Problem p02715 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#define REP(i, n) for (int i = 0; i < (int)(n); i++)
#define REP1(i, a, b) for (int i = a; i <= (int)(b); i++)
#define ALL(x) begin(x), end(x)
#define PB push_back
using namespace std;
typedef int64_t LL;
typedef vector<int> VI;
typedef pair<int, int> PII;

template <class T> inline bool chmax(T &a, const T &b) {
  return b > a ? a = b, true : false;
}
template <class T> inline bool chmin(T &a, const T &b) {
  return b < a ? a = b, true : false;
}
template <class T> using MaxHeap = priority_queue<T>;
template <class T> using MinHeap = priority_queue<T, vector<T>, greater<T>>;
template <class T, class F = less<T>> void sort_uniq(vector<T> &v, F f = F()) {
  sort(begin(v), end(v), f);
  v.resize(unique(begin(v), end(v)) - begin(v));
}

// {{{ ModInt
template <int _MOD> struct ModInt {
  static const auto MOD = _MOD;
  template <class T>
  using integral_only = typename enable_if<is_integral<T>::value>::type;

  int x;
  constexpr ModInt() : x() {}

  template <class T, integral_only<T> * = nullptr> ModInt(T _x = 0) {
    x = _x % MOD;
    if (x < 0)
      x += MOD;
  }
  ModInt operator-() const { return {x == 0 ? 0 : MOD - x}; }
  ModInt &operator+=(const ModInt a) {
    if ((x += a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  ModInt &operator-=(const ModInt a) {
    if ((x += MOD - a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  ModInt &operator*=(const ModInt a) {
    x = (long long)x * a.x % MOD;
    return *this;
  }
  ModInt &operator/=(const ModInt a) { return (*this) *= a.inv(); }

  ModInt operator+(const ModInt a) const { return ModInt(*this) += a; }
  ModInt operator-(const ModInt a) const { return ModInt(*this) -= a; }
  ModInt operator*(const ModInt a) const { return ModInt(*this) *= a; }
  ModInt operator/(const ModInt a) const { return ModInt(*this) /= a; }
  ModInt inv() const {
    // for prime MOD
    return pow(MOD - 2);
  }
  ModInt inv2() const {
    // work for non-prime MOD if gcd(x,MOD) = 1
    int a = x, b = MOD, u = 1, v = 0;
    while (b != 0) {
      int t = a / b;
      a -= t * b;
      u -= t * v;
      swap(a, b);
      swap(u, v);
    }
    return u;
  }
  template <class T, integral_only<T> * = nullptr> ModInt pow(T t) const {
    ModInt r = 1, p = *this;
    while (t) {
      if (t & 1)
        r *= p;
      p *= p;
      t >>= 1;
    }
    return r;
  }

  bool operator==(ModInt rhs) const { return x == rhs.x; }
  bool operator!=(ModInt rhs) const { return x != rhs.x; }
  bool operator<(ModInt rhs) const { return x < rhs.x; }
  bool operator<=(ModInt rhs) const { return x <= rhs.x; }
  bool operator>(ModInt rhs) const { return x > rhs.x; }
  bool operator>=(ModInt rhs) const { return x >= rhs.x; }

  friend string to_string(ModInt i) { return to_string(i.x); }
  friend istream &operator>>(istream &i, ModInt &a) { return i >> a.x; }
  friend ostream &operator<<(ostream &o, const ModInt &a) { return o << a.x; }
};

const int MOD = 1000000007;
using mint = ModInt<MOD>;
// }}}

int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);

  int n, k;
  cin >> n >> k;
  vector<mint> v(n + 1);
  mint ans = 0;
  for (int i = k; i > 0; i--) {
    mint x = k / i;
    v[i] = x.pow(n);
    for (int j = 2 * i; j <= k; j += i) {
      v[i] -= v[j];
    }
    ans += v[i] * i;
  }

