Problem p02573 (C++)

Buggy submission (status: Runtime Error):
/*
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rtll==??>>;::::~~~~......`.``````````````````
..J77!`````````...`..._T,~~~~:::;;>??==lttrrv
tll=??>>;:::~~~~......``````````````````..J7^ ` ``  ` ``
.,```````...._4,.~~~::;;>>?===lttr l=???>;;::~~~......`````HTSu,.``  `..J7!    `
`` .J"~N```````````..?&.~~~~::;;>>?==llt
=??>;;::~~~~.....``````.@????7SJ.?=     `     ` `` .J=....J;
``````````..h..~~~~::;;>??=ll
?>>;::~~~~.....````````.D?>>?>?>?8+.``         `.J"_......_b`
````````.S_.~~~~::;;>??=l
>;;::~~~~....``````````.C>??>>>?>>>?8J.`  ```..Y~..........J;  `  ` ```````
G...~~~~::;>??=
;;::~~~....```````` `..W1>>?>>>>?>>>>>?S,.`.7^.............-N``  `
``````` 6...~~~~::;>??
;:~~~~....``````` ..7` d??>>?>?>>?>>?>>1udMgggNNNNggJ.......([
`````.L...~~~~::;>?
:~~~.....`````` .7`   `K>?>?>>>>>>+ugNMMMB""7<!~~<?7TWMNNa,..w.`  ` `  `
`````,)....~~:::;>
~~~....``````.J^     ` #>>?>>>?jgMMM9=_................-?TMMa,b` `   `  `
````(,...~~~~:;;
~~~....``` .7``   `    @?>>?1uMM#=.........................(TMNa......  `
``````4....~~~::;
~~~...`` .=`` `     `
.b>>jgNH".................`.............?HNmx??17TYY9SA+(..L....~~~::
~....` ,^``     `   `
.b+dN#^............6..-(,-...`............(HMm+>>>>>?>>????zOM_.~~~::
.... .=```  `` `
...JNMM^..........`..._n.(MMN,....`..`.........?MNe<>>?>??>????d^...~~~:
~...v```` `
..-Z""!_..(M@_........`........?7MMMMp.................-TNN+?>>>????1d4-..-(Jk9
..(^`...zY"!_........(MD..............`......JMMMMp....`..`..`......./MNx>??>>?1d!.h7=~(??=
(v_`,R_.............(NF..(/..(&,...`..........?MMMM;..................(MMs>>?1&T"!``....(1=
t..``
4,...........(N@....?,(NMNR_.....`..`....(WMM$..`....`..`..`....._HMe7=```````....~_1
...````.4,........-dM:.....(7MMMNR-.....................`............(.?^ ``
``````....~~~
...``````,4,....`.(NF........(MMMMb..`....--................`....(.7!        `
````....~~:
..````` ` `.5J_...JMt.........?NMMM[...`.HH5._(J7[...`...`...--?^`          `
`````....~~~:
...````` `  ` 7a,.dM{....`...../MMMN......(J=`  .>......._(/=             `  `
`````...~~~:
....```` ``     (4MN{..........._7"^...(/^    `.C....-(J=`                  ` `
```....~~~:
....````` `      JdM[...`...`........`_3..  ..?!..(-7!                  ` `
``````....~~~:: r...``````  ` ``(CJMb..............`......__..-(?^ `  `
`````....~~::; J/...````` `  `,3>+MN-.`..`...`..........._(J=`` `  `
```````....~~~::; _j,..`.```` ``.5>>?dNb...`......`......_-?'` `
`````....~~~::;;
~~j,..```````.D??>>>MM/....`........(-=`                          `  ` `
```````...~~~:::;>
~~~j,...````.E??>??>?MN-.........(J=                            `   ` `
``````....~~~~::;??
:~~~?,...``.@>?>?>>??dMN_....-(?^                        `  `    ` `
````````...~~~~:;;>??
