Problem p03956 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#define _USE_MATH_DEFINES
#include <complex>
#include <functional>
#include <math.h>
using namespace std;

#define rep(i, x) for (ll i = 0; i < x; i++)
#define repn(i, x) for (ll i = 1; i <= x; i++)

typedef long long ll;
const ll INF = 1e17;
const ll MOD = 1000000007;
const ll MAX = 4000001;
const long double eps = 1E-14;

ll max(ll a, ll b) {
  if (a > b) {
    return a;
  }
  return b;
}

ll min(ll a, ll b) {
  if (a > b) {
    return b;
  }
  return a;
}

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

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

struct edge {
  ll ind;
  ll fr;
  ll to;
  ll d;
};

class mint {
  long long x;

public:
  mint(long long 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 {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint &a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint &a) const {
    mint res(*this);
    return res *= 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 {
    mint res(*this);
    return res /= a;
  }

  friend ostream &operator<<(ostream &os, const mint &m) {
    os << m.x;
    return os;
  }
};

mint pw(mint a, ll b) {
  if (b == 0) {
    return 1;
  }
  mint ret = pw(a, b >> 1);
  ret *= ret;
  if (b & 1) {
    ret *= a;
  }
  return ret;
}

typedef vector<ll> vll;
typedef vector<vector<ll>> vvll;
typedef vector<vector<vector<ll>>> vvvll;

typedef vector<mint> vmint;
typedef vector<vector<mint>> vvmint;
typedef vector<vector<vector<mint>>> vvvmint;

/////////////////////////////////////
ll N, M;
vector<vector<edge>> g;

vector<ll> col;
vector<ll> see;

bool dfs(ll u) {
  bool ret = 1;

  for (edge e : g[u]) {
    ll w = e.to;

    if (col[w] == -1) {
      col[w] = (col[u] + 3 + e.d) % 3;
      bool b = dfs(w);
      if (!b) {
        ret = 0;
      }
    } else {
      if (col[w] != (col[u] + 3 + e.d) % 3) {
        ret = 0;
      }
    }
  }

  return ret;
}

vll count(ll u) {
  see[u] = 1;
  vll ret(4, 0);
  ret[col[u]]++;
  ret[3] = g[u].size();

  for (edge e : g[u]) {
    ll w = e.to;
    if (see[w] == 1) {
      continue;
    }
    vll a = count(w);
    rep(j, 4) ret[j] += a[j];
  }

  return ret;
}

int main() {
  cin >> N >> M;
  g.resize(N + 1);

  repn(i, M) {
    ll a, b;
    cin >> a >> b;
    g[a].push_back({i, a, b, 1});
    g[b].push_back({i, b, a, -1});
  }

  col.assign(N + 1, -1);
  see.assign(N + 1, 0);

  ll ans = 0;
  repn(v, N) {
    if (see[v] == 1) {
      continue;
    }
    bool b = dfs(v);
    vll a = count(v);

    if (!b) {
      ll s = a[0] + a[1] + a[2];
      ans += s * s;
    } else {
      if (a[0] == 0 || a[1] == 0 || a[2] == 0) {
        ans += a[3] / 2;
      } else {
        ans += a[0] * a[1] + a[1] * a[2] + a[2] * a[0];
      }
    }
  }

  cout << ans << endl;
}


Fixed submission:
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#define _USE_MATH_DEFINES
#include <complex>
#include <functional>
#include <math.h>
using namespace std;

#define rep(i, x) for (ll i = 0; i < x; i++)
#define repn(i, x) for (ll i = 1; i <= x; i++)

typedef long long ll;
const ll INF = 1e17;
const ll MOD = 1000000007;
const ll MAX = 4000001;
const long double eps = 1E-14;

ll max(ll a, ll b) {
  if (a > b) {
    return a;
  }
  return b;
}

ll min(ll a, ll b) {
  if (a > b) {
    return b;
  }
  return a;
}

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

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

struct edge {
  ll ind;
  ll fr;
  ll to;
  ll d;
};

class mint {
  long long x;

public:
  mint(long long 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 {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint &a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint &a) const {
    mint res(*this);
    return res *= 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 {
    mint res(*this);
    return res /= a;
  }

  friend ostream &operator<<(ostream &os, const mint &m) {
    os << m.x;
    return os;
  }
};

mint pw(mint a, ll b) {
  if (b == 0) {
    return 1;
  }
  mint ret = pw(a, b >> 1);
  ret *= ret;
  if (b & 1) {
    ret *= a;
  }
  return ret;
}

typedef vector<ll> vll;
typedef vector<vector<ll>> vvll;
typedef vector<vector<vector<ll>>> vvvll;

typedef vector<mint> vmint;
typedef vector<vector<mint>> vvmint;
typedef vector<vector<vector<mint>>> vvvmint;

/////////////////////////////////////
ll N, M;
vector<vector<edge>> g;

vector<ll> col;
vector<ll> see;

bool dfs(ll u) {
  bool ret = 1;

  for (edge e : g[u]) {
    ll w = e.to;

    if (col[w] == -1) {
      col[w] = (col[u] + 3 + e.d) % 3;
      bool b = dfs(w);
      if (!b) {
        ret = 0;
      }
    } else {
      if (col[w] != (col[u] + 3 + e.d) % 3) {
        ret = 0;
      }
    }
  }

  return ret;
}

vll count(ll u) {
  see[u] = 1;
  vll ret(4, 0);
  ret[col[u]]++;
  ret[3] = g[u].size();

  for (edge e : g[u]) {
    ll w = e.to;
    if (see[w] == 1) {
      continue;
    }
    vll a = count(w);
    rep(j, 4) ret[j] += a[j];
  }

  return ret;
}

int main() {
  cin >> N >> M;
  g.resize(N + 1);

  repn(i, M) {
    ll a, b;
    cin >> a >> b;
    g[a].push_back({i, a, b, 1});
    g[b].push_back({i, b, a, -1});
  }

  col.assign(N + 1, -1);
  see.assign(N + 1, 0);

  ll ans = 0;
  repn(v, N) {
    if (see[v] == 1) {
      continue;
    }
    col[v] = 0;
    bool b = dfs(v);
    vll a = count(v);

    if (!b) {
      ll s = a[0] + a[1] + a[2];
      ans += s * s;
    } else {
      if (a[0] == 0 || a[1] == 0 || a[2] == 0) {
        ans += a[3] / 2;
      } else {
        ans += a[0] * a[1] + a[1] * a[2] + a[2] * a[0];
      }
    }
  }

  cout << ans << endl;
}