Problem p02715 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bits/stdc++.h>
#include <cmath>
#include <cstdlib>
using namespace std;
using ll = long long;
using P = pair<int, int>;
using Graph = vector<vector<int>>;
#define rep(i, n) for (ll i = 0; i < (n); ++i)
#define rep2(i, n, m) for (ll i = n; i <= m; i++)
#define rep3(i, n, m) for (ll i = n; i >= m; i--)
#define pb push_back
#define eb emplace_back
#define ppb pop_back
const ll INF = 1e18;
inline void chmax(ll &a, ll b) { a = max(a, b); }
inline void chmin(ll &a, ll b) { a = min(a, b); }
// ミント
const int mod = 1e9 + 7;
// 998244353;

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; }
// 昆布
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];
  }
};

mint modpow(ll a, ll b) {
  if (b == 0)
    return 1;
  mint c = modpow(a, b / 2);
  if (b % 2 == 1)
    return c * c * a;
  else
    return c * c;
}

int main() {
  ll n, k;
  cin >> n >> k;
  mint ans = 0;
  vector<mint> B(n + 1);

  for (ll i = k; i >= 1; i--) {
    ll a = k / i;
    mint sum = modpow(a, n);
    for (ll p = i * 2; p <= k; p += i) {
      sum -= B[p];
    }
    B[i] = sum;
    mint j = i;
    ans += sum * j;
  }
  cout << ans << endl;

  return 0;
}


Fixed submission:
#include <algorithm>
#include <bits/stdc++.h>
#include <cmath>
#include <cstdlib>
using namespace std;
using ll = long long;
using P = pair<int, int>;
using Graph = vector<vector<int>>;
#define rep(i, n) for (ll i = 0; i < (n); ++i)
#define rep2(i, n, m) for (ll i = n; i <= m; i++)
#define rep3(i, n, m) for (ll i = n; i >= m; i--)
#define pb push_back
#define eb emplace_back
#define ppb pop_back
const ll INF = 1e18;
inline void chmax(ll &a, ll b) { a = max(a, b); }
inline void chmin(ll &a, ll b) { a = min(a, b); }
// ミント
const int mod = 1e9 + 7;
// 998244353;

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; }
// 昆布
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];
  }
};

mint modpow(ll a, ll b) {
  if (b == 0)
    return 1;
  mint c = modpow(a, b / 2);
  if (b % 2 == 1)
    return c * c * a;
  else
    return c * c;
}

int main() {
  ll n, k;
  cin >> n >> k;
  mint ans = 0;
  vector<mint> B(k + 1);

  for (ll i = k; i >= 1; i--) {
    ll a = k / i;
    mint sum = modpow(a, n);
    for (ll p = i * 2; p <= k; p += i) {
      sum -= B[p];
    }
    B[i] = sum;
    mint j = i;
    ans += sum * j;
  }
  cout << ans << endl;

  return 0;
}