::~~~?/....K??????>>?>dMN-_(7!                               `  ` `
````````....~~~:::>>??l
;:::~~/e.(K==?????????<dM"!                          `    `   ` ` ``
``````...~~~~:::;>??=l
@TT_beginner
*/
#include <bits/stdc++.h>
using namespace std;
#define ok1 cerr << "ok1\n";
#define ok2 cerr << "ok2\n";
#define M LLONG_MAX
#define rep(i, n) for (int i = 0; i < n; ++i)
#define REP(i, s, n) for (int i = (s); i < (n); ++i)
#define repr(i, n) for (int i = n - 1; i >= 0; --i)
#define REPR(i, s, n) for (int i = (s); i >= (n); --(i))
#define all(a) (a).begin(), (a).end()
#define reall(a) (a).rbegin(), (a).rend()
#define pb emplace_back // emplace_backの方が早いが慣れているため
#define DOUBLE fixed << setprecision(15)
#define fi first
#define se second
#define mp make_pair
#define mt make_tuple
const double pi = acos(-1.0L);
typedef vector<int> vi;
typedef vector<string> vs;
typedef long long ll;
typedef vector<ll> vll;
typedef vector<vi> vvi;
typedef vector<vll> vvll;
typedef vector<char> vc;
typedef vector<double> vd;
typedef vector<bool> vb;
typedef deque<ll> dll;
typedef pair<ll, ll> P;
typedef tuple<ll, ll, ll> TLL;
typedef vector<P> vP;
typedef vector<TLL> vTLL;
const ll mod = 1e9 + 7;
// const ll mod = 998244353;
ll dy[4] = {1, 0, -1, 0};
ll dx[4] = {0, 1, 0, -1};
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class A, class B>
ostream &operator<<(ostream &ost, const pair<A, B> &p) {
  ost << "{ " << p.first << ", " << p.second << " }";
  return ost;
}
template <class T> ostream &operator<<(ostream &ost, const vector<T> &v) {
  ost << "{ ";
  for (int i = 0; i < v.size(); i++) {
    if (i)
      ost << ", ";
    ost << v[i];
  }
  ost << " }";
  return ost;
}
template <class A, class B>
ostream &operator<<(ostream &ost, const map<A, B> &v) {
  ost << "{ ";
  for (auto p : v) {
    ost << "{ " << p.first << ", " << p.second << " }";
  }
  ost << " }";
  return ost;
}
bool out_check(ll a, ll b) { return (0 <= a && a < b); }
double pitagoras(ll a, ll b, ll c, ll d) {
  double dx = a - b, dy = c - d;
  return pow(dx * dx + dy * dy, 0.5);
}
ll modpow(ll a, ll b) {
  ll c = 1;
  while (b > 0) {
    if (b & 1) {
      c = a * c % mod;
    }
    a = a * a % mod;
    b >>= 1;
  }
  return c;
}
ll modinv(ll a) { return modpow(a, mod - 2); }
void Yes(bool x) { cout << ((x) ? "Yes\n" : "No\n"); }
void YES(bool x) { cout << ((x) ? "YES\n" : "NO\n"); }
void yes(bool x) { cout << ((x) ? "yes\n" : "no\n"); }
void Yay(bool x) { cout << ((x) ? "Yay!\n" : ":(\n"); }

struct UnionFind {
private:
  vector<long long> data;

public:
  UnionFind(long long size) : data(size, -1) {}
  bool unite(long long x, long long y) {
    x = root(x);
    y = root(y);
    if (x != y) {
      if (data[y] < data[x])
        swap(x, y);
      data[x] += data[y];
      data[y] = x;
    }
    return x != y;
  }
  bool same(long long x, long long y) { return root(x) == root(y); }
  int root(long long x) { return data[x] < 0 ? x : data[x] = root(data[x]); }
  ll size(long long x) { return -data[root(x)]; }
};

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false); // not Interactive
  ll n, m, k, l, r, d = 0, Q, ans = 0, ret = M;
  string s, t;

  cin >> n >> m;
  vll a(m), b(m);
  UnionFind tree(n);
  rep(i, m) {
    cin >> a[i] >> b[i];
    tree.unite(a[i], b[i]);
  }
  rep(i, n) { chmax(ans, tree.size(i + 1)); }
  cout << ans << endl;
}

Fixed submission:
/*
vrrtll==???>;::::~~~~~~......`.`````````````````````...-?777!_.._?7u,~~~::::;>>??=lllttrrzu
rtll==??>>;::::~~~~......`.``````````````````
..J77!`````````...`..._T,~~~~:::;;>??==lttrrv
tll=??>>;:::~~~~......``````````````````..J7^ ` ``  ` ``
.,```````...._4,.~~~::;;>>?===lttr l=???>;;::~~~......`````HTSu,.``  `..J7!    `
`` .J"~N```````````..?&.~~~~::;;>>?==llt
=??>;;::~~~~.....``````.@????7SJ.?=     `     ` `` .J=....J;
``````````..h..~~~~::;;>??=ll
?>>;::~~~~.....````````.D?>>?>?>?8+.``         `.J"_......_b`
````````.S_.~~~~::;;>??=l
>;;::~~~~....``````````.C>??>>>?>>>?8J.`  ```..Y~..........J;  `  ` ```````
G...~~~~::;>??=
;;::~~~....```````` `..W1>>?>>>>?>>>>>?S,.`.7^.............-N``  `
``````` 6...~~~~::;>??
