Problem p02573 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
// #include<atcoder/all>
using namespace std;
// using namespace atcoder;
#define rep(i, n) for (ll i = 0; i < n; i++)
#define repl(i, l, r) for (ll i = (l); i < (r); i++)
#define per(i, n) for (ll i = n - 1; i >= 0; i--)
#define perl(i, r, l) for (ll i = r - 1; i >= l; i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define all(x) (x).begin(), (x).end()
using ll = long long;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
const ll MOD = 1000000007;
const ll MOD9 = 998244353;
const int inf = 1e9 + 10;
const ll INF = 4e18;
const ll dy[8] = {1, 0, -1, 0, 1, 1, -1, -1};
const ll dx[8] = {0, -1, 0, 1, 1, -1, 1, -1};
using Graph = vector<vector<int>>;

// dijkstra　
struct edge {
  ll to, cost;
};
typedef pair<ll, ll> P;
struct graph {
  ll V;
  vector<vector<edge>> G;
  vector<ll> d;

  graph(ll n) { init(n); }

  void init(ll n) {
    V = n;
    G.resize(V);
    d.resize(V);
    rep(i, V) { d[i] = INF; }
  }

  void add_edge(ll s, ll t, ll cost) {
    edge e;
    e.to = t, e.cost = cost;
    G[s].push_back(e);
  }

  void dijkstra(ll s) {
    rep(i, V) { d[i] = INF; }
    d[s] = 0;
    priority_queue<P, vector<P>, greater<P>> que;
    que.push(P(0, s));
    while (!que.empty()) {
      P p = que.top();
      que.pop();
      ll v = p.second;
      if (d[v] < p.first)
        continue;
      for (auto e : G[v]) {
        if (d[e.to] > d[v] + e.cost) {
          d[e.to] = d[v] + e.cost;
          que.push(P(d[e.to], e.to));
        }
      }
    }
  }
};
// dijkstra end

class UnionFind {
public:
  ll par[100005];
  ll depth[100005];
  ll nGroup[100005];

  UnionFind(ll n) { init(n); }

  void init(ll n) {
    for (ll i = 0; i < n; i++) {
      par[i] = i;
      depth[i] = 0;
      nGroup[i] = 1;
    }
  }

  ll root(ll x) {
    if (par[x] == x) {
      return x;
    } else {
      return par[x] = root(par[x]);
    }
  }

  bool same(ll x, ll y) { return root(x) == root(y); }

  void unite(ll x, ll y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return;

    if (depth[x] < depth[y]) {
      par[x] = y;
      nGroup[y] += nGroup[x];
      nGroup[x] = 0;
    } else {
      par[y] = x;
      nGroup[x] += nGroup[y];
      nGroup[y] = 0;
      if (depth[x] == depth[y])
        depth[x]++;
    }
  }
};
// unionfind end

// nCr
const ll MAX = 500010;
ll fac[MAX], finv[MAX], inv[MAX];

void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (ll i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

// nCr end

// tree DP
vector<ll> depth;
vector<ll> f;
vector<ll> g;
void dfs(const Graph &G, ll v, ll p, ll d) {
  depth[v] = d;
  for (auto nv : G[v]) {
    if (nv == p)
      continue;
    dfs(G, nv, v, d + 1);
  }

  f[v] = 1;
  g[v] = 1;
  for (auto c : G[v]) {
    if (c == p)
      continue;
    f[v] *= g[c];
    f[v] %= MOD;
    g[v] *= f[c];
    g[v] %= MOD;
  }
  f[v] += g[v];
  f[v] %= MOD;
}
// tree DP end

