求排列的逆序数[归并排序]

#include <iostream>
#include <string>
#include <math.h>
#include<algorithm>
using namespace std;
int a[1005];
int b[1005];
int num = 0;
void merge(int a[], int l1, int r1, int l2, int r2)
{
	int i = l1, j = l2;
	int temp[1001], index = 0;
	while (i <= r1 && j <= r2)
	{
		if (a[i] <= a[j])
			temp[index++] = a[i++];
		else
		{
			temp[index++] = a[j++];
			num += r1-i+1;
		}
	}
	while (i <= r1)
		temp[index++] = a[i++];
	while (j <= r2)
		temp[index++] = a[j++];
	for (i = 0; i < index; i++)
	{
		a[l1 + i] = temp[i];
	}

}
void MergeSort(int a[], int l, int r)
{
	if (l >= r)
		return;
	int mid = (l+r)/2;
	MergeSort(a, l, mid);
	MergeSort(a, mid + 1, r);
	merge(a, l, mid, mid + 1, r);
}

int main()
{
	int n;
	cin >> n;
	for (int i = 0; i < n; i++)
	{
		cin >> a[i];
	}
	MergeSort(a, 0, n - 1);
	cout << num << endl;
}