P1750 求逆序对

#include<iostream>
using namespace std;
int arr[500010],temp[500010];
long long ans;                                //记录逆序的组数
//归并排序
void merge_sort(int arr[], int temp[], int left, int mid, int right)
{
    int i = left, j = mid + 1;
    int k = left;
    //将分割成的两个有序数组排序，并且求逆序的个数
    while (i <= mid && j <= right)
    {
        if (arr[i] > arr[j])
        {
            ans += mid - i + 1;
            temp[k++] = arr[j++];
        }
        else
        {
            temp[k++] = arr[i++];
        }
    }
    //判断两个数组中的元素是否全部放入最后排序的数组中
    while (i <= mid)
    {
        temp[k++] = arr[i++];
    }
    while (j <= right)
    {
        temp[k++] = arr[j++];
    }
    //复制
    for (i = left; i <= right; i++)
    {
        arr[i] = temp[i];
    }
    return;
}
//将数组分割成有序，即分割至只含一个元素
void merge(int arr[], int left, int right, int temp[])
{
    if (left < right)
    {
        int mid = (left + right) / 2;
        merge(arr, left, mid, temp);
        merge(arr, mid + 1, right,temp);
        merge_sort(arr, temp,left, mid, right);
    }
}
int main()
{
    int n;
    cin >> n;
    int i;
    for (i = 1; i <= n; i++)
    {
        cin >> arr[i];
    }
    merge(arr, 1, n, temp);
    cout << ans << endl;
    return 0;
}