【JSOI2011】bzoj4710 分特产

#include<cstdio>
#include<algorithm>
using namespace std;
#define LL long long
const int p=1000000007,maxn=2000;
int pow(int base,int k)
{
int ret=1;
for (;k;k>>=1,base=(LL)base*base%p)
if (k&1) ret=(LL)ret*base%p;
return ret;
}
int a[maxn+10],fac[maxn+10],inv[maxn+10];
int main()
{
int n,m,mx=1,ans=0,tem;
scanf("%d%d",&n,&m);
for (int i=1;i<=m;i++)
{
scanf("%d",&a[i]);
mx=max(mx,a[i]);
}
mx+=n-1;
fac[0]=inv[0]=1;
for (int i=1;i<=mx;i++) fac[i]=(LL)fac[i-1]*i%p;
inv[mx]=pow(fac[mx],p-2);
for (int i=mx-1;i>=1;i--) inv[i]=(LL)inv[i+1]*(i+1)%p;
for (int i=1;i<=n;i++)
{
tem=(LL)fac
*inv[i]%p*inv[n-i]%p;
for (int j=1;j<=m;j++)
tem=(LL)tem*fac[a[j]+i-1]%p*inv[a[j]]%p*inv[i-1]%p;
if (n-i&1) ans=(ans-tem+p)%p;
else ans=(ans+tem)%p;
}
printf("%d\n",ans);
}