二分归并排序算法求排列的逆序数

最笨的方法时间复杂度为O(n2)，采用利用二分归并排序算法思想，时间复杂度为O(nlogn)

#include

using namespace std;
int a[1000], temp[1000];
long long sum = 0;

void merge(int a[], int s, int m, int e, int temp[])
{
	int pb = 0;
	int p1 = s, p2 = m + 1;
	while (p1 <= m && p2 <= e)
	{
		if (a[p1] < a[p2])
			temp[pb++] = a[p1++];
		else
		{
			temp[pb++] = a[p2++];
			sum += (m - p1 + 1);  // 关键点
		}
	}
	while (p1 <= m)
		temp[pb++] = a[p1++];
	while (p2 <= e)
		temp[pb++] = a[p2++];
	for (int i = 0; i < e - s + 1; i++)
		a[s + i] = temp[i];
}
void mergeSortCount(int a[], int s, int e, int temp[])
{
	if (s < e)
	{
		int m = s + (e - s) / 2;
		mergeSortCount(a, s, m, temp);
		mergeSortCount(a, m + 1, e, temp);
		merge(a, s, m, e, temp);
	}
}
void main()
{
	int n;
	cin >> n;
	for (int i = 0; i < n; i++)
		cin >> a[i];
	mergeSortCount(a, 0, n - 1, temp);
	/*for (int i = 0; i < n; i++)
		cout << a[i] << ' ';
	cout << endl;*/
	cout << sum << endl;
	system("pause");
}