[51nod]1019 逆序数

#include<cstring>
#include<cstdio>
#include<cmath>
#include<iostream>
#include<algorithm>
using namespace std;
#define N 50005
int n;
int a
;
int b
;
long long ans=0;
void mergesort(int l,int r)
{
if(l>=r) return;
int mid=l+r>>1;
mergesort(l,mid);
mergesort(mid+1,r);
int i=l,j=mid+1,k=0;
while(i<=mid && j<=r)
{
if(a[i]<=a[j])  b[k++]=a[i++];
else b[k++]=a[j++],ans+=mid+1-i;
}
while(i<=mid)
b[k++]=a[i++];
while(j<=r)
b[k++]=a[j++];
for(int i=0;i<k;i++)
a[l+i]=b[i];
}
int main()
{
cin>>n;
for(int i=1;i<=n;i++)
{
scanf("%d",a+i);
}
mergesort(1,n);
cout<<ans<<endl;
}