被3整除的子序列

https://ac.nowcoder.com/acm/problem/21302

直接考虑了01背包，目前还不懂为什么会是-1

#include <bits/stdc++.h>
using namespace std;
#define int long long
const int mod = 1e9 + 7;
string s;
int n,a[55],dp[510],ans;
signed main(){
    ios::sync_with_stdio(0);
    cin >> s;
    for(int i = 0; i < s.size(); i++){
        a[i + 1] = s[i] - '0';
        n++;
    }
    dp[0] = 1;
    for(int i = 1; i <= n; i++){
        for(int j = 500; j >= a[i]; j--){
            dp[j] = (dp[j] + dp[j - a[i]]) % mod;
        }
    }
    for(int i = 0; i <= 500; i++){
        if(i % 3 == 0)
            ans = (ans + dp[i]) % mod;
    }
    cout << ans - 1<< endl;
    return 0;
}