【2020模拟赛day5】C. 后缀数组

 

是一个和简单的排列组合

考试时数组开小了........

 

代码

#include<bits/stdc++.h>
using namespace std;
const int maxn=2e5+5;
const int mod=998244353;
typedef long long ll;
int n,a[maxn];
ll ans;
ll inv[maxn<<3],fac[maxn<<3];
void exgcd(int a,int b,int &x,int &y)
{
	if(!b)
	{
		x=1; y=0;
		return;
	}
	exgcd(b,a%b,y,x);
	y-=a/b*x;
	return;
}
ll cinv(int a,int b)
{
	int x,y;
	exgcd(a,b,x,y);
	if(x<0) x+=mod;
	return x;
}
int main()
{
	freopen("horrible.in","r",stdin);
	freopen("horrible.out","w",stdout);
	scanf("%d",&n);
	fac[0]=1; ll sum=0;
	for(int i=1;i<=n;i++)
		scanf("%d",&a[i]),sum+=a[i];
	for(int i=1;i<=sum;i++)
		fac[i]=fac[i-1]*i%mod;
	inv[sum]=cinv(fac[sum],mod);
	for(int i=sum-1;i>=0;i--)
		inv[i]=inv[i+1]*(i+1)%mod;
	ans=fac[sum];
	int x;
	for(int i=1;i<=n;i++)
		ans=ans*inv[a[i]]%mod;
	printf("%lld\n",ans);
	return 0;	
}