Problem p02653 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <functional>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <string>
#define REP(_i, _a, _n) for (int _i = _a; _i <= _n; ++_i)
#define PER(_i, _a, _n) for (int _i = _n; _i >= _a; --_i)
#define hr putchar(10)
#define pb push_back
#define lc (o << 1)
#define rc (lc | 1)
#define mid ((l + r) >> 1)
#define ls lc, l, mid
#define rs rc, mid + 1, r
#define x first
#define y second
#define io std::ios::sync_with_stdio(false)
#define endl '\n'
#define DB(_a)                                                                 \
  ({                                                                           \
    REP(_i, 1, n) cout << _a[_i] << ',';                                       \
    hr;                                                                        \
  })
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
const int P = 1e9 + 7, INF = 0x3f3f3f3f;
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll qpow(ll a, ll n) {
  ll r = 1 % P;
  for (a %= P; n; a = a * a % P, n >>= 1)
    if (n & 1)
      r = r * a % P;
  return r;
}
ll inv(ll x) { return x <= 1 ? 1 : inv(P % x) * (P - P / x) % P; }
inline int rd() {
  int x = 0;
  char p = getchar();
  while (p < '0' || p > '9')
    p = getchar();
  while (p >= '0' && p <= '9')
    x = x * 10 + p - '0', p = getchar();
  return x;
}
// head

const int N = 5e3 + 10;
int n, a, b, f[N], g[N][2];
void add(int &x, ll y) { x = (x + y) % P; }
int main() {
  scanf("%d%d%d", &n, &a, &b);
  if (a > b)
    swap(a, b);
  f[1] = f[2] = 1;
  REP(i, 3, n) {
    f[i] = f[i - 1];
    REP(j, 1, i - a - 1) add(f[i], f[j]);
  }
  REP(i, 1, b - 1) g[i][1] = f[i];
  REP(i, 1, a - 1) g[i][0] = 1;
  REP(i, a, b - 1) REP(j, 1, b - 1 - i) add(g[i + j][1], f[j]);
  REP(i, 1, n) {
    REP(j, 1, a - 1) add(g[i + j][0], g[i][1]);
    REP(j, 1, b - 1) add(g[i + j][1], (ll)g[i][0] * f[j]);
  }
  int ans = (g[n][0] + g[n][1]) % P;
  REP(i, a, b - 1) REP(j, 1, b - 1 - i) add(ans, (ll)g[n - i - j][0] * f[j]);
  ans = (qpow(2, n) - ans) % P;
  if (ans < 0)
    ans += P;
  printf("%d\n", ans);
}


Fixed submission:
#include <algorithm>
#include <bitset>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <functional>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <string>
#define REP(_i, _a, _n) for (int _i = _a; _i <= _n; ++_i)
#define PER(_i, _a, _n) for (int _i = _n; _i >= _a; --_i)
#define hr putchar(10)
#define pb push_back
#define lc (o << 1)
#define rc (lc | 1)
#define mid ((l + r) >> 1)
#define ls lc, l, mid
#define rs rc, mid + 1, r
#define x first
#define y second
#define io std::ios::sync_with_stdio(false)
#define endl '\n'
#define DB(_a)                                                                 \
  ({                                                                           \
    REP(_i, 1, n) cout << _a[_i] << ',';                                       \
    hr;                                                                        \
  })
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
const int P = 1e9 + 7, INF = 0x3f3f3f3f;
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll qpow(ll a, ll n) {
  ll r = 1 % P;
  for (a %= P; n; a = a * a % P, n >>= 1)
    if (n & 1)
      r = r * a % P;
  return r;
}
ll inv(ll x) { return x <= 1 ? 1 : inv(P % x) * (P - P / x) % P; }
inline int rd() {
  int x = 0;
  char p = getchar();
  while (p < '0' || p > '9')
    p = getchar();
  while (p >= '0' && p <= '9')
    x = x * 10 + p - '0', p = getchar();
  return x;
}
// head

const int N = 5e3 + 10;
int n, a, b, f[N], g[N][2];
void add(int &x, ll y) { x = (x + y) % P; }
int main() {
  scanf("%d%d%d", &n, &a, &b);
  if (a > b)
    swap(a, b);
  f[1] = f[2] = 1;
  REP(i, 3, n) {
    f[i] = f[i - 1];
    REP(j, 1, i - a - 1) add(f[i], f[j]);
  }
  REP(i, 1, b - 1) g[i][1] = f[i];
  REP(i, 1, a - 1) g[i][0] = 1;
  REP(i, a, b - 1) REP(j, 1, b - 1 - i) add(g[i + j][1], f[j]);
  REP(i, 1, n) {
    REP(j, 1, min(n - i, a - 1)) add(g[i + j][0], g[i][1]);
    REP(j, 1, min(n - i, b - 1)) add(g[i + j][1], (ll)g[i][0] * f[j]);
  }
  int ans = (g[n][0] + g[n][1]) % P;
  REP(i, a, b - 1) REP(j, 1, b - 1 - i) add(ans, (ll)g[n - i - j][0] * f[j]);
  ans = (qpow(2, n) - ans) % P;
  if (ans < 0)
    ans += P;
  printf("%d\n", ans);
}