以下是询问chatGPT得到的Accepted答案

#include <vector>

using namespace std;

long long merge(vector<int>& nums, int left, int mid, int right) {
    vector<int> tmp(right - left + 1);
    int i = left, j = mid + 1, k = 0;
    long long count = 0;
    while (i <= mid && j <= right) {
        if (nums[i] <= nums[j]) {
            tmp[k++] = nums[i++];
        } else {
            tmp[k++] = nums[j++];
            count += mid - i + 1;
        }
    }
    while (i <= mid) {
        tmp[k++] = nums[i++];
    }
    while (j <= right) {
        tmp[k++] = nums[j++];
    }
    for (k = 0, i = left; i <= right; ++i, ++k) {
        nums[i] = tmp[k];
    }
    return count;
}

long long merge_sort(vector<int>& nums, int left, int right) {
    if (left >= right) {
        return 0;
    }
    int mid = (left + right) >> 1;
    long long count_left = merge_sort(nums, left, mid);
    long long count_right = merge_sort(nums, mid + 1, right);
    long long count = merge(nums, left, mid, right);
    return count_left + count_right + count;
}

int main() {
    int n;
    cin >> n;
    vector<int> nums(n);
    for (int i = 0; i < n; ++i) {
        cin >> nums[i];
    }
    cout << merge_sort(nums, 0, n - 1) << endl;
    return 0;
}

归并排序求逆序对

#include<bits/stdc++.h>
using namespace std;
int a[100001];
int r[100001]; 
long long unsigned ans;
void msort(int s,int t)
{
	if(s==t) return;
	int mid=(s+t)/2;
	msort(s,mid);
	msort(mid+1,t);
	int i=s,j=mid+1,k=s;
	while(i<=mid&&j<=t)
	{
		if(a[i]<=a[j])
		{
			r[k]=a[i];
			k++;
			i++;
		}
		else
		{
			r[k]=a[j];
			k++;
			j++;
			ans+=mid-i+1;
		}
	}
	while(i<=mid)
	{
		r[k]=a[i];
		k++;i++;
	}
	while(j<=t)
	{
		r[k]=a[j];
		k++;
		j++;
	}
	for(int i=s;i<=t;i++) a[i]=r[i];
}
int main()
{
	int n;
	cin>>n;
	for(int i=1;i<=n;i++)
	{
		cin>>a[i];
	}
	msort(1,n);
	cout<<ans;
	return 0;
}