爆零求救

用下面这行就ac while(l<=r) arr[l] = mergearr[l],l++; 但是用这个会爆零 while(l<=r) arr[l] = mergearr[l++]; 求大佬解释一下ToT

#include<cstdio>
#include<algorithm>
using namespace std;
int n;
int arr[500005];
int mergearr[500005];
long long ans;
void merge(int l,int r)
{
    if(l == r) return ;
    int mid = (l+r)/2;
    merge(l,mid);
    merge(mid+1,r);
    int ll=l,rr=mid+1;
    int mergecur=l;
    while(rr<=r&&ll<=mid)
        if(arr[ll]>arr[rr]) (mergearr[mergecur++] = arr[rr++],ans+=mid-ll+1);
        else mergearr[mergecur++] = arr[ll++];
    while(ll<=mid)mergearr[mergecur++] = arr[ll++];
    while(rr<=r)mergearr[mergecur++] = arr[rr++];

   while(l<=r) arr[l] = mergearr[l++];
    // while(l<=r) arr[l] = mergearr[l],l++;

}

int main()
{
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
        scanf("%d",arr+i);

    merge(1,n);
    printf("%lld",ans); 

    return 0;
}