Problem p02715 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>

using namespace std;
using llong = long long int;

#define rep(i, n) for (int i = 0; i < n; ++i)
#define stl_rep(itr, x) for (auto itr = x.begin(); itr != x.end(); ++itr)
#define all(x) x.begin(), x.end()
#define allr(x) x.rbegin(), x.rend()

const static int MOD = 1000000007;
const static int INF = 1000000000;
const static int dx[4] = {1, 0, -1, 0};
const static int dy[4] = {0, 1, 0, -1};

/* --------------- template start --------------- */
template <class Abel> struct BinaryIndexTree {
  vector<Abel> dat[2];
  Abel UNITY_SUM = 0; // to be set

  /* [1, n] */
  BinaryIndexTree(int n) { init(n); }
  void init(int n) {
    for (int iter = 0; iter < 2; ++iter)
      dat[iter].assign(n + 1, UNITY_SUM);
  }

  /* a, b are 1-indexed, [a, b) */
  inline void sub_add(int p, int a, Abel x) {
    for (int i = a; i < (int)dat[p].size(); i += i & -i)
      dat[p][i] = dat[p][i] + x;
  }
  inline void add(int a, int b, Abel x) {
    sub_add(0, a, x * -(a - 1));
    sub_add(1, a, x);
    sub_add(0, b, x * (b - 1));
    sub_add(1, b, x * (-1));
  }

  /* a is 1-indexed, [a, b) */
  inline Abel sub_sum(int p, int a) {
    Abel res = UNITY_SUM;
    for (int i = a; i > 0; i -= i & -i)
      res = res + dat[p][i];
    return res;
  }
  inline Abel sum(int a, int b) {
    return sub_sum(0, b - 1) + sub_sum(1, b - 1) * (b - 1) - sub_sum(0, a - 1) -
           sub_sum(1, a - 1) * (a - 1);
  }

  /* debug */
  void print() {
    for (int i = 1; i < (int)dat[0].size(); ++i)
      cout << sum(i, i + 1) << ",";
    cout << endl;
  }
};

template <typename T> struct UnionFind {
  vector<int> par;
  vector<T> sz;

  UnionFind(int n) {
    par.resize(n);
    sz.resize(n);
    for (int i = 0; i < n; ++i) {
      par[i] = i;
      sz[i] = 1;
    }
  }

  int find_root(int x) { return par[x] == x ? x : par[x] = find_root(par[x]); }

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

  void unite(int x, int y) {
    x = find_root(x);
    y = find_root(y);
    if (x == y)
      return;

    if (sz[x] < sz[y])
      swap(x, y);
    sz[x] += sz[y];
    par[y] = x;
  }

  T size_of_tree(int x) { return sz[find_root(x)]; }
};
/* --------------- template end --------------- */

/* --------------- function start --------------- */
llong my_gcd(llong a, llong b) { return (a ? my_gcd(b % a, a) : b); }
llong my_lcm(llong a, llong b) { return a * b / my_gcd(a, b); }

llong facmod(llong n) {
  if (n == 0)
    return 1;

  llong res = n;
  for (int i = n - 1; i > 0; --i) {
    res = (res * i) % MOD;
  }

  return res;
}
llong powmod(llong a, llong n) {
  llong res = 1;
  while (n > 0) {
    if (n & 1)
      res = (res * a) % MOD;
    a = (a * a) % MOD;
    n >>= 1;
  }

  return res;
}
llong modinv(llong n) { return powmod(n, MOD - 2); }
llong combmod(llong n, llong k) {
  llong res = 1;
  for (int i = 0; i < k; ++i) {
    res = (res * (n - i)) % MOD;
  }
  res = (res * modinv(facmod(k))) % MOD;

  return res;
}

vector<llong> divisors(llong n) {
  vector<llong> ret;
  for (llong i = 1; i * i <= n; i++) {
    if (n % i == 0) {
      ret.push_back(i);
      if ((i * i) != n)
        ret.push_back(n / i);
    }
  }
  sort(begin(ret), end(ret));

  return ret;
}
/* --------------- function end --------------- */

int main(int argc, char *argv[]) {
  cin.tie(0);
  ios::sync_with_stdio(false);

  llong n, k;
  cin >> n >> k;
  vector<llong> nums(k + 1, 0);
  llong ans = 0;
  for (llong x = k; x >= 1LL; --x) {
    // cout << x << " " << ans << endl;
    nums[x] = powmod(k / x, n);
    for (llong xmul = 2LL * x; xmul <= k; xmul += x)
      nums[x] = (nums[x] - nums[xmul] + MOD) % MOD;
    ans += nums[x] * x;
    ans %= MOD;
  }

  cout << ans << endl;

  return 0;
}

Error: 0

Fixed submission:

