归并排序求数组逆序对个数

#include <iostream>
#include <string.h>
#include <stdio.h>
 
using namespace std;
const int N = 1005;
 
int a[N],tmp[N];
int ans;
 
void Merge(int l,int m,int r)
{
    int i = l;
    int j = m + 1;
    int k = l;
    while(i <= m && j <= r)
    {
        if(a[i] > a[j])
        {
            tmp[k++] = a[j++];
            ans += m - i + 1;
        }
        else
        {
            tmp[k++] = a[i++];
        }
    }
    while(i <= m) tmp[k++] = a[i++];
    while(j <= r) tmp[k++] = a[j++];
    for(int i=l;i<=r;i++)
        a[i] = tmp[i];
}
 
void Merge_sort(int l,int r)
{
    if(l < r)
    {
        int m = (l + r) >> 1;
        Merge_sort(l,m);
        Merge_sort(m+1,r);
        Merge(l,m,r);
    }
}
 
int main()
{
    int n,T;
    scanf("%d",&T);
    while(T--)
    {
        scanf("%d",&n);
        for(int i=0;i<n;i++)
            scanf("%d",&a[i]);
        ans = 0;
        Merge_sort(0,n-1);
        printf("%d\n",ans);
    }
    return 0;
}