merge sort implementation with vectors

#include <iostream>
#include <vector>

std::vector<int> merge(std::vector<int> &A, std::vector<int> &B) {
  std::vector<int> M;
  int i = 0, j = 0;

  while (i < A.size() && j < B.size()) {
    if (A.at(i) <= B.at(j)) {
      M.push_back(A.at(i));
      i++;
    } else {
      M.push_back(B.at(j));
      j++;
    }
  }

  while (i < A.size()) {
    M.push_back(A.at(i));
    i++;
  }

  while (j < B.size()) {
    M.push_back(B.at(j));
    j++;
  }

  return M;
}

std::vector<int> merge_sort(std::vector<int> &nums) {
  std::cout << nums.size() << " " << std::endl;
  std::cin.get();
  // Splitto nums in due
  if (nums.size() == 1) {
    return nums;
  }

  // Prima metá di nums
  std::vector<int> left =
      std::vector<int>(nums.begin(), nums.begin() + (nums.size() / 2));

  left = merge_sort(left);

  // Seconda metá di nums
  std::vector<int> right =
      std::vector<int>(nums.begin() + (nums.size() / 2), nums.end());

  right = merge_sort(right);

  return merge(left, right);
}

int main() {
  std::vector<int> nums = {1, 2, 3, 4, 1, 6, 56, 3, -1};

  std::vector<int> A = {1, 3, 6, 7, 9, 11, 23};
  std::vector<int> B = {8, 9, 10, 12, 14, 22};

  std::vector<int> C = merge(A, B);

  std::vector<int> sorted = merge_sort(nums);
  for (int x : sorted) {
    std::cout << x << " ";
  }

  return 0;
}