Submission #3225879

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Source Code Expand

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
#define sqr(x) ((x)*(x))
#define mp make_pair
#define uint unsigned
#define PI pair<int,int>
inline char gc(){
    static char buf[100000],*p1=buf,*p2=buf;
    return p1==p2&&(p2=(p1=buf)+fread(buf,1,100000,stdin),p1==p2)?EOF:*p1++;
}
#define gc getchar
inline ll read(){
	ll x = 0; char ch = gc(); bool positive = 1;
	for (; !isdigit(ch); ch = gc())	if (ch == '-')	positive = 0;
	for (; isdigit(ch); ch = gc())	x = x * 10 + ch - '0';
	return positive ? x : -x;
}
inline void write(ll a){
    if(a<0){
    	a=-a; putchar('-');
	}
    if(a>=10)write(a/10);
    putchar('0'+a%10);
}
inline void writeln(ll a){write(a); puts("");}
inline void wri(ll a){write(a); putchar(' ');}
inline ull rnd(){
	return ((ull)rand()<<30^rand())<<4|rand()%4;
}
const int N=1000005,mod=1000000007;
int n,ans,f[N];
ll fac[N],ni[N];
inline ll p(int a,int b){
	return fac[a]*ni[a-b]%mod;
}
inline int ksm(ll a,int b){
	int ans=1;
	for(;b;b>>=1){
		if(b&1)ans=ans*a%mod;
		a=a*a%mod;
	}
	return ans;
}
int main(){
	n=read();
	for(int i=fac[0]=1;i<=n;i++)fac[i]=fac[i-1]*i%mod; ni[n]=ksm(fac[n],mod-2);
	for(int i=n;i;i--)ni[i-1]=ni[i]*i%mod;
	for(int i=(n+1)/2;i<n;i++){
		f[i]=p(i-1,n-1-i)*fac[i]%mod;
		ans=(ans+(ll)(f[i]-f[i-1])*i)%mod;
	}
	cout<<(ans+mod)%mod<<endl;
}

Submission Info

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