c++归并排序

void merge(int a[], int first, int mid, int last)
{
	int* ptemp = new int[last - first + 1]{ 0 };
	int left_fitst = first;
	int left_last = mid;
	int right_first = mid + 1;
	int right_last = last;

	int k = 0;
	for (k = 0;left_fitst <= left_last && right_first <= right_last;k++)
	{
		if (a[left_fitst] <= a[right_first])
		{
			ptemp[k] = a[left_fitst++];

		}
		else
		{
			ptemp[k] = a[right_first++];
		}

	}
	if (left_fitst <= left_last)
	{
		for (int j = left_fitst;j <= left_last;j++)
		{
			ptemp[k] = a[j];
			k++;
		}


	}
	if (right_first <= right_last)
	{
		for (int j = right_first;j <= right_last;j++)
		{
			ptemp[k] = a[j];
			k++;
		}


	}
	for (int i = 0;i < last - first + 1;i++)
	{
		a[first + i] = ptemp[i];
	}
	delete[]ptemp;
}

void merge_sort(int a[], int first, int last)
{
	int mid = 0;
	if (first < last)
	{
		mid = (first + last) / 2;
		merge_sort(a, first, mid);
		merge_sort(a, mid + 1, last);
		merge(a, first, mid, last);
	}




}