  cout << ans << endl;
  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
#define REP(i, n) for (int i = 0; i < (int)(n); i++)
#define REP1(i, a, b) for (int i = a; i <= (int)(b); i++)
#define ALL(x) begin(x), end(x)
#define PB push_back
using namespace std;
typedef int64_t LL;
typedef vector<int> VI;
typedef pair<int, int> PII;

template <class T> inline bool chmax(T &a, const T &b) {
  return b > a ? a = b, true : false;
}
template <class T> inline bool chmin(T &a, const T &b) {
  return b < a ? a = b, true : false;
}
template <class T> using MaxHeap = priority_queue<T>;
template <class T> using MinHeap = priority_queue<T, vector<T>, greater<T>>;
template <class T, class F = less<T>> void sort_uniq(vector<T> &v, F f = F()) {
  sort(begin(v), end(v), f);
  v.resize(unique(begin(v), end(v)) - begin(v));
}

// {{{ ModInt
template <int _MOD> struct ModInt {
  static const auto MOD = _MOD;
  template <class T>
  using integral_only = typename enable_if<is_integral<T>::value>::type;

  int x;
  constexpr ModInt() : x() {}

  template <class T, integral_only<T> * = nullptr> ModInt(T _x = 0) {
    x = _x % MOD;
    if (x < 0)
      x += MOD;
  }
  ModInt operator-() const { return {x == 0 ? 0 : MOD - x}; }
  ModInt &operator+=(const ModInt a) {
    if ((x += a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  ModInt &operator-=(const ModInt a) {
    if ((x += MOD - a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  ModInt &operator*=(const ModInt a) {
    x = (long long)x * a.x % MOD;
    return *this;
  }
  ModInt &operator/=(const ModInt a) { return (*this) *= a.inv(); }

  ModInt operator+(const ModInt a) const { return ModInt(*this) += a; }
  ModInt operator-(const ModInt a) const { return ModInt(*this) -= a; }
  ModInt operator*(const ModInt a) const { return ModInt(*this) *= a; }
  ModInt operator/(const ModInt a) const { return ModInt(*this) /= a; }
  ModInt inv() const {
    // for prime MOD
    return pow(MOD - 2);
  }
  ModInt inv2() const {
    // work for non-prime MOD if gcd(x,MOD) = 1
    int a = x, b = MOD, u = 1, v = 0;
    while (b != 0) {
      int t = a / b;
      a -= t * b;
      u -= t * v;
      swap(a, b);
      swap(u, v);
    }
    return u;
  }
  template <class T, integral_only<T> * = nullptr> ModInt pow(T t) const {
    ModInt r = 1, p = *this;
    while (t) {
      if (t & 1)
        r *= p;
      p *= p;
      t >>= 1;
    }
    return r;
  }

  bool operator==(ModInt rhs) const { return x == rhs.x; }
  bool operator!=(ModInt rhs) const { return x != rhs.x; }
  bool operator<(ModInt rhs) const { return x < rhs.x; }
  bool operator<=(ModInt rhs) const { return x <= rhs.x; }
  bool operator>(ModInt rhs) const { return x > rhs.x; }
  bool operator>=(ModInt rhs) const { return x >= rhs.x; }

  friend string to_string(ModInt i) { return to_string(i.x); }
  friend istream &operator>>(istream &i, ModInt &a) { return i >> a.x; }
  friend ostream &operator<<(ostream &o, const ModInt &a) { return o << a.x; }
};

const int MOD = 1000000007;
using mint = ModInt<MOD>;
// }}}

int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);

  int n, k;
  cin >> n >> k;
  vector<mint> v(k + 1);
  mint ans = 0;
  for (int i = k; i > 0; i--) {
    mint x = k / i;
    v[i] = x.pow(n);
    for (int j = 2 * i; j <= k; j += i) {
      v[i] -= v[j];
    }
    ans += v[i] * i;
  }

  cout << ans << endl;
  return 0;
}