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

//----AUTHOR:kkdrummer----/
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
// #include <boost/multiprecision/cpp_int.hpp>
// using namespace boost::multiprecision;
using namespace __gnu_pbds;
using namespace std;
typedef long long ll;
typedef long double ld;
typedef unordered_set<ll> usll;
typedef unordered_multiset<ll> umsll;
typedef multiset<ll> msll;
typedef set<ll> sll;
typedef vector<ll> vll;
typedef pair<ll, ll> pll;
typedef vector<pll> vpll;
typedef priority_queue<ll> pqll;
typedef vector<int> vi;
typedef set<int> si;
typedef multiset<int> msi;
typedef unordered_multiset<int> umsi;
typedef unordered_set<int> usi;
typedef pair<int, int> pi;
typedef vector<pi> vpi;
typedef priority_queue<int> pqi;
typedef tree<int, null_type, less<int>, rb_tree_tag,
             tree_order_statistics_node_update>
    ind_si;
typedef tree<ll, null_type, less<ll>, rb_tree_tag,
             tree_order_statistics_node_update>
    ind_sll;
typedef tree<int, null_type, less_equal<int>, rb_tree_tag,
             tree_order_statistics_node_update>
    ind_msi;
typedef tree<ll, null_type, less_equal<ll>, rb_tree_tag,
             tree_order_statistics_node_update>
    ind_msll;
#define in insert
#define fi first
#define se second
#define pb push_back
#define mp make_pair
#define be begin
#define en end
#define itr iterator
#define io                                                                     \\
  ios_base::sync_with_stdio(false);                                            \\
  cin.tie(NULL);                                                               \\
  cout.tie(NULL);
#define mo 1000000007
#define inf 8222372026854775807
#define ninf -inf
#define ima 1147483647
#define imi -ima
#define oncnt __builtin_popcount
#define zerobegin __builtin_clz
#define zeroend __builtin_ctz
#define parity __builtin_parity
#define all(x) x.be(), x.en()
#define eps 1e-9
#define coutd cout << setprecision(15) << fixed
#define mems(dp, x) memset(dp, x, sizeof(dp))
#define fbo find_by_order
#define ook order_of_key
#define upb upper_bound
#define lowb lower_bound
#define lte(v, x) (upb(all(v), x) - v.be())
#define gte(v, x) (v.end() - lowb(all(v), x))
#define gt(v, x) (v.en() - upb(all(v), x))
#define lt(v, x) (lowb(all(v), x) - v.be())
const ld PI = 3.1415926535897932384626433832792884197169399375105820974944;
inline ll modpow(ll x, ll n) {
  if (n == 0)
    return 1;
  if (x == 0)
    return 0;
  if (n == 1)
    return (x % mo);
  ll u = (modpow(x, n / 2));
  u = (u * u) % mo;
  if (n % 2 != 0)
    u = (u * x % mo) % mo;
  return u;
}
inline ll modinv(ll x) { return modpow(x, mo - 2); }
inline ll mmul(ll a, ll b) {
  if (a >= mo)
    a = a % mo;
  if (b >= mo)
    b = b % mo;
  if (a * b >= mo)
    return (a * b) % mo;
  return (a * b);
}
inline ll madd(ll a, ll b) {
  if (a >= mo)
    a = a % mo;
  if (b >= mo)
    b = b % mo;
  if (a + b >= mo)
    return (a + b) % mo;
  return (a + b);
}
inline ll msub(ll a, ll b) {
  if (a >= mo)
    a = a % mo;
  if (b >= mo)
    b = b % mo;
  return (((a - b) % mo + mo) % mo);
}
inline ll mdiv(ll a, ll bb) {
  if (a >= mo)
    a = a % mo;
  ll b = modinv(bb);
  if (b >= mo)
    b = b % mo;
  if (a * b >= mo)
    return (a * b) % mo;
  return (a * b);
}
inline ll gcd(ll a, ll b) { return __gcd(a, b); }
inline ll lcm(ll a, ll b) { return (a * b) / gcd(a, b); }

