P1908归并排序求逆序对

#include
#define ll long long
#define INF 0x3f3f3f3f
using namespace std;

int n,a[500010],c[500010];
ll ans=0;

void msort(int b,int e){
    if(b==e)
        return;
    int mid=(b+e)/2,i=b,j=mid+1,k=b;
    msort(b,mid),msort(mid+1,e);
    while(i<=mid&&j<=e)
        if(a[i]<=a[j])
            c[k++]=a[i++];
        else{
            c[k++]=a[j++];
            ans+=mid-i+1;
        }
    while(i<=mid)
        c[k++]=a[i++];
    while(j<=e)
        c[k++]=a[j++];
    for(int l=b;l<=e;l++)
        a[l]=c[l];
}

int main(){
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
        scanf("%d",&a[i]);
    msort(1,n);
    for(int i=1;i<=n;i++)
        cout<