Problem p02715 (C++)

Buggy submission (status: Runtime Error):
#include "bits/stdc++.h"

#define MOD 1000000007
#define rep(i, n) for (ll i = 0; i < (n); i++)
#define rrep(i, n) for (ll i = (n)-1; i >= 0; i--)
#define ALL(v) v.begin(), v.end()
#define rALL(v) v.rbegin(), v.rend()
#define FOR(i, j, k) for (ll i = j; i < k; i++)
#define DUMP(i, v)                                                             \
  for (ll i = 0; i < v.size(); i++)                                            \
  cerr << v[i] << " "

using namespace std;
typedef long long int ll;
typedef vector<ll> llvec;
typedef vector<double> dvec;
typedef pair<ll, ll> P;
typedef long double ld;
struct edge {
  ll x, c;
};

ll mod(ll a, ll mod) {
  ll res = a % mod;
  if (res < 0)
    res = res + mod;
  return res;
}

ll modpow(ll a, ll n, ll mod) {
  ll res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}

ll modinv(ll a, ll mod) { return modpow(a, mod - 2, mod); }

ll gcd(ll a, ll b) {
  ll r = a % b;
  if (r == 0)
    return b;
  else
    return gcd(b, a % b);
}

bool is_prime(ll n) {
  ll i = 2;
  if (n == 1)
    return false;
  if (n == 2)
    return true;
  bool res = true;
  while (i * i < n) {
    if (n % i == 0) {
      res = false;
    }
    i = i + 1;
  }

  // if(i==1)res = false;
  if (n % i == 0)
    res = false;
  return res;
}

struct UnionFind {
  ll N;
  llvec p;
  llvec cnt;
  UnionFind(ll n) {
    N = n;
    p = llvec(N);
    cnt = llvec(N, 0);
    rep(i, N) {
      p[i] = i;
      cnt[i] = 1;
    }
  }

  void con(ll a, ll b) {
    P at = root(a);
    P bt = root(b);
    if (at.second != bt.second) {
      if (at.first > bt.first) {
        swap(at, bt);
        swap(a, b);
      }
      p[at.second] = bt.second;
      cnt[bt.second] += cnt[at.second];
      p[a] = bt.second;
    }
  }

  P root(ll a) {
    ll atmp = a;
    ll c = 0;
    while (atmp != p[atmp]) {
      atmp = p[atmp];
      c++;
    }
    p[a] = atmp;
    return {c, atmp};
  }

  bool is_con(ll a, ll b) {
    P at = root(a);
    P bt = root(b);
    return at.second == bt.second;
  }
};

struct dijkstra {
  ll N;
  llvec d;
  vector<vector<edge>> e;
  dijkstra(ll n) {
    N = n;
    // d = llvec(N, 1e18);
    e = vector<vector<edge>>(N);
  }

  void add_edge(ll from, ll to, ll cost) { e[from].push_back({to, cost}); }

  void run(ll start) {
    priority_queue<P, vector<P>, greater<P>> que;
    que.push({0, start});
    d = llvec(N, 1e18);
    d[start] = 0;
    while (!que.empty()) {
      P q = que.top();
      que.pop();
      ll dc = q.first;
      ll x = q.second;
      if (dc > d[x]) {
        continue;
      } else {
        for (auto ip : e[x]) {
          if (d[ip.x] <= d[x] + ip.c) {
            continue;
          } else {
            d[ip.x] = d[x] + ip.c;
            que.push({d[ip.x], ip.x});
          }
        }
      }
    }
  }
};

/**************************************
** A main function starts from here  **
***************************************/
int main() {
  ll N, K;
  cin >> N >> K;
  ll ans = 0;
  llvec d(K, 0);
  ll cnt = 0;
  rrep(i, K + 1) {
    if (i == 1)
      break;
    ll num = K / i;
    d[i] = modpow(num, N, MOD);
    ll j = 2;
    while (i * j <= K) {
      d[i] = mod(d[i] - d[i * j], MOD);
      j++;
    }
    cnt = mod(cnt + d[i], MOD);
    ans = mod(ans + d[i] * i, MOD);
  }
  ans = mod(ans + mod(modpow(K, N, MOD) - cnt, MOD), MOD);
  cout << ans;
  return 0;
}


