Submission #3229538

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Source Code Expand

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
#define Rep(i,a,b) for(register int i=(a);i<=int(b);++i)
#define Dep(i,a,b) for(register int i=(a);i>=int(b);--i)
#define rep(i,a,b) for(register int i=(a);i<int(b);++i)
#define mem(x,v) memset(x,v,sizeof(x))
#define gc getchar
#define pc putchar
#define fi first
#define queue QQQ
#define se second
#define debug(x) cout << #x" = " << x << endl;
#define pp(x,y) cout << "pp: " << x << " " << y << endl;
#define rank __RAnK
inline ll read(){
	register ll x=0,f=1;register char c=gc();
	for(;!isdigit(c);c=gc())if(c=='-')f=-1;
	for(;isdigit(c);c=gc())x=(x<<1)+(x<<3)+(c^48);
	return x*f;
}
#define rd read
void write(ll x){if(x<0)x=-x,pc('-');if(x>=10)write(x/10);putchar(x%10+'0');}
void writeln(ll x){write(x);puts("");}
const int maxn = 2e5+233; 
const int mod = 1e9+7;
int fac[maxn],inv[maxn],f[maxn],n;
inline int qpow(int a,int b){
	int ans=1;
	for(;b;b>>=1,a=1ll*a*a%mod)
		if(b&1) ans=1ll*ans*a%mod;
	return ans;
}
inline int C(int n,int m){
	return 1ll * fac[n] * inv[m] % mod * inv[n-m] % mod;
}
int main(){
	n=rd();
	fac[0] = 1;
	Rep(i,1,n) fac[i] = 1ll * fac[i-1] * i % mod;
	inv[n] = qpow(fac[n],mod-2);
	Dep(i,n-1,0) inv[i] = 1ll * inv[i+1] * (i+1) % mod;
	Dep(i,n-1,(n+1)/2){
		f[i] = 1ll * C(i-1,n-1-i) * fac[i] % mod * fac[n-1-i] % mod;
		f[i+1] = (f[i+1] - f[i] + mod) % mod;
	}
	int ans = 0;
	Dep(i,n-1,(n+1)/2) ans = (ans + 1ll * i * f[i] % mod) % mod;
	writeln(ans);
}
//f[i]表示答案小于等于i的方案数：i步可以完成
//也就是相当于有i个变量 

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