Problem p02702 (python)

Fixed submission:
from collections import defaultdict
S = input()
MOD = 2019

cnt = [0] * MOD
cnt[0] = 1

now = 0
ans = 0
for d, n in enumerate(map(int, S[::-1])):
    now = (n * pow(10, d, MOD) + now) % MOD
    ans += cnt[now]
    cnt[now] += 1

print(ans)

Buggy submission (verdict: Runtime Error):
#include <bits/stdc++.h>
using namespace std;

template <class A, class B>
string to_string(pair<A, B> p);

template <class A, class B, class C>
string to_string(tuple<A, B, C> p);

template <class A, class B, class C, class D>
string to_string(tuple<A, B, C, D> p);

string to_string(const string& s) {
    return '"' + s + '"';
}

string to_string(const char* s) {
    return to_string((string) s);
}

string to_string(bool b) {
    return (b ? "true" : "false");
}

string to_string(vector<bool> v) {
    bool first = true;
    string res = "{";
    for (int i = 0; i < static_cast<int>(v.size()); i++) {
        if (!first) {
            res += ", ";
        }
        first = false;
        res += to_string(v[i]);
    }
    res += "}";
    return res;
}

template <size_t N>
string to_string(bitset<N> v) {
    string res = "";
    for (size_t i = 0; i < N; i++) {
        res += static_cast<char>('0' + v[i]);
    }
    return res;
}

template <class A>
string to_string(A v) {
    bool first = true;
    string res = "{";
    for (const auto &x : v) {
        if (!first) {
            res += ", ";
        }
        first = false;
        res += to_string(x);
    }
    res += "}";
    return res;
}

template <class A, class B>
string to_string(pair<A, B> p) {
    return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template <class A, class B, class C>
string to_string(tuple<A, B, C> p) {
    return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ")";
}

template <class A, class B, class C, class D>
string to_string(tuple<A, B, C, D> p) {
    return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}

void debug_out() { cerr << endl; }

template <class Head, class... Tail>
void debug_out(Head H, Tail... T) {
    cerr << " " << to_string(H);
    debug_out(T...);
}

#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

using Int = unsigned int;
using llong = long long;
using Llong = unsigned long long;
using ldouble = long double;
using intV = vector<int>;
using intVV = vector<vector<int>>;
using llongV = vector<long long>;
using llongVV = vector<vector<long long>>;

template<class T = int>
using asc_pque = priority_queue<T, vector<T>, greater<T>>;
template<class T = int>
using desc_pque = priority_queue<T, vector<T>, less<T>>;

const llong MOD = 1000000007;
const int IINF = 1000000000;
const llong LINF = 100000000000000000LL;

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }

#define FOR(i, n) for (llong i = 0LL; i < llong(n); i++)
#define FORS(i, a, b) for (llong i = llong(a); i < llong(b); i++)
#define bisect_right(vec, a) upper_bound(vec.begin(), vec.end(), a) - vec.begin()
#define bisect_left(vec, a) lower_bound(vec.begin(), vec.end(), a) - vec.begin()
#define GET(i, p) get<i>(p)

int dp[2019] = {0};
int oldDp[2019] = {0};

void calc(int n) {
    FOR(i, 2019) {
        oldDp[i] = dp[i];
    }
    FOR(i, 2019) {
        dp[(i + n) % 2019] = oldDp[i];
    }
}

int main(void) {
    cin.tie(0);
    ios::sync_with_stdio(false);

    string S;
    cin >> S;

    int ans = 0;
    int pow = 1;
    dp[0] = 1;

    for(auto s = S.rbegin(); s != S.rend(); s++) {
        int n = (*s - '0') * pow % 2019;
        calc(n);
        ans += dp[0];
        dp[0] += 1;
        pow = pow * 10 % 2019;
    }

    cout << ans << endl;

    return 0;
}