Problem p02702 (C++) — buggy submission, verdict: Time Limit Exceeded

#include <bits/stdc++.h>
#define FOR(i, a, n) for (int(i) = (int)(a); (i) < (int)(n); ++(i))
#define FORe(i, a, n) for (int(i) = (int)(a); (i) <= (int)(n); ++(i))
#define rFOR(i, a, n) for (int(i) = (int)(n)-1; (i) >= (int)(a); --(i))
#define rFORe(i, a, n) for (int(i) = (int)(n); (i) >= (int)(a); --(i))
#define REP(i, n) FOR((i), 0, (n))
#define REPe(i, n) FORe((i), 1, (n))
#define REPo(i, n) FORe((i), 0, (n))
#define rREP(i, n) rFOR((i), 0, (n))
#define rREPe(i, n) rFORe((i), 1, (n))
#define rREPo(i, n) rFORe((i), 0, (n))
#define rep(i, n) FOR((i), 0, (n))
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()
#define fi first
#define se second
#define dup(x, y) (((x) + (y)-1) / (y))
#define v(T) vector<T>
#define vv(T) v(v(T))
using namespace std;
using ll = long long;
using P = pair<int, int>;
using T = tuple<int, int, int>;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
using vs = vector<string>;
using vp = vector<P>;
using vt = vector<T>;

template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}

// auto mod int
const int mod = 2019;
// const int mod = 998244353;
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; }

ll comb(ll a) { return a * (a - 1) / 2; }

int main() {
  string s;
  cin >> s;
  int n = s.size();
  vl cnt(2019);
  cnt[0]++;
  mint tmp = 0;
  mint digit = 1;
  reverse(all(s));
  REP(i, n) {
    tmp += mint(s[i] - '0') * digit;
    cnt[tmp.x]++;
    digit *= 10;
  }
  ll ans = 0;
  REP(i, 2019) { ans += comb(cnt[i]); }
  cout << ans << endl;

  return 0;
}

Error: TLE

Fixed submission:

#include <bits/stdc++.h>
#define FOR(i, a, n) for (int(i) = (int)(a); (i) < (int)(n); ++(i))
#define FORe(i, a, n) for (int(i) = (int)(a); (i) <= (int)(n); ++(i))
#define rFOR(i, a, n) for (int(i) = (int)(n)-1; (i) >= (int)(a); --(i))
#define rFORe(i, a, n) for (int(i) = (int)(n); (i) >= (int)(a); --(i))
#define REP(i, n) FOR((i), 0, (n))
#define REPe(i, n) FORe((i), 1, (n))
#define REPo(i, n) FORe((i), 0, (n))
#define rREP(i, n) rFOR((i), 0, (n))
#define rREPe(i, n) rFORe((i), 1, (n))
#define rREPo(i, n) rFORe((i), 0, (n))
#define rep(i, n) FOR((i), 0, (n))
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()
#define fi first
#define se second
#define dup(x, y) (((x) + (y)-1) / (y))
#define v(T) vector<T>
#define vv(T) v(v(T))
using namespace std;
using ll = long long;
using P = pair<int, int>;
using T = tuple<int, int, int>;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
using vs = vector<string>;
using vp = vector<P>;
using vt = vector<T>;

template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}

// auto mod int
const int mod = 2019;
// const int mod = 998244353;
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; }

int main() {
  string s;
  cin >> s;
  ll ans = 0;
  REP(i, s.size()) {
    mint tmp = 0;
    FOR(j, i, s.size()) {
      tmp *= 10;
      tmp += (s[j] - '0');
      // cout << tmp.x << endl;
      if (tmp.x % 2019 == 0)
        ans++;
    }
  }
  cout << ans << endl;

  return 0;
}