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

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef double db;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<int, ll> pil;
typedef pair<ll, int> pli;
#define Fi first
#define Se second
#define _Out(a) cerr << #a << " = " << (a) << endl
const int INF = 0x3f3f3f3f, MAXN = 3e5 + 7;
const ll LINF = 0x3f3f3f3f3f3f3f3f, MOD = 1e9 + 7;
const db Pi = acos(-1), EPS = 1e-6;
void test() { cerr << "\\n"; }
template <typename T, typename... Args> void test(T x, Args... args) {
  cerr << x << " ";
  test(args...);
}
inline ll qpow(ll a, ll b) {
  return b ? ((b & 1) ? a * qpow(a * a % MOD, b >> 1) % MOD
                      : qpow(a * a % MOD, b >> 1)) %
                 MOD
           : 1;
}
inline ll qpow(ll a, ll b, ll c) {
  return b ? ((b & 1) ? a * qpow(a * a % c, b >> 1, c) % c
                      : qpow(a * a % c, b >> 1, c)) %
                 c
           : 1;
}
inline ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
inline ll cede(ll a, ll b) {
  if (b < 0)
    return cede(-a, -b);
  if (a < 0)
    return a / b;
  return (a + b - 1) / b;
}
inline ll flde(ll a, ll b) {
  if (b < 0)
    return flde(-a, -b);
  if (a < 0)
    return (a - b + 1) / b;
  return a / b;
}
inline int sign(db x) { return x < -EPS ? -1 : x > EPS; }
inline int dbcmp(db l, db r) { return sign(l - r); }
namespace Fast_IO { // orz laofu
const int MAXL((1 << 18) + 1);
int iof, iotp;
char ioif[MAXL], *ioiS, *ioiT, ioof[MAXL],
    *iooS = ioof, *iooT = ioof + MAXL - 1, ioc, iost[55];
char Getchar() {
  if (ioiS == ioiT) {
    ioiS = ioif;
    ioiT = ioiS + fread(ioif, 1, MAXL, stdin);
    return (ioiS == ioiT ? EOF : *ioiS++);
  } else
    return (*ioiS++);
}
void Write() {
  fwrite(ioof, 1, iooS - ioof, stdout);
  iooS = ioof;
}
void Putchar(char x) {
  *iooS++ = x;
  if (iooS == iooT)
    Write();
}
inline int read() {
  int x = 0;
  for (iof = 1, ioc = Getchar(); ioc < '0' || ioc > '9';)
    iof = ioc == '-' ? -1 : 1, ioc = Getchar();
  for (x = 0; ioc <= '9' && ioc >= '0'; ioc = Getchar())
    x = (x << 3) + (x << 1) + (ioc ^ 48);
  return x * iof;
}
inline long long read_ll() {
  long long x = 0;
  for (iof = 1, ioc = Getchar(); ioc < '0' || ioc > '9';)
    iof = ioc == '-' ? -1 : 1, ioc = Getchar();
  for (x = 0; ioc <= '9' && ioc >= '0'; ioc = Getchar())
    x = (x << 3) + (x << 1) + (ioc ^ 48);
  return x * iof;
}
template <class Int> void Print(Int x, char ch = '\\0') {
  if (!x)
    Putchar('0');
  if (x < 0)
    Putchar('-'), x = -x;
  while (x)
    iost[++iotp] = x % 10 + '0', x /= 10;
  while (iotp)
    Putchar(iost[iotp--]);
  if (ch)
    Putchar(ch);
}
void Getstr(char *s, int &l) {
  for (ioc = Getchar(); ioc < 'a' || ioc > 'z';)
    ioc = Getchar();
  for (l = 0; ioc <= 'z' && ioc >= 'a'; ioc = Getchar())
    s[l++] = ioc;
  s[l] = 0;
}
void Putstr(const char *s) {
  for (int i = 0, n = strlen(s); i < n; ++i)
    Putchar(s[i]);
}
} // namespace Fast_IO
using namespace Fast_IO;

int fa[MAXN], siz[MAXN];
int fifa(int x) { return fa[x] == x ? x : fa[x] = fifa(fa[x]); }
void work() {
  int n = read(), m = read();
  for (int i = 1; i <= n; i++)
    fa[i] = i, siz[i] = 1;
  int ans = 1;
  for (int i = 1; i <= m; i++) {
    int x = read(), y = read();
    int fx = fifa(x), fy = fifa(y);
    if (fx != fy) {
      siz[fy] += siz[fx];
      ans = max(ans, siz[fy]);
      fa[fx] = fy;
    }
  }
  printf("%d\\n", ans);
}

int main() {
  // std::ios::sync_with_stdio(false);
  // int T=read();for(int cas=1;cas<=T;cas++)
  // int T;scanf("%d",&T);for(int i=1;i<=T;i++)
  work();
  // Write();
}
/*
3
6 9 2 1 1 1 4 1 3 3
6 9 2 1 1 1 5 1 4 3
6 9 2 1 1 1 5 1 5 3
6 9 2 1 1 1 8 1 6 3
6 9 2 1 1 1 8 1 7 4

