求逆序對個數
在歸併排序（從大到小）的合併過程前,對兩個分支進行逆序對數的計算(該計算過程為O(n))

#include <cstdio>
#include <iostream>
#include <algorithm>
#define N 100005
using namespace std;

int a[N], b[N];

void Merge(int a[], int s, int m, int e, int tmp[])
{
	int p = 0, p1 = s, p2 = m + 1;
	//開始合併
	while (p1 <= m && p2 <= e)
	{
		if (a[p1] > a[p2])
			tmp[p++] = a[p1++];
		else
			tmp[p++] = a[p2++];
	}

	while (p1 <= m)
		tmp[p++] = a[p1++];
	while (p2 <= m)
		tmp[p++] = a[p2++];

	for (int i = 0; i < p; i++)//把結果拷貝到全域性陣列a中
		a[i + s] = tmp[i];
}

long long cal(int a[], int s, int m, int e, int tmp[])
{
	int i = s, j = m + 1;
	long long result = 0;
	while (i <= m && j <= e)
	{
		if (a[i] > a[j])
		{
			result += e - j + 1;
			i++;
		}
		else
			j++;
	}
	return result;
}

long long Merge_sort(int a[], int s, int e, int tmp[])
{
	long long cnt = 0;
	if (e > s)
	{
		int m = s + (e - s) / 2;
		cnt += Merge_sort(a, s, m, tmp);
		cnt += Merge_sort(a, m + 1, e, tmp);
		cnt += cal(a, s, m, e, tmp);
		Merge(a, s, m, e, tmp);
	}
	return cnt;
}

int main()
{
	long long ans = 0;
	int n;
	scanf("%d", &n);
	for (int i = 0; i < n; i++)
		scanf("%d", &a[i]);
	ans = Merge_sort(a, 0, n - 1, b);
	printf("%lld\n", ans);
	return 0;
}