Problem p02892 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;

#define REP(i, m, n) for (int i = (m); i < (int)(n); i++)
#define RREP(i, m, n) for (int i = (int)(n - 1); i >= m; i--)
#define rep(i, n) REP(i, 0, n)
#define rrep(i, n) RREP(i, 0, n)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define fi first
#define se second
#define debug(...)                                                             \
  {                                                                            \
    cerr << "[L" << __LINE__ << "] ";                                          \
    _debug(__VA_ARGS__);                                                       \
  }

template <typename T> string join(const vector<T> &v, string del = ", ") {
  stringstream s;
  for (auto x : v)
    s << del << x;
  return s.str().substr(del.size());
}
template <typename T> ostream &operator<<(ostream &o, const vector<T> &v) {
  if (v.size())
    o << "[" << join(v) << "]";
  return o;
}
template <typename T>
ostream &operator<<(ostream &o, const vector<vector<T>> &vv) {
  int l = vv.size();
  if (l) {
    o << endl;
    rep(i, l) o << (i == 0 ? "[ " : ",\n  ") << vv[i]
                << (i == l - 1 ? " ]" : "");
  }
  return o;
}
inline void _debug() { cerr << endl; }
template <class First, class... Rest>
void _debug(const First &first, const Rest &...rest) {
  cerr << first << " ";
  _debug(rest...);
}

typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<vl> vvl;

const double PI = (1 * acos(0.0));
const double EPS = 1e-9;
const int INF = 0x3f3f3f3f;
const ll INFL = 0x3f3f3f3f3f3f3f3fLL;
const ll mod = 1e9 + 7;

inline void finput(string filename) { freopen(filename.c_str(), "r", stdin); }

vi g[300];
vi cost;
bool dfs(int v, int c = 1) {
  cost[v] = c;
  for (auto u : g[v]) {
    if (cost[u] == 0 && !dfs(v, c + 1))
      return false;
    if (abs(cost[v] - cost[u]) % 2 == 0)
      return false;
  }
  return true;
}

int bfs(int s) {
  int res = 0;
  queue<int> Q;
  Q.emplace(s);
  cost[s] = 1;
  while (!Q.empty()) {
    int v = Q.front();
    Q.pop();
    res = max(res, cost[v]);
    for (auto u : g[v]) {
      if (cost[u] == 0) {
        Q.emplace(u);
        cost[u] = cost[v] + 1;
      }
    }
  }
  return res;
}

int main() {
  ios_base::sync_with_stdio(0);
  // finput("./input");
  int n;
  cin >> n;
  vector<string> s(n);
  rep(i, n) cin >> s[i];
  cost = vi(n);

  rep(i, n) rep(j, n) {
    if (s[i][j] == '1')
      g[i].push_back(j);
  }
  fill(all(cost), 0);
  if (!dfs(0)) {
    cout << -1 << endl;
  } else {
    int ans = 0;
    rep(i, n) {
      fill(all(cost), 0);
      ans = max(ans, bfs(i));
    }
    cout << ans << endl;
  }
  return 0;
}

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

#define REP(i, m, n) for (int i = (m); i < (int)(n); i++)
#define RREP(i, m, n) for (int i = (int)(n - 1); i >= m; i--)
#define rep(i, n) REP(i, 0, n)
#define rrep(i, n) RREP(i, 0, n)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define fi first
#define se second
#define debug(...)                                                             \
  {                                                                            \
    cerr << "[L" << __LINE__ << "] ";                                          \
    _debug(__VA_ARGS__);                                                       \
  }

template <typename T> string join(const vector<T> &v, string del = ", ") {
  stringstream s;
  for (auto x : v)
    s << del << x;
  return s.str().substr(del.size());
}
template <typename T> ostream &operator<<(ostream &o, const vector<T> &v) {
  if (v.size())
    o << "[" << join(v) << "]";
  return o;
}
template <typename T>
ostream &operator<<(ostream &o, const vector<vector<T>> &vv) {
  int l = vv.size();
  if (l) {
    o << endl;
    rep(i, l) o << (i == 0 ? "[ " : ",\n  ") << vv[i]
                << (i == l - 1 ? " ]" : "");
  }
  return o;
}
inline void _debug() { cerr << endl; }
template <class First, class... Rest>
void _debug(const First &first, const Rest &...rest) {
  cerr << first << " ";
  _debug(rest...);
}

typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<vl> vvl;

const double PI = (1 * acos(0.0));
const double EPS = 1e-9;
const int INF = 0x3f3f3f3f;
const ll INFL = 0x3f3f3f3f3f3f3f3fLL;
const ll mod = 1e9 + 7;

inline void finput(string filename) { freopen(filename.c_str(), "r", stdin); }

vi g[300];
vi cost;
bool dfs(int v, int c = 1) {
  cost[v] = c;
  for (auto u : g[v]) {
    if (cost[u] == 0 && !dfs(u, c + 1))
      return false;
    if (abs(cost[v] - cost[u]) % 2 == 0)
      return false;
  }
  return true;
}

int bfs(int s) {
  int res = 0;
  queue<int> Q;
  Q.emplace(s);
  cost[s] = 1;
  while (!Q.empty()) {
    int v = Q.front();
    Q.pop();
    res = max(res, cost[v]);
    for (auto u : g[v]) {
      if (cost[u] == 0) {
        Q.emplace(u);
        cost[u] = cost[v] + 1;
      }
    }
  }
  return res;
}

int main() {
  ios_base::sync_with_stdio(0);
  // finput("./input");
  int n;
  cin >> n;
  vector<string> s(n);
  rep(i, n) cin >> s[i];
  cost = vi(n);

  rep(i, n) rep(j, n) {
    if (s[i][j] == '1')
      g[i].push_back(j);
  }
  fill(all(cost), 0);
  if (!dfs(0)) {
    cout << -1 << endl;
  } else {
    int ans = 0;
    rep(i, n) {
      fill(all(cost), 0);
      ans = max(ans, bfs(i));
    }
    cout << ans << endl;
  }
  return 0;
}