Problem p02647 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>

using namespace std;
using ll = long long;
const int mod = 1e9 + 7;

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; }

struct combination {
  vector<mint> fact, ifact;

  combination(int n) : fact(n + 1), ifact(n + 1) {
    assert(n < mod);
    fact[0] = 1;
    for (int i = 1; i <= n; ++i)
      fact[i] = fact[i - 1] * i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i)
      ifact[i - 1] = ifact[i] * i;
  }

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

struct UnionFind {
  vector<int> d;

  UnionFind(int n = 0) : d(n, -1) {}

  int find(int x) {
    if (d[x] < 0)
      return x;
    return d[x] = find(d[x]);
  }

  bool unite(int x, int y) {
    x = find(x);
    y = find(y);
    if (x == y)
      return false;
    if (d[x] > d[y])
      swap(x, y);
    d[x] += d[y];
    d[y] = x;
    return true;
  }

  bool same(int x, int y) { return find(x) == find(y); }

  int size(int x) { return -d[find(x)]; }
};

template <typename T> struct BIT {
  int n;
  vector<T> bit[2];

  BIT(int n_) { init(n_); }

  void init(int n_) {
    n = n_ + 1;
    for (int p = 0; p < 2; p++)
      bit[p].assign(n, 0);
  }

  void add_sub(int p, int i, T x) {
    for (int idx = i; idx < n; idx += (idx & -idx)) {
      bit[p][idx] += x;
    }
  }

  void add(int l, int r, T x) {
    add_sub(0, l, -x * (l - 1));
    add_sub(0, r, x * (r - 1));
    add_sub(1, l, x);
    add_sub(1, r, -x);
  }

  T sum_sub(int p, int i) {
    T s(0);
    for (int idx = i; idx > 0; idx -= (idx & -idx)) {
      s += bit[p][idx];
    }
    return s;
  }

  T sum(int i) { return sum_sub(0, i) + sum_sub(1, i) * i; }
};

struct WeightedUnionFind {
  vector<int> data;
  vector<ll> ws;

  WeightedUnionFind() {}

  WeightedUnionFind(int sz) : data(sz, -1), ws(sz) {}

  int find(int k) {
    if (data[k] < 0)
      return k;
    auto par = find(data[k]);
    ws[k] += ws[data[k]];
    return data[k] = par;
  }

  ll weight(int t) {
    find(t);
    return ws[t];
  }

  bool unite(int x, int y, ll w) {
    w += weight(x);
    w -= weight(y);
    x = find(x), y = find(y);
    if (x == y)
      return false;
    if (data[x] > data[y]) {
      swap(x, y);
      w *= -1;
    }
    data[x] += data[y];
    data[y] = x;
    ws[y] = w;
    return true;
  }

  ll diff(int x, int y) { return weight(y) - weight(x); }
};

ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }

ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }

int main() {
  int n, k;
  cin >> n >> k;
  vector<int> A(n + 10);
  for (int i = 0; i < n; ++i) {
    cin >> A[i];
  }
  for (int l = 0; l < k; ++l) {
    vector<int> b(n + 10), c(n + 10);
    bool done = true;
    for (int i = 0; i < n; ++i) {
      b[max(0, i - A[i])]++;
      c[min(n, i + A[i]) + 1]++;
      if (A[i] < n)
        done = false;
    }
    if (done)
      break;
    int x = 0;
    for (int i = 0; i < n; ++i) {
      x += b[i] - c[i];
      A[i] = x;
    }
  }
  for (int i = 0; i < n; ++i) {
    cout << A[i] << " ";
  }
  cout << endl;
  return 0;
}


Error: WA

Fixed submission:

#include <bits/stdc++.h>

using namespace std;
using ll = long long;
const int mod = 1e9 + 7;

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; }

struct combination {
  vector<mint> fact, ifact;

  combination(int n) : fact(n + 1), ifact(n + 1) {
    assert(n < mod);
    fact[0] = 1;
    for (int i = 1; i <= n; ++i)
      fact[i] = fact[i - 1] * i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i)
      ifact[i - 1] = ifact[i] * i;
  }

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

struct UnionFind {
  vector<int> d;

  UnionFind(int n = 0) : d(n, -1) {}

  int find(int x) {
    if (d[x] < 0)
      return x;
    return d[x] = find(d[x]);
  }

  bool unite(int x, int y) {
    x = find(x);
    y = find(y);
    if (x == y)
      return false;
    if (d[x] > d[y])
      swap(x, y);
    d[x] += d[y];
    d[y] = x;
    return true;
  }

  bool same(int x, int y) { return find(x) == find(y); }

  int size(int x) { return -d[find(x)]; }
};

template <typename T> struct BIT {
  int n;
  vector<T> bit[2];

  BIT(int n_) { init(n_); }

  void init(int n_) {
    n = n_ + 1;
    for (int p = 0; p < 2; p++)
      bit[p].assign(n, 0);
  }

  void add_sub(int p, int i, T x) {
    for (int idx = i; idx < n; idx += (idx & -idx)) {
      bit[p][idx] += x;
    }
  }

  void add(int l, int r, T x) {
    add_sub(0, l, -x * (l - 1));
    add_sub(0, r, x * (r - 1));
    add_sub(1, l, x);
    add_sub(1, r, -x);
  }

  T sum_sub(int p, int i) {
    T s(0);
    for (int idx = i; idx > 0; idx -= (idx & -idx)) {
      s += bit[p][idx];
    }
    return s;
  }

  T sum(int i) { return sum_sub(0, i) + sum_sub(1, i) * i; }
};

struct WeightedUnionFind {
  vector<int> data;
  vector<ll> ws;

  WeightedUnionFind() {}

  WeightedUnionFind(int sz) : data(sz, -1), ws(sz) {}

  int find(int k) {
    if (data[k] < 0)
      return k;
    auto par = find(data[k]);
    ws[k] += ws[data[k]];
    return data[k] = par;
  }

  ll weight(int t) {
    find(t);
    return ws[t];
  }

  bool unite(int x, int y, ll w) {
    w += weight(x);
    w -= weight(y);
    x = find(x), y = find(y);
    if (x == y)
      return false;
    if (data[x] > data[y]) {
      swap(x, y);
      w *= -1;
    }
    data[x] += data[y];
    data[y] = x;
    ws[y] = w;
    return true;
  }

  ll diff(int x, int y) { return weight(y) - weight(x); }
};

ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }

ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }

int main() {
  int n, k;
  cin >> n >> k;
  vector<int> A(n + 10);
  for (int i = 0; i < n; ++i) {
    cin >> A[i];
  }
  for (int l = 0; l < k; ++l) {
    vector<int> b(n), c(n);
    bool done = true;
    for (int i = 0; i < n; ++i) {
      b[max(0, i - A[i])]++;
      c[min(n, i + A[i]) + 1]++;
      if (A[i] < n)
        done = false;
    }
    if (done)
      break;
    int x = 0;
    for (int i = 0; i < n; ++i) {
      x += b[i] - c[i];
      A[i] = x;
    }
  }
  for (int i = 0; i < n; ++i) {
    cout << A[i] << " ";
  }
  cout << endl;
  return 0;
}