洛谷 1966 [NOIP2013] 火柴排队 逆序对

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<map>
using namespace std;

const int mod=99999997,MAXN=200001;
int a[MAXN],r[MAXN];
int n,ans;
int s2[MAXN],ma[MAXN];
map<int,int>mp;
struct hh {int num,pos;}s1[MAXN];
bool cmp(hh a,hh b)
{
return a.num<b.num;
}

void merge_sort(int f,int t)
{
if(f==t) return;
int mid=(f+t)>>1;
merge_sort(f,mid),merge_sort(mid+1,t);
int i=f,j=mid+1,k=f;
while(i<=mid && j<=t)
{
if(a[i]>a[j])
ans=(ans+mid-i+1)%mod,r[k++]=a[j++];
else r[k++]=a[i++];
}
while(i<=mid) r[k++]=a[i++];
while(j<=t) r[k++]=a[j++];
for(int i=f;i<=t;i++) a[i]=r[i];
return;

}

void solve()
{
cin>>n;
for(int i=1;i<=n;i++) scanf("%d",&s1[i].num),s1[i].pos=i;
for(int i=1;i<=n;i++) scanf("%d",&s2[i]),a[i]=s2[i];
sort(s1+1,s1+n+1,cmp);
sort(s2+1,s2+n+1);

for(int i=1;i<=n;i++) mp[s2[i]]=s1[i].pos;
for(int i=1;i<=n;i++)
a[i]=mp[a[i]];

merge_sort(1,n);
cout<<ans%mod;
return;
}

int main()
{
solve();
return 0;
}