归并排序，开long long还是50

代码如下

#include<bits/stdc++.h>
using namespace std;
int n,a[500000],b[500000];
long long ans;
void msort(int l,int r)
{
    if(l==r)  
        return;
    int mid=(l+r)/2,i=l,j=mid+1,k=l;
    msort(l,mid),msort(mid+1,r);
    while(i<=mid&&j<=r)
        if(a[i]<=a[j])
            b[k++]=a[i++];
        else
            b[k++]=a[j++],ans+=mid-i+1;
    while(i<=mid)
        b[k++]=a[i++];
    while(j<=r)
        b[k++]=a[j++];
    for(int i=l;l<=r;l++)
        a[l]=b[l];
} 
int main()
{
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
        scanf("%d",&a[i]);
    msort(1,n);
    printf("%lld",ans);
    return 0;
}