有谁告诉我这为甚会RE

#include <bits/stdc++.h>
using namespace std;
int n,a[100010],tmp[100010];
long long ans;
void merge_sort(int l,int r)
{
    if (l==r) return;
    int mid=(l+r)/2,p,i,j;
    merge_sort(l,mid);
    merge_sort(mid+1,r);
    p=0; i=l; j=mid+1;
    while (i<=mid&&j<=r)
        if (a[i]<=a[j])
            tmp[p++]=a[i++];
        else {
            ans+=mid-i+1;
            tmp[p++]=a[j++];
        }
    while (i<=mid) tmp[p++]=a[i++];
    while (j<=r) tmp[p++]=a[j++];
    memcpy(a+l,tmp,sizeof(int)*(r-l+1));
}
int main()
{
    cin>>n;
    for (int i=0;i<n;++i) scanf("%d",&a[i]);
    merge_sort(0,n-1);
    cout<<ans<<endl;
    return 0;
}

有谁告诉我这为甚会RE