bzoj 4710: [Jsoi2011]分特产

题意：

题解：

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<iostream>
#define LL long long
using namespace std;
const LL mod=1000000007;
LL C[2010][2010],n,m,a[1010];
void pre()
{
for(LL i=0;i<=2000;i++)
{
C[i][0]=1;
for(LL j=1;j<=i;j++) C[i][j]=(C[i-1][j]+C[i-1][j-1])%mod;
}
}
LL solve(LL x)
{
LL ans=1;
for(LL i=1;i<=m;i++)
ans=ans*C[x+a[i]-1][x-1]%mod;
return ans;
}
int main()
{
pre();
scanf("%lld %lld",&n,&m);
for(LL i=1;i<=m;i++) scanf("%lld",&a[i]);
LL ans=0;
for(LL i=1;i<=n;i++)
{
LL tmp=solve(i);
if((i&1)==(n&1)) ans=(ans+tmp*C
[i]%mod)%mod;
else ans=(ans-tmp*C
[i]%mod)%mod;
(ans+=mod)%=mod;
}
printf("%lld",ans);
}