【CodeVS1688】求逆序对

#include<cstdio>
using namespace std;
const int N=1000010;
int a[N],b[N],c[N];
int n;
long long ans;
void MergeSort(int l,int r)
{
    if (l==r) return;
    int mid=(l+r)/2;
    MergeSort(l,mid);    MergeSort(mid+1,r);
    int i=l,j=mid+1,p=l;
    while (i<=mid && j<=r)
    {
        if (a[i]>a[j]) c[p++]=a[j++],ans+=mid-i+1;
            else c[p++]=a[i++];
    }
    while (i<=mid) c[p++]=a[i++];
    while (j<=r) c[p++]=a[j++];
    for (int i=l;i<=r;i++) a[i]=c[i];
}
int main()
{
    scanf("%d",&n);
    for (int i=1;i<=n;i++) scanf("%d",&a[i]);
    MergeSort(1,n);
    printf("%lld",ans);
}