Problem p02635 (C++)

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

template <int MOD_> struct modnum {
  static constexpr int MOD = MOD_;
  static_assert(MOD_ > 0, "MOD must be positive");

private:
  using ll = long long;

  int v;

  static int minv(int a, int m) {
    a %= m;
    assert(a);
    return a == 1 ? 1 : int(m - ll(minv(m, a)) * ll(m) / a);
  }

public:
  modnum() : v(0) {}
  modnum(ll v_) : v(int(v_ % MOD)) {
    if (v < 0)
      v += MOD;
  }
  explicit operator int() const { return v; }
  friend std::ostream &operator<<(std::ostream &out, const modnum &n) {
    return out << int(n);
  }
  friend std::istream &operator>>(std::istream &in, modnum &n) {
    ll v_;
    in >> v_;
    n = modnum(v_);
    return in;
  }

  friend bool operator==(const modnum &a, const modnum &b) {
    return a.v == b.v;
  }
  friend bool operator!=(const modnum &a, const modnum &b) {
    return a.v != b.v;
  }

  modnum inv() const {
    modnum res;
    res.v = minv(v, MOD);
    return res;
  }
  friend modnum inv(const modnum &m) { return m.inv(); }
  modnum neg() const {
    modnum res;
    res.v = v ? MOD - v : 0;
    return res;
  }
  friend modnum neg(const modnum &m) { return m.neg(); }

  modnum operator-() const { return neg(); }
  modnum operator+() const { return modnum(*this); }

  modnum &operator++() {
    v++;
    if (v == MOD)
      v = 0;
    return *this;
  }
  modnum &operator--() {
    if (v == 0)
      v = MOD;
    v--;
    return *this;
  }
  modnum &operator+=(const modnum &o) {
    v += o.v;
    if (v >= MOD)
      v -= MOD;
    return *this;
  }
  modnum &operator-=(const modnum &o) {
    v -= o.v;
    if (v < 0)
      v += MOD;
    return *this;
  }
  modnum &operator*=(const modnum &o) {
    v = int(ll(v) * ll(o.v) % MOD);
    return *this;
  }
  modnum &operator/=(const modnum &o) { return *this *= o.inv(); }

  friend modnum operator++(modnum &a, int) {
    modnum r = a;
    ++a;
    return r;
  }
  friend modnum operator--(modnum &a, int) {
    modnum r = a;
    --a;
    return r;
  }
  friend modnum operator+(const modnum &a, const modnum &b) {
    return modnum(a) += b;
  }
  friend modnum operator-(const modnum &a, const modnum &b) {
    return modnum(a) -= b;
  }
  friend modnum operator*(const modnum &a, const modnum &b) {
    return modnum(a) *= b;
  }
  friend modnum operator/(const modnum &a, const modnum &b) {
    return modnum(a) /= b;
  }
};

template <typename T> T pow(T a, long long b) {
  assert(b >= 0);
  T r = 1;
  while (b) {
    if (b & 1)
      r *= a;
    b >>= 1;
    a *= a;
  }
  return r;
}

using num = modnum<998244353>;

vector<num> fact, ifact;

void init() {
  int N = 1100000;
  fact = {1};
  for (int i = 1; i < N; i++)
    fact.push_back(i * fact[i - 1]);
  ifact.resize(N);
  ifact.back() = 1 / fact.back();
  for (int i = N - 1; i > 0; i--)
    ifact[i - 1] = i * ifact[i];
}

num ncr(int n, int k) {
  if (k < 0 || k > n)
    return 0;
  return fact[n] * ifact[k] * ifact[n - k];
}

int main() {
  ios_base::sync_with_stdio(false), cin.tie(nullptr);
  init();

  string s;
  int k;
  cin >> s >> k;
  s += "0";
  vector<int> cnt;
  int cur = 0;
  int ntot = 0;
  for (char c : s) {
    if (c == '1') {
      cur++;
      ntot++;
    }
    if (c == '0') {
      cnt.push_back(cur);
      cur = 0;
    }
  }
  vector<vector<num>> dp(ntot + 1, vector<num>(k + 1, 0));
  dp[0][0] = 1;
  int ctot = 0;
  for (int c : cnt) {
    ctot += c;
    vector<vector<num>> z(ntot + 1, vector<num>(k + 1, 0));
    for (int a = 0; a <= ntot; a++) {
      for (int b = 0; b <= k; b++) {
        for (int na = 0; a + na <= ntot; na++) {
          if (a + na < ctot)
            continue;
          int nb = b + max(na - c, 0);
          if (nb > k)
            continue;
          z[a + na][nb] += dp[a][b];
        }
      }
    }
    dp = z;
  }
  num ans = 0;
  for (int i = 0; i <= k; i++)
    ans += dp[ntot][i];
  cout << ans << '\n';
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;