Fixed submission:
#include "bits/stdc++.h"

#define MOD 1000000007
#define rep(i, n) for (ll i = 0; i < (n); i++)
#define rrep(i, n) for (ll i = (n)-1; i >= 0; i--)
#define ALL(v) v.begin(), v.end()
#define rALL(v) v.rbegin(), v.rend()
#define FOR(i, j, k) for (ll i = j; i < k; i++)
#define DUMP(i, v)                                                             \
  for (ll i = 0; i < v.size(); i++)                                            \
  cerr << v[i] << " "

using namespace std;
typedef long long int ll;
typedef vector<ll> llvec;
typedef vector<double> dvec;
typedef pair<ll, ll> P;
typedef long double ld;
struct edge {
  ll x, c;
};

ll mod(ll a, ll mod) {
  ll res = a % mod;
  if (res < 0)
    res = res + mod;
  return res;
}

ll modpow(ll a, ll n, ll mod) {
  ll res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}

ll modinv(ll a, ll mod) { return modpow(a, mod - 2, mod); }

ll gcd(ll a, ll b) {
  ll r = a % b;
  if (r == 0)
    return b;
  else
    return gcd(b, a % b);
}

bool is_prime(ll n) {
  ll i = 2;
  if (n == 1)
    return false;
  if (n == 2)
    return true;
  bool res = true;
  while (i * i < n) {
    if (n % i == 0) {
      res = false;
    }
    i = i + 1;
  }

  // if(i==1)res = false;
  if (n % i == 0)
    res = false;
  return res;
}

struct UnionFind {
  ll N;
  llvec p;
  llvec cnt;
  UnionFind(ll n) {
    N = n;
    p = llvec(N);
    cnt = llvec(N, 0);
    rep(i, N) {
      p[i] = i;
      cnt[i] = 1;
    }
  }

  void con(ll a, ll b) {
    P at = root(a);
    P bt = root(b);
    if (at.second != bt.second) {
      if (at.first > bt.first) {
        swap(at, bt);
        swap(a, b);
      }
      p[at.second] = bt.second;
      cnt[bt.second] += cnt[at.second];
      p[a] = bt.second;
    }
  }

  P root(ll a) {
    ll atmp = a;
    ll c = 0;
    while (atmp != p[atmp]) {
      atmp = p[atmp];
      c++;
    }
    p[a] = atmp;
    return {c, atmp};
  }

  bool is_con(ll a, ll b) {
    P at = root(a);
    P bt = root(b);
    return at.second == bt.second;
  }
};

struct dijkstra {
  ll N;
  llvec d;
  vector<vector<edge>> e;
  dijkstra(ll n) {
    N = n;
    // d = llvec(N, 1e18);
    e = vector<vector<edge>>(N);
  }

  void add_edge(ll from, ll to, ll cost) { e[from].push_back({to, cost}); }

  void run(ll start) {
    priority_queue<P, vector<P>, greater<P>> que;
    que.push({0, start});
    d = llvec(N, 1e18);
    d[start] = 0;
    while (!que.empty()) {
      P q = que.top();
      que.pop();
      ll dc = q.first;
      ll x = q.second;
      if (dc > d[x]) {
        continue;
      } else {
        for (auto ip : e[x]) {
          if (d[ip.x] <= d[x] + ip.c) {
            continue;
          } else {
            d[ip.x] = d[x] + ip.c;
            que.push({d[ip.x], ip.x});
          }
        }
      }
    }
  }
};

/**************************************
** A main function starts from here  **
***************************************/
int main() {
  ll N, K;
  cin >> N >> K;
  ll ans = 0;
  llvec d(K + 1, 0);
  ll cnt = 0;
  rrep(i, K + 1) {
    if (i == 1)
      break;
    ll num = K / i;
    d[i] = modpow(num, N, MOD);
    ll j = 2;
    while (i * j <= K) {
      d[i] = mod(d[i] - d[i * j], MOD);
      j++;
    }
    cnt = mod(cnt + d[i], MOD);
    ans = mod(ans + d[i] * i, MOD);
  }
  ans = mod(ans + mod(modpow(K, N, MOD) - cnt, MOD), MOD);
  cout << ans;
  return 0;
}