CE求助

我的评测记录

i,j,k用作数组下标类型无效?

代码：

#include<iostream>
#include<cstdio>
using namespace std;
const int N=2505;
int a[N],r[N],n;
long long ans;
void merge_sort(int l,int r)
{
    if(l==r)return;
    int mid=(l+r)/2;
    merge_sort(l,mid);
    merge_sort(mid+1,r);
    int i=l,j=mid+1,k=l;
    while(i<=mid&&j<=r)
    {
        if(a[i]<=a[j])r[k++]=a[i++];
        else r[k++]=a[j++],ans+=(long long)mid-i+1;
    }
    while(i<=mid)r[k]=a[i],i++,k++;
    while(j<=r)r[k]=a[j],k++,j++;
    for(int i=l;i<=r;++i)a[i]=r[i];
}
int main()
{
    scanf("%d",&n);
    for(int i=1;i<=n;i++)scanf("%d",&a[i]);
    merge_sort(1,n);
    printf("%lld\n",ans);
    return 0;
}