排列逆序对计数

https://pan.baidu.com/s/1mRenOxuGQ9gEpW7WsJXf4Q    7uqg 

组合计数，结合数位DP的思想。

考试话3小时做出来了，结果没开LL，50分，很崩溃

https://www.cnblogs.com/universeplayer/p/10565654.html

#include<bits/stdc++.h>
using namespace std;
#define rg register
typedef long long ll;
#define gmax(a,b) a=max(a,b)
#define gmin(a,b) a=min(a,b)
#define FOR(i,a,b) for(rg int i=a;i<=b;++i)
#define For(i,a,b) for(rg int i=a;i>=b;--i)
#define gc pa==pb&&(pb=(pa=buf)+fread(buf,1,100000,stdin),pa==pb)?EOF:*pa++
static char buf[100000],*pa(buf),*pb(buf);
inline int rd()
{
    rg int x(0),w(1);
    rg char c(gc);
    while(c<'0' || c>'9')
    {
        if(c=='-') w=-1;
        c=gc;
    }
    while(c>='0' && c<='9') x=x*10+c-48,c=gc;
    return x*w;
}

const int N=1000005;
const ll MOD=998244353,ny=499122177;
ll f[N],g[N],tn[N],jc[N];
int c[N],a[N],n;
ll ans;

void add(int x,int y)
{
    for(;x<=n;x+=x&(-x)) c[x]+=y;
}

ll ask(int x)
{
    ll res=0;
    for(;x;x-=x&(-x)) res+=c[x];
    return res;
}

int main()
{
    freopen("sort.in","r",stdin);
    freopen("sort.out","w",stdout);
    n=rd();FOR(i,1,n) a[i]=rd();
    jc[0]=1;FOR(i,1,n) jc[i]=(i*jc[i-1])%MOD;
    FOR(i,2,n) f[i]=((i*f[i-1])%MOD+(1LL*i*(i-1)/2)%MOD*jc[i-1]%MOD)%MOD;
    tn[1]=1;add(a[n],1);
    FOR(i,2,n)
        g[i]=ask(a[n-i+1]),add(a[n-i+1],1),
        tn[i]=((g[i]*jc[i-1])%MOD+tn[i-1])%MOD;
    FOR(i,1,n) ans=(ans+g[i]*f[i-1]%MOD+g[i]*(g[i]-1)%MOD*ny%MOD*jc[i-1]%MOD+g[i]*tn[i-1]%MOD)%MOD;
    cout<<ans;
    return 0;
}