用vector实现逆序对数量统计和归并排序

#include <iostream>
#include <algorithm>
#include <cmath>
#include <cstring>
#include <cstdio>
#include <vector>

using namespace std;
vector<int> num;
int answer;

vector<int> Merge(vector<int> ,vector<int> );
vector<int> msort(vector<int> a)
{
	int len=a.size();
	if (len==1) return a;
	int mid=len/2;
	vector<int> al,ar;
	for (int i=0;i<mid;i++) al.push_back(a[i]);
	for (int i=mid;i<len;i++) ar.push_back(a[i]);
	al=msort(al);
	ar=msort(ar);
	return Merge(al,ar);
}

vector<int> Merge(vector<int> a,vector<int> b)
{
	vector<int> ans;
	ans.clear();
	int n=a.size();
	int m=b.size();
	int p_a=0;
	int p_b=0;
	while (p_a<n && p_b<m) {
		if (a[p_a]<=b[p_b]) {
			ans.push_back(a[p_a]);
			p_a++;
			answer+=p_b;
		}
		else {
			ans.push_back(b[p_b]);
			p_b++;
		}
	}
	if (p_a<n) {
		for (int i=p_a;i<n;i++) ans.push_back(a[i]);
		answer+=(n-p_a)*m;
	}
	if (p_b<m) {
		for (int i=p_b;i<m;i++) ans.push_back(b[i]);
	}
	return ans;
}

int main()
{
	int n,ls;
	cin>>n;
	for (int i=1;i<=n;i++) {
		cin>>ls;
		num.push_back(ls);
	}
	
	msort(num);
	
	cout<<answer<<endl;
	return 0;
}