Problem p02702 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

#define REP(i, n) for (int i = 0; i < (n); ++i)
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define FORR(i, a, b) for (int i = (b)-1; i >= (a); --i)

#define DEBUG(x) cout << #x << ": " << (x) << '\n'
#define DEBUG_VEC(v)                                                           \
  cout << #v << ":";                                                           \
  REP(i, v.size()) cout << ' ' << v[i];                                        \
  cout << '\n'
#define ALL(a) (a).begin(), (a).end()

template <typename T> inline void CHMAX(T &a, const T b) {
  if (a < b)
    a = b;
}
template <typename T> inline void CHMIN(T &a, const T b) {
  if (a > b)
    a = b;
}

constexpr ll MOD = 1000000007ll;
// constexpr ll MOD=998244353ll;
#define FIX(a) ((a) % MOD + MOD) % MOD

const double EPS = 1e-9;
#define EQ0(x) (abs((x)) < EPS)
#define EQ(a, b) (abs((a) - (b)) < EPS)

ll sum[2525];

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  // cout<<setprecision(10)<<fixed;
  string s;
  cin >> s;
  int n = s.size();
  sum[n] = 0;
  int ten = 1;
  FORR(i, 0, n) {
    int x = s[i] - '0';
    sum[i] = (x * ten + sum[i + 1]) % 2019;
    ten = (ten * 10) % 2019;
  }
  // REP(i, n+1){
  //   cout<<sum[i]<<" \n"[i==n];
  // }
  sort(sum, sum + n + 1);
  ll ans = 0;
  int l = 0, r = 0;
  while (l < n + 1) {
    while (r < n + 1 && sum[l] == sum[r]) {
      ++r;
    }
    ans += (r - l) * (r - l - 1) / 2;
    // DEBUG(l);DEBUG(r);
    l = r;
  }
  cout << ans << '\n';
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

#define REP(i, n) for (int i = 0; i < (n); ++i)
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define FORR(i, a, b) for (int i = (b)-1; i >= (a); --i)

#define DEBUG(x) cout << #x << ": " << (x) << '\n'
#define DEBUG_VEC(v)                                                           \
  cout << #v << ":";                                                           \
  REP(i, v.size()) cout << ' ' << v[i];                                        \
  cout << '\n'
#define ALL(a) (a).begin(), (a).end()

template <typename T> inline void CHMAX(T &a, const T b) {
  if (a < b)
    a = b;
}
template <typename T> inline void CHMIN(T &a, const T b) {
  if (a > b)
    a = b;
}

constexpr ll MOD = 1000000007ll;
// constexpr ll MOD=998244353ll;
#define FIX(a) ((a) % MOD + MOD) % MOD

const double EPS = 1e-9;
#define EQ0(x) (abs((x)) < EPS)
#define EQ(a, b) (abs((a) - (b)) < EPS)

ll sum[252521];

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  // cout<<setprecision(10)<<fixed;
  string s;
  cin >> s;
  int n = s.size();
  sum[n] = 0;
  int ten = 1;
  FORR(i, 0, n) {
    int x = s[i] - '0';
    sum[i] = (x * ten + sum[i + 1]) % 2019;
    ten = (ten * 10) % 2019;
  }
  // REP(i, n+1){
  //   cout<<sum[i]<<" \n"[i==n];
  // }
  sort(sum, sum + n + 1);
  ll ans = 0;
  int l = 0, r = 0;
  while (l < n + 1) {
    while (r < n + 1 && sum[l] == sum[r]) {
      ++r;
    }
    ans += (r - l) * (r - l - 1) / 2;
    // DEBUG(l);DEBUG(r);
    l = r;
  }
  cout << ans << '\n';
  return 0;
}