vi ad[200001];
vi adjr[200001];
bool visited[200001] = {false};
int n, m;

void dfsfillorder(stack<int> &s, int u) {
  if (visited[u])
    return;
  visited[u] = true;
  for (int i = 0; i < ad[u].size(); i++)
    dfsfillorder(s, ad[u][i]);
  s.push(u);
}

void rev() {
  for (int i = 1; i <= n; i++)
    for (int j = 0; j < ad[i].size(); j++)
      adjr[ad[i][j]].pb(i);
}

void dfsutil(int x, vi &vx) {
  if (visited[x])
    return;
  vx.pb(x);
  visited[x] = true;
  for (int i = 0; i < adjr[x].size(); i++)
    dfsutil(adjr[x][i], vx);
}

vector<vector<int>> getSCC(int v) {
  vector<vector<int>> cc;
  stack<int> s;
  for (int i = 1; i <= v; i++)
    dfsfillorder(s, i);

  rev();

  for (int i = 1; i <= v; i++)
    visited[i] = false;

  while (!s.empty()) {
    int x = s.top();
    s.pop();
    if (!visited[x]) {
      vi vx;
      dfsutil(x, vx);
      cc.pb(vx);
    }
  }

  return cc;
}

int main() {
  io int testcases = 1; //          cin>>testcases;
  while (testcases--) {

    cin >> n >> m;
    int a, b;
    set<pi> s;
    for (int i = 1; i <= m; i++) {
      cin >> a >> b;
      if (a > b)
        swap(a, b);
      pi p = mp(a, b);
      if (s.find(p) != s.end())
        continue;
      s.in(p);
      ad[a].pb(b);
      ad[b].pb(a);
    }

    vector<vector<int>> cc = getSCC(n);
    int ma = 0;
    for (int i = 0; i < cc.size(); i++) {
      ma = max(ma, (int)cc[i].size());
    }

    cout << ma;
  }
  return 0;
}


Error: WA

Fixed submission:

//----AUTHOR:kkdrummer----/
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
// #include <boost/multiprecision/cpp_int.hpp>
// using namespace boost::multiprecision;
using namespace __gnu_pbds;
using namespace std;
typedef long long ll;
typedef long double ld;
typedef unordered_set<ll> usll;
typedef unordered_multiset<ll> umsll;
typedef multiset<ll> msll;
typedef set<ll> sll;
typedef vector<ll> vll;
typedef pair<ll, ll> pll;
typedef vector<pll> vpll;
typedef priority_queue<ll> pqll;
typedef vector<int> vi;
typedef set<int> si;
typedef multiset<int> msi;
typedef unordered_multiset<int> umsi;
typedef unordered_set<int> usi;
typedef pair<int, int> pi;
typedef vector<pi> vpi;
typedef priority_queue<int> pqi;
typedef tree<int, null_type, less<int>, rb_tree_tag,
             tree_order_statistics_node_update>
    ind_si;
typedef tree<ll, null_type, less<ll>, rb_tree_tag,
             tree_order_statistics_node_update>
    ind_sll;
typedef tree<int, null_type, less_equal<int>, rb_tree_tag,
             tree_order_statistics_node_update>
    ind_msi;
typedef tree<ll, null_type, less_equal<ll>, rb_tree_tag,
             tree_order_statistics_node_update>
    ind_msll;
#define in insert
#define fi first
#define se second
#define pb push_back
#define mp make_pair
#define be begin
#define en end
#define itr iterator
#define io                                                                     \\
  ios_base::sync_with_stdio(false);                                            \\
  cin.tie(NULL);                                                               \\
  cout.tie(NULL);
#define mo 1000000007
#define inf 8222372026854775807
#define ninf -inf
#define ima 1147483647
#define imi -ima
#define oncnt __builtin_popcount
#define zerobegin __builtin_clz
#define zeroend __builtin_ctz
#define parity __builtin_parity
#define all(x) x.be(), x.en()
#define eps 1e-9
#define coutd cout << setprecision(15) << fixed
#define mems(dp, x) memset(dp, x, sizeof(dp))
#define fbo find_by_order
#define ook order_of_key
#define upb upper_bound
#define lowb lower_bound
#define lte(v, x) (upb(all(v), x) - v.be())
#define gte(v, x) (v.end() - lowb(all(v), x))
#define gt(v, x) (v.en() - upb(all(v), x))
#define lt(v, x) (lowb(all(v), x) - v.be())
const ld PI = 3.1415926535897932384626433832792884197169399375105820974944;
inline ll modpow(ll x, ll n) {
  if (n == 0)
    return 1;
  if (x == 0)
    return 0;
  if (n == 1)
    return (x % mo);
  ll u = (modpow(x, n / 2));
  u = (u * u) % mo;
  if (n % 2 != 0)
    u = (u * x % mo) % mo;
  return u;
}
inline ll modinv(ll x) { return modpow(x, mo - 2); }
inline ll mmul(ll a, ll b) {
  if (a >= mo)
    a = a % mo;
  if (b >= mo)
    b = b % mo;
  if (a * b >= mo)
    return (a * b) % mo;
  return (a * b);
}
inline ll madd(ll a, ll b) {
  if (a >= mo)
    a = a % mo;
  if (b >= mo)
    b = b % mo;
  if (a + b >= mo)
    return (a + b) % mo;
  return (a + b);
}
inline ll msub(ll a, ll b) {
  if (a >= mo)
    a = a % mo;
  if (b >= mo)
    b = b % mo;
  return (((a - b) % mo + mo) % mo);
}
inline ll mdiv(ll a, ll bb) {
  if (a >= mo)
    a = a % mo;
  ll b = modinv(bb);
  if (b >= mo)
    b = b % mo;
  if (a * b >= mo)
    return (a * b) % mo;
  return (a * b);
}
inline ll gcd(ll a, ll b) { return __gcd(a, b); }
inline ll lcm(ll a, ll b) { return (a * b) / gcd(a, b); }

vi ad[100001];
vi adjr[100001];
bool visited[100001] = {false};
int n, m;

void dfsfillorder(stack<int> &s, int u) {
  if (visited[u])
    return;
  visited[u] = true;
  for (int i = 0; i < ad[u].size(); i++)
    dfsfillorder(s, ad[u][i]);
  s.push(u);
}

void rev() {
  for (int i = 1; i <= n; i++)
    for (int j = 0; j < ad[i].size(); j++)
      adjr[ad[i][j]].pb(i);
}

void dfsutil(int x, vi &vx) {
  if (visited[x])
    return;
  vx.pb(x);
  visited[x] = true;
  for (int i = 0; i < adjr[x].size(); i++)
    dfsutil(adjr[x][i], vx);
}

vector<vector<int>> getSCC(int v) {
  vector<vector<int>> cc;
  stack<int> s;
  for (int i = 1; i <= v; i++)
    dfsfillorder(s, i);

  rev();

  for (int i = 1; i <= v; i++)
    visited[i] = false;

  while (!s.empty()) {
    int x = s.top();
    s.pop();
    if (!visited[x]) {
      vi vx;
      dfsutil(x, vx);
      cc.pb(vx);
    }
  }

  return cc;
}

int main() {
  io int testcases = 1; //          cin>>testcases;
  while (testcases--) {

    cin >> n >> m;
    int a, b;
    set<pi> s;
    for (int i = 1; i <= m; i++) {
      cin >> a >> b;
      if (a > b)
        swap(a, b);
      pi p = mp(a, b);
      if (s.find(p) != s.end())
        continue;
      s.in(p);
      ad[a].pb(b);
      ad[b].pb(a);
    }

    vector<vector<int>> cc = getSCC(n);
    int ma = 0;
    for (int i = 0; i < cc.size(); i++) {
      ma = max(ma, (int)cc[i].size());
    }

    cout << ma;
  }
  return 0;
}