template <ll MOD> struct Fp {
  ll val;
  constexpr Fp(ll v = 0) noexcept : val(v % MOD) {
    if (val < 0)
      val += MOD;
  }
  constexpr ll getmod() { return MOD; }
  constexpr Fp operator-() const noexcept { return val ? MOD - val : 0; }
  constexpr Fp operator+(const Fp &r) const noexcept { return Fp(*this) += r; }
  constexpr Fp operator-(const Fp &r) const noexcept { return Fp(*this) -= r; }
  constexpr Fp operator*(const Fp &r) const noexcept { return Fp(*this) *= r; }
  constexpr Fp operator/(const Fp &r) const noexcept { return Fp(*this) /= r; }
  constexpr Fp &operator+=(const Fp &r) noexcept {
    val += r.val;
    if (val >= MOD)
      val -= MOD;
    return *this;
  }
  constexpr Fp &operator-=(const Fp &r) noexcept {
    val -= r.val;
    if (val < 0)
      val += MOD;
    return *this;
  }
  constexpr Fp &operator*=(const Fp &r) noexcept {
    val = val * r.val % MOD;
    return *this;
  }
  constexpr Fp &operator/=(const Fp &r) noexcept {
    ll a = r.val, b = MOD, u = 1, v = 0;
    while (b) {
      ll t = a / b;
      a -= t * b;
      swap(a, b);
      u -= t * v;
      swap(u, v);
    }
    val = val * u % MOD;
    if (val < 0)
      val += MOD;
    return *this;
  }
  constexpr bool operator==(const Fp &r) const noexcept {
    return this->val == r.val;
  }
  constexpr bool operator!=(const Fp &r) const noexcept {
    return this->val != r.val;
  }
  friend constexpr ostream &operator<<(ostream &os, const Fp<MOD> &x) noexcept {
    return os << x.val;
  }
  friend constexpr Fp<MOD> modpow(const Fp<MOD> &a, ll n) noexcept {
    if (n == 0)
      return 1;
    auto t = modpow(a, n / 2);
    t = t * t;
    if (n & 1)
      t = t * a;
    return t;
  }
};
using mint = Fp<MOD>;

mint calc(ll N, ll K) {
  mint res = 1;
  for (ll n = 0; n < K; ++n) {
    res *= (N - n);
    res /= (n + 1);
  }
  return res;
}
mint COM(ll n, ll k) {
  if (k == 0)
    return 1;
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

int main() {
  ll n, m;
  cin >> n >> m;
  UnionFind tree(n);
  for (ll i = 0; i < m; i++) {
    ll a, b;
    cin >> a >> b;
    a--;
    b--;
    tree.unite(a, b);
  }
  ll k = 0;
  rep(i, n) { k = max(k, tree.nGroup[i]); }
  cout << k << endl;
}

Fixed submission:
#include <bits/stdc++.h>
// #include<atcoder/all>
using namespace std;
// using namespace atcoder;
#define rep(i, n) for (ll i = 0; i < n; i++)
#define repl(i, l, r) for (ll i = (l); i < (r); i++)
#define per(i, n) for (ll i = n - 1; i >= 0; i--)
#define perl(i, r, l) for (ll i = r - 1; i >= l; i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define all(x) (x).begin(), (x).end()
using ll = long long;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
const ll MOD = 1000000007;
const ll MOD9 = 998244353;
const int inf = 1e9 + 10;
const ll INF = 4e18;
const ll dy[8] = {1, 0, -1, 0, 1, 1, -1, -1};
const ll dx[8] = {0, -1, 0, 1, 1, -1, 1, -1};
using Graph = vector<vector<int>>;

// dijkstra　
struct edge {
  ll to, cost;
};
typedef pair<ll, ll> P;
struct graph {
  ll V;
  vector<vector<edge>> G;
  vector<ll> d;

  graph(ll n) { init(n); }

  void init(ll n) {
    V = n;
    G.resize(V);
    d.resize(V);
    rep(i, V) { d[i] = INF; }
  }

  void add_edge(ll s, ll t, ll cost) {
    edge e;
    e.to = t, e.cost = cost;
    G[s].push_back(e);
  }

  void dijkstra(ll s) {
    rep(i, V) { d[i] = INF; }
    d[s] = 0;
    priority_queue<P, vector<P>, greater<P>> que;
    que.push(P(0, s));
    while (!que.empty()) {
      P p = que.top();
      que.pop();
      ll v = p.second;
      if (d[v] < p.first)
        continue;
      for (auto e : G[v]) {
        if (d[e.to] > d[v] + e.cost) {
          d[e.to] = d[v] + e.cost;
          que.push(P(d[e.to], e.to));
        }
      }
    }
  }
};
// dijkstra end

class UnionFind {
public:
  ll par[200005];
  ll depth[200005];
  ll nGroup[200005];

