++;
    }
  }
  while (i < left.size()){
    res.push_back(left[i]);
    i++;
  }

  while (j < right.size()){
    res.push_back(right[j]);
    j++;
  }

  return res