template <int MOD_> struct modnum {
  static constexpr int MOD = MOD_;
  static_assert(MOD_ > 0, "MOD must be positive");

private:
  using ll = long long;

  int v;

  static int minv(int a, int m) {
    a %= m;
    assert(a);
    return a == 1 ? 1 : int(m - ll(minv(m, a)) * ll(m) / a);
  }

public:
  modnum() : v(0) {}
  modnum(ll v_) : v(int(v_ % MOD)) {
    if (v < 0)
      v += MOD;
  }
  explicit operator int() const { return v; }
  friend std::ostream &operator<<(std::ostream &out, const modnum &n) {
    return out << int(n);
  }
  friend std::istream &operator>>(std::istream &in, modnum &n) {
    ll v_;
    in >> v_;
    n = modnum(v_);
    return in;
  }

  friend bool operator==(const modnum &a, const modnum &b) {
    return a.v == b.v;
  }
  friend bool operator!=(const modnum &a, const modnum &b) {
    return a.v != b.v;
  }

  modnum inv() const {
    modnum res;
    res.v = minv(v, MOD);
    return res;
  }
  friend modnum inv(const modnum &m) { return m.inv(); }
  modnum neg() const {
    modnum res;
    res.v = v ? MOD - v : 0;
    return res;
  }
  friend modnum neg(const modnum &m) { return m.neg(); }

  modnum operator-() const { return neg(); }
  modnum operator+() const { return modnum(*this); }

  modnum &operator++() {
    v++;
    if (v == MOD)
      v = 0;
    return *this;
  }
  modnum &operator--() {
    if (v == 0)
      v = MOD;
    v--;
    return *this;
  }
  modnum &operator+=(const modnum &o) {
    v += o.v;
    if (v >= MOD)
      v -= MOD;
    return *this;
  }
  modnum &operator-=(const modnum &o) {
    v -= o.v;
    if (v < 0)
      v += MOD;
    return *this;
  }
  modnum &operator*=(const modnum &o) {
    v = int(ll(v) * ll(o.v) % MOD);
    return *this;
  }
  modnum &operator/=(const modnum &o) { return *this *= o.inv(); }

  friend modnum operator++(modnum &a, int) {
    modnum r = a;
    ++a;
    return r;
  }
  friend modnum operator--(modnum &a, int) {
    modnum r = a;
    --a;
    return r;
  }
  friend modnum operator+(const modnum &a, const modnum &b) {
    return modnum(a) += b;
  }
  friend modnum operator-(const modnum &a, const modnum &b) {
    return modnum(a) -= b;
  }
  friend modnum operator*(const modnum &a, const modnum &b) {
    return modnum(a) *= b;
  }
  friend modnum operator/(const modnum &a, const modnum &b) {
    return modnum(a) /= b;
  }
};

template <typename T> T pow(T a, long long b) {
  assert(b >= 0);
  T r = 1;
  while (b) {
    if (b & 1)
      r *= a;
    b >>= 1;
    a *= a;
  }
  return r;
}

using num = modnum<998244353>;

vector<num> fact, ifact;

void init() {
  int N = 1100000;
  fact = {1};
  for (int i = 1; i < N; i++)
    fact.push_back(i * fact[i - 1]);
  ifact.resize(N);
  ifact.back() = 1 / fact.back();
  for (int i = N - 1; i > 0; i--)
    ifact[i - 1] = i * ifact[i];
}

num ncr(int n, int k) {
  if (k < 0 || k > n)
    return 0;
  return fact[n] * ifact[k] * ifact[n - k];
}

int main() {
  ios_base::sync_with_stdio(false), cin.tie(nullptr);
  init();

  string s;
  int k;
  cin >> s >> k;
  s += "0";
  vector<int> cnt;
  int cur = 0;
  int ntot = 0;
  for (char c : s) {
    if (c == '1') {
      cur++;
      ntot++;
    }
    if (c == '0') {
      cnt.push_back(cur);
      cur = 0;
    }
  }
  k = min(k, 400);
  vector<vector<num>> dp(ntot + 1, vector<num>(k + 1, 0));
  dp[0][0] = 1;
  int ctot = 0;
  for (int c : cnt) {
    ctot += c;
    vector<vector<num>> z(ntot + 1, vector<num>(k + 1, 0));
    for (int a = 0; a <= ntot; a++) {
      for (int b = 0; b <= k; b++) {
        for (int na = 0; a + na <= ntot; na++) {
          if (a + na < ctot)
            continue;
          int nb = b + max(na - c, 0);
          if (nb > k)
            continue;
          z[a + na][nb] += dp[a][b];
        }
      }
    }
    dp = z;
  }
  num ans = 0;
  for (int i = 0; i <= k; i++)
    ans += dp[ntot][i];
  cout << ans << '\n';
}