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
1 #include<bits/stdc++.h> 2 using namespace std; 3 #define rg register 4 typedef long long ll; 5 #define gmax(a,b) a=max(a,b) 6 #define gmin(a,b) a=min(a,b) 7 #define FOR(i,a,b) for(rg int i=a;i<=b;++i) 8 #define For(i,a,b) for(rg int i=a;i>=b;--i) 9 #define gc pa==pb&&(pb=(pa=buf)+fread(buf,1,100000,stdin),pa==pb)?EOF:*pa++ 10 static char buf[100000],*pa(buf),*pb(buf); 11 inline int rd() 12 { 13 rg int x(0),w(1); 14 rg char c(gc); 15 while(c<'0' || c>'9') 16 { 17 if(c=='-') w=-1; 18 c=gc; 19 } 20 while(c>='0' && c<='9') x=x*10+c-48,c=gc; 21 return x*w; 22 } 23 24 const int N=1000005; 25 const ll MOD=998244353,ny=499122177; 26 ll f[N],g[N],tn[N],jc[N]; 27 int c[N],a[N],n; 28 ll ans; 29 30 void add(int x,int y) 31 { 32 for(;x<=n;x+=x&(-x)) c[x]+=y; 33 } 34 35 ll ask(int x) 36 { 37 ll res=0; 38 for(;x;x-=x&(-x)) res+=c[x]; 39 return res; 40 } 41 42 int main() 43 { 44 freopen("sort.in","r",stdin); 45 freopen("sort.out","w",stdout); 46 n=rd();FOR(i,1,n) a[i]=rd(); 47 jc[0]=1;FOR(i,1,n) jc[i]=(i*jc[i-1])%MOD; 48 FOR(i,2,n) f[i]=((i*f[i-1])%MOD+(1LL*i*(i-1)/2)%MOD*jc[i-1]%MOD)%MOD; 49 tn[1]=1;add(a[n],1); 50 FOR(i,2,n) 51 g[i]=ask(a[n-i+1]),add(a[n-i+1],1), 52 tn[i]=((g[i]*jc[i-1])%MOD+tn[i-1])%MOD; 53 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; 54 cout<<ans; 55 return 0; 56 }
