Problem p02555 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)
#define reps(i, n) for (int i = 1, i##_len = (n); i <= i##_len; ++i)
#define rrep(i, n) for (int i = ((int)(n)-1); i >= 0; --i)
#define rreps(i, n) for (int i = ((int)(n)); i > 0; --i)
#define ALL(x) (x).begin(), (x).end()
#define SZ(x) ((int)(x).size())
#define pb push_back
#define mp make_pair
#define P pair<int, int>
#define optimize_cin()                                                         \
  cin.tie(0);                                                                  \
  ios::sync_with_stdio(false)
using namespace std;
using ll = long long;
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}

static const ll mod = 1000000007LL;

const int MAX = 700;
ll fac[MAX], finv[MAX], inv[MAX];

void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % mod;
    inv[i] = mod - inv[mod % i] * (mod / i) % mod;
    finv[i] = finv[i - 1] * inv[i] % mod;
  }
}

ll com(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % mod) % mod;
}

int main() {
  int s;
  cin >> s;

  if (s <= 2) {
    cout << 0 << endl;
    return 0;
  }

  COMinit();

  int num = s / 3;
  ll ans = 0;
  reps(i, num) { ans = (ans + com(s - 2 * i - 1, i - 1)) % mod; }

  cout << ans << endl;
}


Fixed submission:
#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)
#define reps(i, n) for (int i = 1, i##_len = (n); i <= i##_len; ++i)
#define rrep(i, n) for (int i = ((int)(n)-1); i >= 0; --i)
#define rreps(i, n) for (int i = ((int)(n)); i > 0; --i)
#define ALL(x) (x).begin(), (x).end()
#define SZ(x) ((int)(x).size())
#define pb push_back
#define mp make_pair
#define P pair<int, int>
#define optimize_cin()                                                         \
  cin.tie(0);                                                                  \
  ios::sync_with_stdio(false)
using namespace std;
using ll = long long;
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}

static const ll mod = 1000000007LL;

const int MAX = 10000;
ll fac[MAX], finv[MAX], inv[MAX];

void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % mod;
    inv[i] = mod - inv[mod % i] * (mod / i) % mod;
    finv[i] = finv[i - 1] * inv[i] % mod;
  }
}

ll com(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % mod) % mod;
}

int main() {
  int s;
  cin >> s;

  if (s <= 2) {
    cout << 0 << endl;
    return 0;
  }

  COMinit();

  int num = s / 3;
  ll ans = 0;
  reps(i, num) { ans = (ans + com(s - 2 * i - 1, i - 1)) % mod; }

  cout << ans << endl;
}