5 6 4 1 1 4 3 999 1 10
10 10 4 1 1 3 7 999 1 100
10 10 4 1 1 8 9 999 1 100
*/

Error: 0

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef double db;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<int, ll> pil;
typedef pair<ll, int> pli;
#define Fi first
#define Se second
#define _Out(a) cerr << #a << " = " << (a) << endl
const int INF = 0x3f3f3f3f, MAXN = 1e3 + 7;
const ll LINF = 0x3f3f3f3f3f3f3f3f, MOD = 1e9 + 7;
const db Pi = acos(-1), EPS = 1e-6;
void test() { cerr << "\\n"; }
template <typename T, typename... Args> void test(T x, Args... args) {
  cerr << x << " ";
  test(args...);
}
inline ll qpow(ll a, ll b) {
  return b ? ((b & 1) ? a * qpow(a * a % MOD, b >> 1) % MOD
                      : qpow(a * a % MOD, b >> 1)) %
                 MOD
           : 1;
}
inline ll qpow(ll a, ll b, ll c) {
  return b ? ((b & 1) ? a * qpow(a * a % c, b >> 1, c) % c
                      : qpow(a * a % c, b >> 1, c)) %
                 c
           : 1;
}
inline ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
inline ll cede(ll a, ll b) {
  if (b < 0)
    return cede(-a, -b);
  if (a < 0)
    return a / b;
  return (a + b - 1) / b;
}
inline ll flde(ll a, ll b) {
  if (b < 0)
    return flde(-a, -b);
  if (a < 0)
    return (a - b + 1) / b;
  return a / b;
}
inline int sign(db x) { return x < -EPS ? -1 : x > EPS; }
inline int dbcmp(db l, db r) { return sign(l - r); }
namespace Fast_IO { // orz laofu
const int MAXL((1 << 18) + 1);
int iof, iotp;
char ioif[MAXL], *ioiS, *ioiT, ioof[MAXL],
    *iooS = ioof, *iooT = ioof + MAXL - 1, ioc, iost[55];
char Getchar() {
  if (ioiS == ioiT) {
    ioiS = ioif;
    ioiT = ioiS + fread(ioif, 1, MAXL, stdin);
    return (ioiS == ioiT ? EOF : *ioiS++);
  } else
    return (*ioiS++);
}
void Write() {
  fwrite(ioof, 1, iooS - ioof, stdout);
  iooS = ioof;
}
void Putchar(char x) {
  *iooS++ = x;
  if (iooS == iooT)
    Write();
}
inline int read() {
  int x = 0;
  for (iof = 1, ioc = Getchar(); ioc < '0' || ioc > '9';)
    iof = ioc == '-' ? -1 : 1, ioc = Getchar();
  for (x = 0; ioc <= '9' && ioc >= '0'; ioc = Getchar())
    x = (x << 3) + (x << 1) + (ioc ^ 48);
  return x * iof;
}
inline long long read_ll() {
  long long x = 0;
  for (iof = 1, ioc = Getchar(); ioc < '0' || ioc > '9';)
    iof = ioc == '-' ? -1 : 1, ioc = Getchar();
  for (x = 0; ioc <= '9' && ioc >= '0'; ioc = Getchar())
    x = (x << 3) + (x << 1) + (ioc ^ 48);
  return x * iof;
}
template <class Int> void Print(Int x, char ch = '\\0') {
  if (!x)
    Putchar('0');
  if (x < 0)
    Putchar('-'), x = -x;
  while (x)
    iost[++iotp] = x % 10 + '0', x /= 10;
  while (iotp)
    Putchar(iost[iotp--]);
  if (ch)
    Putchar(ch);
}
void Getstr(char *s, int &l) {
  for (ioc = Getchar(); ioc < 'a' || ioc > 'z';)
    ioc = Getchar();
  for (l = 0; ioc <= 'z' && ioc >= 'a'; ioc = Getchar())
    s[l++] = ioc;
  s[l] = 0;
}
void Putstr(const char *s) {
  for (int i = 0, n = strlen(s); i < n; ++i)
    Putchar(s[i]);
}
} // namespace Fast_IO
using namespace Fast_IO;

int fa[MAXN], siz[MAXN];
int fifa(int x) { return fa[x] == x ? x : fa[x] = fifa(fa[x]); }
void work() {
  int n = read(), m = read();
  for (int i = 1; i <= n; i++)
    fa[i] = i, siz[i] = 1;
  int ans = 1;
  for (int i = 1; i <= m; i++) {
    int x = read(), y = read();
    int fx = fifa(x), fy = fifa(y);
    if (fx != fy) {
      siz[fy] += siz[fx];
      ans = max(ans, siz[fy]);
      fa[fx] = fy;
    }
  }
  printf("%d\\n", ans);
}

int main() {
  // std::ios::sync_with_stdio(false);
  // int T=read();for(int cas=1;cas<=T;cas++)
  // int T;scanf("%d",&T);for(int i=1;i<=T;i++)
  work();
  // Write();
}
/*
3
6 9 2 1 1 1 4 1 3 3
6 9 2 1 1 1 5 1 4 3
6 9 2 1 1 1 5 1 5 3
6 9 2 1 1 1 8 1 6 3
6 9 2 1 1 1 8 1 7 4

5 6 4 1 1 4 3 999 1 10
10 10 4 1 1 3 7 999 1 100
10 10 4 1 1 8 9 999 1 100
*/