Codeforces1000D-Yet Another Problem On a Subsequence

#include<bits/stdc++.h>//序列类dp问题，找到序列所满足的条件进行转移 
using namespace std;
typedef long long ll;
ll MOD=998244353;
ll C[1010][1010];
ll dp[1010];
ll a[1010];
void init()
{
    for(int i=0;i<=1000;i++)
    {
        C[i][0]=C[i][i]=1;
        for(int j=1;j<i;j++)
          C[i][j]=(C[i-1][j]+C[i-1][j-1])%MOD;
    }
}
int main()
{
    int n;
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
       scanf("%lld",&a[i]);
    init();
    ll ans=0;
    dp[n+1]=1;
    for(int i=n-1;i>=1;i--)
    {
        if(a[i]<=0||a[i]>n-i)
           continue;
        for(int j=i+a[i]+1;j<=n+1;j++)
        {
            dp[i]+=C[j-i-1][a[i]]*dp[j];
            dp[i]%=MOD;
        }
    }
    for(int i=1;i<=n-1;i++)
    {
        ans+=dp[i];
        ans%=MOD;
    }
    printf("%lld\n",ans);
}