蒟蒻求助，热（RE）了一片

#include <bits/stdc++.h>
using namespace std;
int s1[2000],s2[2000];
long long ans;
void merge(int l,int mid,int r)
{
    if (l==r) return;
    merge(l,(l+mid)/2,mid);
    merge(mid+1,(mid+1+r)/2,r);
    int i=l,j=mid+1,k=l;
    while(i<=mid && j<=r)
    {
        if (s1[i]<=s1[j]) s2[k++]=s1[i++];//从大到小">",从小到大"<" 
        else
        {
            s2[k++]=s1[j++];
            ans+=mid-i+1;
        }
    }
    while(i<=mid) s2[k++]=s1[i++];
    while(j<=r) s2[k++]=s1[j++];
    for(i=l;i<=r;i++) s1[i]=s2[i];
}
/*
void MergeSort(int L,int R)
{
    int Mid;
    if(L<R)
    {
        Mid=(L+R)/2;
        MergeSort(L,Mid);
        MergeSort(Mid+1,R);
        merge(L,Mid,R);
    }
}
*/
int main()
{
    //逆序对
    int n,i;
    scanf("%d",&n);
    for(i=1;i<=n;i++) scanf("%d",&s1[i]);
    merge(1,(1+n)/2,n);
    printf("%lld",ans);
}