#include <bits/stdc++.h>

using namespace std;
using llong = long long int;

#define rep(i, n) for (int i = 0; i < n; ++i)
#define stl_rep(itr, x) for (auto itr = x.begin(); itr != x.end(); ++itr)
#define all(x) x.begin(), x.end()
#define allr(x) x.rbegin(), x.rend()

const static int MOD = 1000000007;
const static int INF = 1000000000;
const static int dx[4] = {1, 0, -1, 0};
const static int dy[4] = {0, 1, 0, -1};

/* --------------- template start --------------- */
template <class Abel> struct BinaryIndexTree {
  vector<Abel> dat[2];
  Abel UNITY_SUM = 0; // to be set

  /* [1, n] */
  BinaryIndexTree(int n) { init(n); }
  void init(int n) {
    for (int iter = 0; iter < 2; ++iter)
      dat[iter].assign(n + 1, UNITY_SUM);
  }

  /* a, b are 1-indexed, [a, b) */
  inline void sub_add(int p, int a, Abel x) {
    for (int i = a; i < (int)dat[p].size(); i += i & -i)
      dat[p][i] = dat[p][i] + x;
  }
  inline void add(int a, int b, Abel x) {
    sub_add(0, a, x * -(a - 1));
    sub_add(1, a, x);
    sub_add(0, b, x * (b - 1));
    sub_add(1, b, x * (-1));
  }

  /* a is 1-indexed, [a, b) */
  inline Abel sub_sum(int p, int a) {
    Abel res = UNITY_SUM;
    for (int i = a; i > 0; i -= i & -i)
      res = res + dat[p][i];
    return res;
  }
  inline Abel sum(int a, int b) {
    return sub_sum(0, b - 1) + sub_sum(1, b - 1) * (b - 1) - sub_sum(0, a - 1) -
           sub_sum(1, a - 1) * (a - 1);
  }

  /* debug */
  void print() {
    for (int i = 1; i < (int)dat[0].size(); ++i)
      cout << sum(i, i + 1) << ",";
    cout << endl;
  }
};

template <typename T> struct UnionFind {
  vector<int> par;
  vector<T> sz;

  UnionFind(int n) {
    par.resize(n);
    sz.resize(n);
    for (int i = 0; i < n; ++i) {
      par[i] = i;
      sz[i] = 1;
    }
  }

  int find_root(int x) { return par[x] == x ? x : par[x] = find_root(par[x]); }

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

  void unite(int x, int y) {
    x = find_root(x);
    y = find_root(y);
    if (x == y)
      return;

    if (sz[x] < sz[y])
      swap(x, y);
    sz[x] += sz[y];
    par[y] = x;
  }

  T size_of_tree(int x) { return sz[find_root(x)]; }
};
/* --------------- template end --------------- */

/* --------------- function start --------------- */
llong my_gcd(llong a, llong b) { return (a ? my_gcd(b % a, a) : b); }
llong my_lcm(llong a, llong b) { return a * b / my_gcd(a, b); }

llong facmod(llong n) {
  if (n == 0)
    return 1;

  llong res = n;
  for (int i = n - 1; i > 0; --i) {
    res = (res * i) % MOD;
  }

  return res;
}
llong powmod(llong a, llong n) {
  llong res = 1;
  while (n > 0) {
    if (n & 1)
      res = (res * a) % MOD;
    a = (a * a) % MOD;
    n >>= 1;
  }

  return res;
}
llong modinv(llong n) { return powmod(n, MOD - 2); }
llong combmod(llong n, llong k) {
  llong res = 1;
  for (int i = 0; i < k; ++i) {
    res = (res * (n - i)) % MOD;
  }
  res = (res * modinv(facmod(k))) % MOD;

  return res;
}

vector<llong> divisors(llong n) {
  vector<llong> ret;
  for (llong i = 1; i * i <= n; i++) {
    if (n % i == 0) {
      ret.push_back(i);
      if ((i * i) != n)
        ret.push_back(n / i);
    }
  }
  sort(begin(ret), end(ret));

  return ret;
}
/* --------------- function end --------------- */

int main(int argc, char *argv[]) {
  cin.tie(0);
  ios::sync_with_stdio(false);

  llong n, k;
  cin >> n >> k;
  vector<llong> nums(n + 1);
  nums[0] = 0;
  for (llong x = 1; x <= k; ++x)
    nums[x] = powmod(k / x, n);

  llong ans = 0;
  for (llong x = 1; x <= k; ++x) {
    llong xmul = 2 * x;
    while (xmul <= k) {
      nums[x] -= nums[xmul];
      xmul += x;
    }
    ans += nums[x] * x;
    ans %= MOD;
  }

  cout << ans << endl;

  return 0;
}