Problem p02573 (C++)

Buggy submission (status: Runtime Error):
/// Bismillahir Rahmanir Rahim
/* Mohammad Morsalin
   Dept of ICE, NSTU
*/
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
#define ll long long
#define pb push_back
#define mp make_pair
#define endl "\n"
#define int long long
#define f0(n) for (int i = 0; i < n; i++)
#define ms(x) memset(x, 0, sizeof(x))
#define ms2d(x, m, n) memset(x, 0, sizeof(x[0][0]) * m * n)
#define uniq(vec)                                                              \
  vec.resize(distance(vec.begin(), unique(vec.begin(), vec.end())))
#define ordered_set                                                            \
  tree<int, null_type, less<int>, rb_tree_tag,                                 \
       tree_order_statistics_node_update>
#define pi pair<int, int>
#define tc(t)                                                                  \
  int t;                                                                       \
  cin >> t;                                                                    \
  while (t--)
#define bits(n) __builtin_popcount(n)
#define maxpq priority_queue<int>
#define minpq priority_queue<int, vector<int>, greater<int>>
#define ins insert
#define ALL(v) v.begin(), v.end()
#define highest(x) numeric_limits<x>::max()
#define lowest(x) numeric_limits<x>::min()
#define Inf INFINITY
#define minv(v) *min_element(v.begin(), v.end())
#define maxv(v) *max_element(v.begin(), v.end())
#define fi first
#define se second
#define PI acos(-1)
#define sz(a) (int)a.size();
#define IOS ios::sync_with_stdio(false);
using namespace std;
int gcd(int a, int b) {
  if (a == 0)
    return b;
  return gcd(b % a, a);
}
typedef tree<pair<int, int>, null_type, less<pair<int, int>>, rb_tree_tag,
             tree_order_statistics_node_update>
    ordered_multiset;
int dx8[] = {0, 0, 1, 1, 1, -1, -1, -1};
int dy8[] = {1, -1, 1, -1, 0, 0, -1, 1};
int dx4[] = {0, 0, 1, -1};
int dy4[] = {1, -1, 0, 0};
const long long MOD = 1000000007;
double sq(double x) { return x * x; }
template <typename T> inline T Bigmod(T base, T power, T MOD) {
  T ret = 1;
  while (power) {
    if (power & 1)
      ret = (ret * base) % MOD;
    base = (base * base) % MOD;
    power >>= 1;
  }
  return ret;
}

bool sortinrev(const pair<int, int> &a, const pair<int, int> &b) {
  return (a.first > b.first);
}
int arr[100005];
int Size[100005];
int n, m;
void init() {
  for (int i = 0; i <= n; i++) {
    arr[i] = i;
    Size[i] = 1;
  }
}
int root(int i) {
  while (arr[i] != i) {
    i = arr[arr[i]];
    i = arr[i];
  }
  return i;
}
void w_union(int a, int b) {
  int root_a = root(a);
  int root_b = root(b);
  if (root_a == root_b)
    return;
  if (Size[root_a] < Size[root_b]) {
    arr[root_a] = arr[root_b];
    Size[root_b] += Size[root_a];
  } else {
    arr[root_b] = arr[root_a];
    Size[root_a] += Size[root_b];
  }
}
signed main() {
  IOS cin >> n >> m;
  init();
  for (int i = 0; i < m; i++) {
    int a, b;
    cin >> a >> b;
    w_union(a, b);
  }
  int maxx = 0;
  for (int i = 1; i <= n; i++) {
    int rooti = root(i);
    maxx = max(maxx, (ll)Size[rooti]);
  }
  cout << maxx << endl;
  return 0;
}
/// Alhamdulillah


Fixed submission:
/// Bismillahir Rahmanir Rahim
/* Mohammad Morsalin
   Dept of ICE, NSTU
*/
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
#define ll long long
#define pb push_back
#define mp make_pair
#define endl "\n"
#define int long long
#define f0(n) for (int i = 0; i < n; i++)
#define ms(x) memset(x, 0, sizeof(x))
#define ms2d(x, m, n) memset(x, 0, sizeof(x[0][0]) * m * n)
#define uniq(vec)                                                              \
  vec.resize(distance(vec.begin(), unique(vec.begin(), vec.end())))
#define ordered_set                                                            \
  tree<int, null_type, less<int>, rb_tree_tag,                                 \
       tree_order_statistics_node_update>
#define pi pair<int, int>
#define tc(t)                                                                  \
  int t;                                                                       \
  cin >> t;                                                                    \
  while (t--)
#define bits(n) __builtin_popcount(n)
#define maxpq priority_queue<int>
#define minpq priority_queue<int, vector<int>, greater<int>>
#define ins insert
#define ALL(v) v.begin(), v.end()
#define highest(x) numeric_limits<x>::max()
#define lowest(x) numeric_limits<x>::min()
#define Inf INFINITY
#define minv(v) *min_element(v.begin(), v.end())
#define maxv(v) *max_element(v.begin(), v.end())
#define fi first
#define se second
#define PI acos(-1)
#define sz(a) (int)a.size();
#define IOS ios::sync_with_stdio(false);
using namespace std;
int gcd(int a, int b) {
  if (a == 0)
    return b;
  return gcd(b % a, a);
}
typedef tree<pair<int, int>, null_type, less<pair<int, int>>, rb_tree_tag,
             tree_order_statistics_node_update>
    ordered_multiset;
int dx8[] = {0, 0, 1, 1, 1, -1, -1, -1};
int dy8[] = {1, -1, 1, -1, 0, 0, -1, 1};
int dx4[] = {0, 0, 1, -1};
int dy4[] = {1, -1, 0, 0};
const long long MOD = 1000000007;
double sq(double x) { return x * x; }
template <typename T> inline T Bigmod(T base, T power, T MOD) {
  T ret = 1;
  while (power) {
    if (power & 1)
      ret = (ret * base) % MOD;
    base = (base * base) % MOD;
    power >>= 1;
  }
  return ret;
}

bool sortinrev(const pair<int, int> &a, const pair<int, int> &b) {
  return (a.first > b.first);
}
int arr[200005];
int Size[200005];
int n, m;
void init() {
  for (int i = 0; i <= n; i++) {
    arr[i] = i;
    Size[i] = 1;
  }
}
int root(int i) {
  while (arr[i] != i) {
    i = arr[arr[i]];
    i = arr[i];
  }
  return i;
}
void w_union(int a, int b) {
  int root_a = root(a);
  int root_b = root(b);
  if (root_a == root_b)
    return;
  if (Size[root_a] < Size[root_b]) {
    arr[root_a] = arr[root_b];
    Size[root_b] += Size[root_a];
  } else {
    arr[root_b] = arr[root_a];
    Size[root_a] += Size[root_b];
  }
}
signed main() {
  IOS cin >> n >> m;
  init();
  for (int i = 0; i < m; i++) {
    int a, b;
    cin >> a >> b;
    w_union(a, b);
  }
  int maxx = 0;
  for (int i = 1; i <= n; i++) {
    int rooti = root(i);
    maxx = max(maxx, (ll)Size[rooti]);
  }
  cout << maxx << endl;
  return 0;
}
/// Alhamdulillah