考虑以下C++代码实现的归并排序算法：
void merge(int arr[], int left, int mid, int right) { int n1 = mid - left + 1; int n2 = right - mid; int L[n1], R[n2]; for (int i = 0; i < n1; i++) L[i] = arr[left + i]; for (int j = 0; j < n2; j++) R[j] = arr[mid + 1 + j]; int i = 0, j = 0, k = left; while (i < n1 && j < n2) { if (L[i] <= R[j]) { arr[k] = L[i]; i++; } else { arr[k] = R[j]; j++; } k++; } while (i < n1) { arr[k] = L[i]; i++; k++; } while (j < n2) { arr[k] = R[j]; j++; k++; }} void merge_sort(int arr[], int left, int right) { if (left < right) { int mid = left + (right - left) / 2; merge_sort(arr, left, mid); merge_sort(arr, mid + 1, right); merge(arr, left, mid, right); }}对长度为n的数组arr，挑用函数merge_sort(a, 0, n-1)，在排序过程中merge函数的递归调用次数大约是
（ ）。