求逆序对（分治法）

int a[100005],temp[100005]; //排序的辅助数组
long long int sum;
void memrge(int left,int mid,int right)
{
    int i=left,j=mid+1,k=left;
    while(i<=mid&&j<=right)
    {
        if(a[i]>a[j])
        {
            temp[k++]=a[j++];
            sum+=mid-i+1;
        }
        else temp[k++]=a[i++];
    }
    while(i<=mid) temp[k++]=a[i++];
    while(j<=right) temp[k++]=a[j++];
    for(int i=left;i<=right;i++) a[i]=temp[i];
}
int fen(int left,int right)
{
    if(left<right)
    {
        int mid=(left+right)/2;
        fen(left,mid);
        fen(mid+1,right);
        memrge(left,mid,right);
    }
}
int main()
{
    int n,m,k;
    cin>>n;
    for(int i=0;i<n;i++) scanf("%d",&a[i]);
    fen(0,n-1);
    cout<<sum<<endl;
    return 0;
}