;:~~~~....``````` ..7` d??>>?>?>>?>>?>>1udMgggNNNNggJ.......([
`````.L...~~~~::;>?
:~~~.....`````` .7`   `K>?>?>>>>>>+ugNMMMB""7<!~~<?7TWMNNa,..w.`  ` `  `
`````,)....~~:::;>
~~~....``````.J^     ` #>>?>>>?jgMMM9=_................-?TMMa,b` `   `  `
````(,...~~~~:;;
~~~....``` .7``   `    @?>>?1uMM#=.........................(TMNa......  `
``````4....~~~::;
~~~...`` .=`` `     `
.b>>jgNH".................`.............?HNmx??17TYY9SA+(..L....~~~::
~....` ,^``     `   `
.b+dN#^............6..-(,-...`............(HMm+>>>>>?>>????zOM_.~~~::
.... .=```  `` `
...JNMM^..........`..._n.(MMN,....`..`.........?MNe<>>?>??>????d^...~~~:
~...v```` `
..-Z""!_..(M@_........`........?7MMMMp.................-TNN+?>>>????1d4-..-(Jk9
..(^`...zY"!_........(MD..............`......JMMMMp....`..`..`......./MNx>??>>?1d!.h7=~(??=
(v_`,R_.............(NF..(/..(&,...`..........?MMMM;..................(MMs>>?1&T"!``....(1=
t..``
4,...........(N@....?,(NMNR_.....`..`....(WMM$..`....`..`..`....._HMe7=```````....~_1
...````.4,........-dM:.....(7MMMNR-.....................`............(.?^ ``
``````....~~~
...``````,4,....`.(NF........(MMMMb..`....--................`....(.7!        `
````....~~:
..````` ` `.5J_...JMt.........?NMMM[...`.HH5._(J7[...`...`...--?^`          `
`````....~~~:
...````` `  ` 7a,.dM{....`...../MMMN......(J=`  .>......._(/=             `  `
`````...~~~:
....```` ``     (4MN{..........._7"^...(/^    `.C....-(J=`                  ` `
```....~~~:
....````` `      JdM[...`...`........`_3..  ..?!..(-7!                  ` `
``````....~~~:: r...``````  ` ``(CJMb..............`......__..-(?^ `  `
`````....~~::; J/...````` `  `,3>+MN-.`..`...`..........._(J=`` `  `
```````....~~~::; _j,..`.```` ``.5>>?dNb...`......`......_-?'` `
`````....~~~::;;
~~j,..```````.D??>>>MM/....`........(-=`                          `  ` `
```````...~~~:::;>
~~~j,...````.E??>??>?MN-.........(J=                            `   ` `
``````....~~~~::;??
:~~~?,...``.@>?>?>>??dMN_....-(?^                        `  `    ` `
````````...~~~~:;;>??
