Problem p02702 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)
#define rep2(i, x, n) for (int i = x, i##_len = (n); i < i##_len; ++i)
#define all(n) begin(n), end(n)
using ll = long long;
using P = pair<int, int>;
using vi = vector<int>;
using vl = vector<ll>;
using vs = vector<string>;
using vc = vector<char>;
using vb = vector<bool>;
using vd = vector<double>;
vi dir = {-1, 0, 1, 0, -1, -1, 1, 1, -1};

const int mod = 2019;
struct mint {
  ll x; // typedef long long ll;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}
  mint operator-() const { return mint(-x); }
  mint &operator+=(const mint a) {
    if ((x += a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= mod;
    return *this;
  }
  mint operator+(const mint a) const { return mint(*this) += a; }
  mint operator-(const mint a) const { return mint(*this) -= a; }
  mint operator*(const mint a) const { return mint(*this) *= a; }
  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  // for prime mod
  mint inv() const { return pow(mod - 2); }
  mint &operator/=(const mint a) { return *this *= a.inv(); }
  mint operator/(const mint a) const { return mint(*this) /= a; }
};
istream &operator>>(istream &is, const mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }
mint fact(int n) {
  if (n == 1 || n == 0)
    return 1;
  mint res = n;
  return res * fact(n - 1);
}

int main() {
  string s;
  cin >> s;
  int n = s.size();
  vl a(n + 1, 0);
  rep(i, n) a[i + 1] = (a[i] * 10 + (s[i] - '0')) % 2019;
  map<ll, ll> mp;
  ll ans = 0;
  vector<mint> p(n + 1, 1);
  rep(i, n) p[i + 1] = p[i] * 10;
  vl cnt(2019, 0);
  rep(i, n + 1) {
    mint now = p[n - i];
    now *= a[i];
    ll x = now.x;
    ans += cnt[x];
    cnt[x]++;
  }
  cout << ans << endl;
}

Error: -6
Fatal glibc error: malloc assertion failure in sysmalloc: (old_top == initial_top (av) && old_size == 0) || ((unsigned long) (old_size) >= MINSIZE && prev_inuse (old_top) && ((unsigned long) old_end & (pagesize - 1)) == 0)


Fixed submission:

#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)
#define rep2(i, x, n) for (int i = x, i##_len = (n); i < i##_len; ++i)
#define all(n) begin(n), end(n)
using ll = long long;
using P = pair<int, int>;
using vi = vector<int>;
using vl = vector<ll>;
using vs = vector<string>;
using vc = vector<char>;
using vb = vector<bool>;
using vd = vector<double>;
vi dir = {-1, 0, 1, 0, -1, -1, 1, 1, -1};

const int mod = 2019;
struct mint {
  ll x; // typedef long long ll;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}
  mint operator-() const { return mint(-x); }
  mint &operator+=(const mint a) {
    if ((x += a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= mod;
    return *this;
  }
  mint operator+(const mint a) const { return mint(*this) += a; }
  mint operator-(const mint a) const { return mint(*this) -= a; }
  mint operator*(const mint a) const { return mint(*this) *= a; }
  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  // for prime mod
  mint inv() const { return pow(mod - 2); }
  mint &operator/=(const mint a) { return *this *= a.inv(); }
  mint operator/(const mint a) const { return mint(*this) /= a; }
};
istream &operator>>(istream &is, const mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }
mint fact(int n) {
  if (n == 1 || n == 0)
    return 1;
  mint res = n;
  return res * fact(n - 1);
}

int main() {
  string s;
  cin >> s;
  int n = s.size();
  vl a(n, 0);
  rep(i, n) a[i + 1] = (a[i] * 10 + (s[i] - '0')) % 2019;
  map<ll, ll> mp;
  ll ans = 0;
  vector<mint> p(n + 1, 1);
  rep(i, n) p[i + 1] = p[i] * 10;
  vl cnt(2019, 0);
  rep(i, n + 1) {
    mint now = p[n - i];
    now *= a[i];
    ans += cnt[now.x];
    cnt[now.x]++;
  }
  cout << ans << endl;
}