Problem p02573 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
using namespace std;
// #pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math,O3")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
// using namespace __gnu_pbds;
#define fastio()                                                               \
  ios_base::sync_with_stdio(0);                                                \
  cin.tie(0);                                                                  \
  cout.tie(0)
#define pb push_back
#define show(x) cout << (#x) << " : " << x << endl;
#define ll long long
#define ld long double
#define fill(a, val) memset(a, val, sizeof(a))
#define mp make_pair
#define ff first
#define ss second
#define pii pair<ll, ll>
#define sq(x) ((x) * (x))
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
#define endl "\n"
#define int long long
#define printclock                                                             \
  cerr << "Time : " << 1000 * (ld)clock() / (ld)CLOCKS_PER_SEC << "ms\n";
const ll MOD = 1000 * 1000 * 1000 + 7;
const ll INF = 1ll * 1000 * 1000 * 1000 * 1000 * 1000 * 1000 + 7;
const ll MOD2 = 998244353;
const ll N = 1000 * 100 + 10;
const ll N2 = 70;
const ld PI = 3.141592653589793;
// template<class T> using oset=tree<T, null_type, less<T>, rb_tree_tag,
// tree_order_statistics_node_update>;
ll gcd(ll a, ll b) {
  if (!b)
    return a;
  return gcd(b, a % b);
}
ll power(ll x, ll y, ll p = LLONG_MAX) {
  ll res = 1;
  x %= p;
  while (y > 0) {
    if (y & 1)
      res = (res * x) % p;
    y = y >> 1;
    x = (x * x) % p;
  }
  return res;
}
ll lcm(ll a, ll b) { return (a * b) / gcd(a, b); }
int a[N], si[N];
void initialize(int n) {
  for (int i = 0; i < n; ++i) {
    a[i] = i;
    si[i] = 1;
  }
}
int root(int i) {
  while (a[i] != i)
    i = a[i];
  return i;
}
bool find(int A, int B) { return (root(A) == root(B)); }
void weighted_union(int A, int B) {
  int root_A = root(A);
  int root_B = root(B);
  if (si[root_A] < si[root_B]) {
    a[root_A] = a[root_B];
    si[root_B] += si[root_A];
  } else {
    a[root_B] = a[root_A];
    si[root_A] += si[root_B];
  }
}
signed main() {
  fastio();
// cout<<fixed<<setprecision(20);
// CHECK for LONG LONG and LONG DOUBLE
//*comment for all except cc/cf
#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif //*/
  int n, m;
  cin >> n >> m;
  initialize(n);
  set<pii> s;
  for (int i = 0; i < m; ++i) {
    int x, y;
    cin >> x >> y;
    x--;
    y--;
    s.insert({max(x, y), min(x, y)});
  }
  for (auto x : s) {
    if (!find(x.ff, x.ss))
      weighted_union(x.ff, x.ss);
  }
  int ans = 1;
  for (int i = 0; i < n; ++i) {
    ans = max(ans, si[root(i)]);
  }
  cout << ans;

  printclock;
  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
using namespace std;
// #pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math,O3")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
// using namespace __gnu_pbds;
#define fastio()                                                               \
  ios_base::sync_with_stdio(0);                                                \
  cin.tie(0);                                                                  \
  cout.tie(0)
#define pb push_back
#define show(x) cout << (#x) << " : " << x << endl;
#define ll long long
#define ld long double
#define fill(a, val) memset(a, val, sizeof(a))
#define mp make_pair
#define ff first
#define ss second
#define pii pair<ll, ll>
#define sq(x) ((x) * (x))
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
#define endl "\n"
#define int long long
#define printclock                                                             \
  cerr << "Time : " << 1000 * (ld)clock() / (ld)CLOCKS_PER_SEC << "ms\n";
const ll MOD = 1000 * 1000 * 1000 + 7;
const ll INF = 1ll * 1000 * 1000 * 1000 * 1000 * 1000 * 1000 + 7;
const ll MOD2 = 998244353;
const ll N = 2000 * 100 + 10;
const ll N2 = 70;
const ld PI = 3.141592653589793;
// template<class T> using oset=tree<T, null_type, less<T>, rb_tree_tag,
// tree_order_statistics_node_update>;
ll gcd(ll a, ll b) {
  if (!b)
    return a;
  return gcd(b, a % b);
}
ll power(ll x, ll y, ll p = LLONG_MAX) {
  ll res = 1;
  x %= p;
  while (y > 0) {
    if (y & 1)
      res = (res * x) % p;
    y = y >> 1;
    x = (x * x) % p;
  }
  return res;
}
ll lcm(ll a, ll b) { return (a * b) / gcd(a, b); }
int a[N], si[N];
void initialize(int n) {
  for (int i = 0; i < n; ++i) {
    a[i] = i;
    si[i] = 1;
  }
}
int root(int i) {
  while (a[i] != i)
    i = a[i];
  return i;
}
bool find(int A, int B) { return (root(A) == root(B)); }
void weighted_union(int A, int B) {
  int root_A = root(A);
  int root_B = root(B);
  if (si[root_A] < si[root_B]) {
    a[root_A] = a[root_B];
    si[root_B] += si[root_A];
  } else {
    a[root_B] = a[root_A];
    si[root_A] += si[root_B];
  }
}
signed main() {
  fastio();
// cout<<fixed<<setprecision(20);
// CHECK for LONG LONG and LONG DOUBLE
//*comment for all except cc/cf
#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif //*/
  int n, m;
  cin >> n >> m;
  initialize(n);
  set<pii> s;
  for (int i = 0; i < m; ++i) {
    int x, y;
    cin >> x >> y;
    x--;
    y--;
    s.insert({max(x, y), min(x, y)});
  }
  for (auto x : s) {
    if (!find(x.ff, x.ss))
      weighted_union(x.ff, x.ss);
  }
  int ans = 1;
  for (int i = 0; i < n; ++i) {
    ans = max(ans, si[root(i)]);
  }
  cout << ans;

  printclock;
  return 0;
}