求调QwQ

如下代码

样例 10
输出 9
答案 89

请各位巨佬们看一看哪里错了QwQ

#include <iostream>
#include <vector>
using namespace std;
vector < vector <int> > ans(5010);

vector<int> add(vector <int> a, vector <int> b) {
    vector < int > c;
    vector < int > ans;
    for (int i = 0; i < max(a.size(), b.size()); i++) {
        ans.push_back(a[i]+b[i]);
        ans[i+1] = ans[i]/10;
        ans[i] = ans[i] % 10;
    }
    for (int i = ans.size()-1; i >= 0; i--) {
        c.push_back(ans[i]);
    }
    return c;
}

signed main() {
    int n;
    cin >> n;
    ans[1].push_back(1);
    ans[2].push_back(2);
    for (int i = 3; i <= n; i++) {
        ans[i] = add(ans[i-1], ans[i-2]);
    }
    bool flag = true;
    for (int i = 0; i < ans[n].size(); i++) {
        if (flag) {
            if (ans[n][i] == 0) continue;
            else flag = 0;
        }
        cout << ans[n][i];
    }

    return 0;
}