Problem p02573 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>
using namespace std;

#define fast                                                                   \\
  ios_base::sync_with_stdio(false);                                            \\
  cin.tie(NULL);                                                               \\
  cout.tie(NULL);
#define ll long long int
#define ull unsigned long long
#define ld long double

#define mod 1000000007
#define EULER 2.7182818284
#define INF 1000000000

#define pb push_back
#define ins insert
#define IT iterator
#define PQ priority_queue
#define nl "\\n"
#define fi first
#define se second
#define maxe max_element
#define mine min_element
#define er erase
#define lb lower_bound
#define ub upper_bound

#define vi vector<int>
#define vll vector<ll>
#define vb vector<bool>
#define pi pair<int, int>
#define pll pair<ll, ll>
#define all(v) v.begin(), v.end()
#define mpi map<int, int>
#define mpl map<ll, ll>
#define umpi unordered_map<int, int>
#define umpl unordered_map<ll, ll>
#define si set<int>
#define sl set<ll>
#define msi multi_set<int>
#define msl multi_set<ll>

#define mem(v, i)                                                              \\
  memset(                                                                      \\
      v, i,                                                                    \\
      sizeof(                                                                  \\
          v)) // v is array. To set all elements to 0 or -1 only. mem(arr,0);

#define repab(i, a, b) for (int i = (a); i <= (b); i++)
#define reprab(i, a, b) for (int i = (a); i >= (b); i--)
#define repll(i, a, b) for (ll i = (a); i <= (b); i++)
#define rep(i, n) for (int i = (0); i < (n); i++)
#define repr(i, n) for (int i = (n); i >= 0; i--)
#define bs bitset
#define bpc(a) __builtin_popcount(a)
#define tc                                                                     \\
  int t;                                                                       \\
  cin >> t;                                                                    \\
  while (t--)

int dx[] = {1, -1, 0, 0};
int dy[] = {0, 0, 1, -1};

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

ll mypow(ll a, ll b) {
  ll res = 1;
  while (b > 0) {
    if (b & 1)
      res = res * a;
    a = a * a;
    b >>= 1;
  }
  return res;
}
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}

int par[200005], r[200005];
int mx = 1;
int find(int a) {
  if (par[a] < 0)
    return a;
  return par[a] = find(par[a]);
}
void merge(int a, int b) {
  if (r[a] > r[b]) {
    par[b] = a;
    r[a] += r[b];
    mx = max(mx, r[a]);
  } else {
    par[a] = b;
    r[b] += r[a];
    mx = max(mx, r[b]);
  }
}
int main() {

  fast int n, m;
  cin >> n >> m;
  rep(i, n + 1) {
    par[i] = -1;
    r[i] = 1;
  }
  rep(i, m) {
    int x, y;
    cin >> x >> y;
    int a = find(x), b = find(y);
    if (a != b)
      merge(a, b);
  }
  cout << mx << nl;

  return 0;
}

Error: WA

Fixed submission:

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace __gnu_pbds;
using namespace std;

#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
#pragma GCC optimization("unroll-loops")

#define fast                                                                   \\
  ios_base::sync_with_stdio(false);                                            \\
  cin.tie(NULL);                                                               \\
  cout.tie(NULL);
#define ll long long int
#define ull unsigned long long
#define ld long double

#define mod 1000000007
#define EULER 2.7182818284
#define INF 1000000000

#define pb push_back
#define ins insert
#define IT iterator
#define PQ priority_queue
#define nl "\\n"
#define sp " "
#define fi first
#define se second
#define maxe max_element
#define mine min_element
#define er erase
#define lb lower_bound
#define ub upper_bound

#define vi vector<int>
#define vll vector<ll>
#define vb vector<bool>
#define pi pair<int, int>
#define pll pair<ll, ll>
#define all(v) v.begin(), v.end()
#define mpi map<int, int>
#define mpl map<ll, ll>
#define umpi unordered_map<int, int>
#define umpl unordered_map<ll, ll>
#define si set<int>
#define sl set<ll>
#define msi multi_set<int>
#define msl multi_set<ll>

#define mem(v, i)                                                              \\
  memset(                                                                      \\
      v, i,                                                                    \\
      sizeof(                                                                  \\
          v)) // v is array. To set all elements to 0 or -1 only. mem(arr,0);

#define repab(i, a, b) for (int i = (a); i <= (b); i++)
#define reprab(i, a, b) for (int i = (a); i >= (b); i--)
#define repll(i, a, b) for (ll i = (a); i <= (b); i++)
#define rep(i, n) for (int i = (0); i < (n); i++)
#define repr(i, n) for (int i = (n); i >= 0; i--)
#define bs bitset
#define bpc(a) __builtin_popcount(a)
#define tc                                                                     \\
  int t;                                                                       \\
  cin >> t;                                                                    \\
  while (t--)
template <typename T> void TIME(T start, T end) {
#ifndef ONLINE_JUDGE
  double time_taken =
      chrono::duration_cast<chrono::nanoseconds>(end - start).count();
  time_taken *= 1e-9;
  cout << "\\nExecuted in: " << fixed << time_taken << setprecision(9);
  cout << " sec";
#endif
}

int dx[] = {1, -1, 0, 0};
int dy[] = {0, 0, 1, -1};

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

ll mypow(ll a, ll b) {
  ll res = 1;
  while (b > 0) {
    if (b & 1)
      res = res * a;
    a = a * a;
    b >>= 1;
  }
  return res;
}
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
//..............................!!!!!!Get ready to
//fight!!!!!!............................
int par[200005], r[20005];
int mx = -1;
int find(int a) {
  if (par[a] < 0)
    return a;
  return par[a] = find(par[a]);
}
void merge(int a, int b) {
  if (r[a] > r[b]) {
    par[b] = a;
    r[a] += r[b];
    mx = max(mx, r[a]);
  } else {
    par[a] = b;
    r[b] += r[a];
    mx = max(mx, r[b]);
  }
}
int main() {
  auto start = chrono::high_resolution_clock::now();
  fast int n, m;
  cin >> n >> m;
  rep(i, n + 1) {
    par[i] = -1;
    r[i] = 1;
  }
  rep(i, m) {
    int x, y;
    cin >> x >> y;
    int a = find(x), b = find(y);
    if (a != b)
      merge(a, b);
  }
  cout << mx << nl;
  auto end = chrono::high_resolution_clock::now();
  TIME(start, end);
  return 0;
}