  UnionFind(ll n) { init(n); }

  void init(ll n) {
    for (ll i = 0; i < n; i++) {
      par[i] = i;
      depth[i] = 0;
      nGroup[i] = 1;
    }
  }

  ll root(ll x) {
    if (par[x] == x) {
      return x;
    } else {
      return par[x] = root(par[x]);
    }
  }

  bool same(ll x, ll y) { return root(x) == root(y); }

  void unite(ll x, ll y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return;

    if (depth[x] < depth[y]) {
      par[x] = y;
      nGroup[y] += nGroup[x];
      nGroup[x] = 0;
    } else {
      par[y] = x;
      nGroup[x] += nGroup[y];
      nGroup[y] = 0;
      if (depth[x] == depth[y])
        depth[x]++;
    }
  }
};
// unionfind end

// nCr
const ll MAX = 500010;
ll fac[MAX], finv[MAX], inv[MAX];

void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (ll i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

// nCr end

// tree DP
vector<ll> depth;
vector<ll> f;
vector<ll> g;
void dfs(const Graph &G, ll v, ll p, ll d) {
  depth[v] = d;
  for (auto nv : G[v]) {
    if (nv == p)
      continue;
    dfs(G, nv, v, d + 1);
  }

  f[v] = 1;
  g[v] = 1;
  for (auto c : G[v]) {
    if (c == p)
      continue;
    f[v] *= g[c];
    f[v] %= MOD;
    g[v] *= f[c];
    g[v] %= MOD;
  }
  f[v] += g[v];
  f[v] %= MOD;
}
// tree DP end

template <ll MOD> struct Fp {
  ll val;
  constexpr Fp(ll v = 0) noexcept : val(v % MOD) {
    if (val < 0)
      val += MOD;
  }
  constexpr ll getmod() { return MOD; }
  constexpr Fp operator-() const noexcept { return val ? MOD - val : 0; }
  constexpr Fp operator+(const Fp &r) const noexcept { return Fp(*this) += r; }
  constexpr Fp operator-(const Fp &r) const noexcept { return Fp(*this) -= r; }
  constexpr Fp operator*(const Fp &r) const noexcept { return Fp(*this) *= r; }
  constexpr Fp operator/(const Fp &r) const noexcept { return Fp(*this) /= r; }
  constexpr Fp &operator+=(const Fp &r) noexcept {
    val += r.val;
    if (val >= MOD)
      val -= MOD;
    return *this;
  }
  constexpr Fp &operator-=(const Fp &r) noexcept {
    val -= r.val;
    if (val < 0)
      val += MOD;
    return *this;
  }
  constexpr Fp &operator*=(const Fp &r) noexcept {
    val = val * r.val % MOD;
    return *this;
  }
  constexpr Fp &operator/=(const Fp &r) noexcept {
    ll a = r.val, b = MOD, u = 1, v = 0;
    while (b) {
      ll t = a / b;
      a -= t * b;
      swap(a, b);
      u -= t * v;
      swap(u, v);
    }
    val = val * u % MOD;
    if (val < 0)
      val += MOD;
    return *this;
  }
  constexpr bool operator==(const Fp &r) const noexcept {
    return this->val == r.val;
  }
  constexpr bool operator!=(const Fp &r) const noexcept {
    return this->val != r.val;
  }
  friend constexpr ostream &operator<<(ostream &os, const Fp<MOD> &x) noexcept {
    return os << x.val;
  }
  friend constexpr Fp<MOD> modpow(const Fp<MOD> &a, ll n) noexcept {
    if (n == 0)
      return 1;
    auto t = modpow(a, n / 2);
    t = t * t;
    if (n & 1)
      t = t * a;
    return t;
  }
};
using mint = Fp<MOD>;

mint calc(ll N, ll K) {
  mint res = 1;
  for (ll n = 0; n < K; ++n) {
    res *= (N - n);
    res /= (n + 1);
  }
  return res;
}
mint COM(ll n, ll k) {
  if (k == 0)
    return 1;
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

int main() {
  ll n, m;
  cin >> n >> m;
  UnionFind tree(n);
  for (ll i = 0; i < m; i++) {
    ll a, b;
    cin >> a >> b;
    a--;
    b--;
    tree.unite(a, b);
  }
  ll k = 0;
  rep(i, n) { k = max(k, tree.nGroup[i]); }
  cout << k << endl;
}