1688 求逆序对

#include<cstdio>
#include<iostream>
#include<algorithm>
#define N 100001
using namespace std;
int n,a[N];
long long ans;
void gsort(int l,int r)
{
     if(l==r) return ;
    int mid=(l+r)/2;int tmp[N];
    gsort(l,mid),gsort(mid+1,r);
    int i=l,j=mid+1,k=l;
    while(i<=mid&&j<=r)
    {
        if(a[i]<=a[j]) tmp[k++]=a[i++];
        else
        {
            ans+=mid-i+1;
            tmp[k++]=a[j++];
        }
    }
    while(i<=mid) tmp[k++]=a[i++];
    while(j<=r) tmp[k++]=a[j++];
    for(int i=l;i<=r;i++) a[i]=tmp[i];
}
int main()
{
    scanf("%d",&n);
    for(int i=1;i<=n;i++) scanf("%d",&a[i]);
    gsort(1,n);
    cout<<ans;
    return 0;
}