::~~~?/....K??????>>?>dMN-_(7!                               `  ` `
````````....~~~:::>>??l
;:::~~/e.(K==?????????<dM"!                          `    `   ` ` ``
``````...~~~~:::;>??=l
@TT_beginner
*/
#include <bits/stdc++.h>
using namespace std;
#define ok1 cerr << "ok1\n";
#define ok2 cerr << "ok2\n";
#define M LLONG_MAX
#define rep(i, n) for (int i = 0; i < n; ++i)
#define REP(i, s, n) for (int i = (s); i < (n); ++i)
#define repr(i, n) for (int i = n - 1; i >= 0; --i)
#define REPR(i, s, n) for (int i = (s); i >= (n); --(i))
#define all(a) (a).begin(), (a).end()
#define reall(a) (a).rbegin(), (a).rend()
#define pb emplace_back // emplace_backの方が早いが慣れているため
#define DOUBLE fixed << setprecision(15)
#define fi first
#define se second
#define mp make_pair
#define mt make_tuple
const double pi = acos(-1.0L);
typedef vector<int> vi;
typedef vector<string> vs;
typedef long long ll;
typedef vector<ll> vll;
typedef vector<vi> vvi;
typedef vector<vll> vvll;
typedef vector<char> vc;
typedef vector<double> vd;
typedef vector<bool> vb;
typedef deque<ll> dll;
typedef pair<ll, ll> P;
typedef tuple<ll, ll, ll> TLL;
typedef vector<P> vP;
typedef vector<TLL> vTLL;
const ll mod = 1e9 + 7;
// const ll mod = 998244353;
ll dy[4] = {1, 0, -1, 0};
ll dx[4] = {0, 1, 0, -1};
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class A, class B>
ostream &operator<<(ostream &ost, const pair<A, B> &p) {
  ost << "{ " << p.first << ", " << p.second << " }";
  return ost;
}
template <class T> ostream &operator<<(ostream &ost, const vector<T> &v) {
  ost << "{ ";
  for (int i = 0; i < v.size(); i++) {
    if (i)
      ost << ", ";
    ost << v[i];
  }
  ost << " }";
  return ost;
}
template <class A, class B>
ostream &operator<<(ostream &ost, const map<A, B> &v) {
  ost << "{ ";
  for (auto p : v) {
    ost << "{ " << p.first << ", " << p.second << " }";
  }
  ost << " }";
  return ost;
}
bool out_check(ll a, ll b) { return (0 <= a && a < b); }
double pitagoras(ll a, ll b, ll c, ll d) {
  double dx = a - b, dy = c - d;
  return pow(dx * dx + dy * dy, 0.5);
}
ll modpow(ll a, ll b) {
  ll c = 1;
  while (b > 0) {
    if (b & 1) {
      c = a * c % mod;
    }
    a = a * a % mod;
    b >>= 1;
  }
  return c;
}
ll modinv(ll a) { return modpow(a, mod - 2); }
void Yes(bool x) { cout << ((x) ? "Yes\n" : "No\n"); }
void YES(bool x) { cout << ((x) ? "YES\n" : "NO\n"); }
void yes(bool x) { cout << ((x) ? "yes\n" : "no\n"); }
void Yay(bool x) { cout << ((x) ? "Yay!\n" : ":(\n"); }

struct UnionFind {
private:
  vector<long long> data;

public:
  UnionFind(long long size) : data(size, -1) {}
  bool unite(long long x, long long y) {
    x = root(x);
    y = root(y);
    if (x != y) {
      if (data[y] < data[x])
        swap(x, y);
      data[x] += data[y];
      data[y] = x;
    }
    return x != y;
  }
  bool same(long long x, long long y) { return root(x) == root(y); }
  int root(long long x) { return data[x] < 0 ? x : data[x] = root(data[x]); }
  ll size(long long x) { return -data[root(x)]; }
};

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false); // not Interactive
  ll n, m, k, l, r, d = 0, Q, ans = 0, ret = M;
  string s, t;

  cin >> n >> m;
  vll a(m), b(m);
  UnionFind tree(n + 1);
  rep(i, m) {
    cin >> a[i] >> b[i];
    tree.unite(a[i], b[i]);
  }
  rep(i, n) { chmax(ans, tree.size(i + 1)); }
  cout << ans << endl;
}