Problem p02892 (C++)

Buggy submission (status: Time Limit Exceeded):
#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using datas = pair<ll, ll>;
using ddatas = pair<double, double>;
using tdata = pair<ll, datas>;
using vec = vector<ll>;
using mat = vector<vec>;
using pvec = vector<datas>;
using pmat = vector<pvec>;
#define For(i, a, b) for (i = a; i < b; i++)
#define bFor(i, a, b) for (i = a; i >= b; i--)
#define rep(i, N) For(i, 0, N)
#define rep1(i, N) For(i, 1, N)
#define brep(i, N) bFor(i, N - 1, 0)
#define all(v) (v).begin(), (v).end()
#define allr(v) (v).rbegin(), (v).rend()
#define vsort(v) sort(all(v))
#define vrsort(v) sort(allr(v))
#define endl "\n"
#define pb push_back
#define output(v)                                                              \
  do {                                                                         \
    bool f = 0;                                                                \
    for (auto outi : v) {                                                      \
      cout << (f ? " " : "") << outi;                                          \
      f = 1;                                                                   \
    }                                                                          \
    cout << endl;                                                              \
  } while (0)
const ll mod = 1000000007;
const ll inf = 1LL << 60;
template <class T> inline bool chmax(T &a, T b) {
  bool x = a < b;
  if (x)
    a = b;
  return x;
}
template <class T> inline bool chmin(T &a, T b) {
  bool x = a > b;
  if (x)
    a = b;
  return x;
}

double distance(ddatas &x, ddatas &y) {
  double a = x.first - y.first, b = x.second - y.second;
  return sqrt(a * a + b * b);
}

ll modinv(ll a) {
  ll b = mod, u = 1, v = 0, t;
  while (b) {
    t = a / b;
    a -= t * b;
    swap(a, b);
    u -= t * v;
    swap(u, v);
  }
  return (u + mod) % mod;
}

ll moddevide(ll a, ll b) { return (a * modinv(b)) % mod; }

vec modncrlistp, modncrlistm;

ll modncr(ll n, ll r) {
  ll i, size = modncrlistp.size();
  if (size <= n) {
    modncrlistp.resize(n + 1);
    modncrlistm.resize(n + 1);
    if (!size) {
      modncrlistp[0] = modncrlistm[0] = 1;
      size++;
    }
    For(i, size, n + 1) {
      modncrlistp[i] = modncrlistp[i - 1] * i % mod;
      modncrlistm[i] = modinv(modncrlistp[i]);
    }
  }
  return modncrlistp[n] * modncrlistm[r] % mod * modncrlistm[n - r] % mod;
}

ll modpow(ll a, ll n) {
  ll res = 1;
  while (n) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}

ll gcd(ll a, ll b) {
  if (!b)
    return a;
  return (a % b == 0) ? b : gcd(b, a % b);
}
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }

ll countdigits(ll n) {
  ll ans = 0;
  while (n) {
    n /= 10;
    ans++;
  }
  return ans;
}

ll sumdigits(ll n) {
  ll ans = 0;
  while (n) {
    ans += n % 10;
    n /= 10;
  }
  return ans;
}

int main() {
  ll i, j, k, n, ans = 0;
  cin >> n;
  string s;
  mat G(n), g(n, vec(n));
  vec color(n, -1);
  rep(i, n) {
    cin >> s;
    rep(j, n) {
      g[i][j] = (s[j] == '1') ? 1 : inf;
      if (g[i][j] == 1) {
        G[i].pb(j);
      }
    }
    g[i][i] = 0;
  }
  color[0] = 0;
  queue<datas> que;
  for (auto i : G[0]) {
    que.push(datas(i, 1));
  }
  while (!que.empty()) {
    auto x = que.front();
    que.pop();
    if (color[x.first] != -1 && color[x.first] != x.second) {
      cout << -1 << endl;
      return 0;
    }
    color[x.first] = x.second;
    for (auto i : G[x.first]) {
      if (color[i] != -1) {
        if (color[i] == x.second) {
          cout << -1 << endl;
          return 0;
        }
      } else {
        que.push(datas(i, 1 - x.second));
      }
    }
  }
  rep(k, n) {
    rep(i, n) {
      rep(j, n) { g[i][j] = min(g[i][j], g[i][k] + g[k][j]); }
    }
  }
  rep(i, n) {
    ll res = *max_element(all(g[i]));
    ans = max(res, ans);
  }
  cout << ans + 1 << endl;
  return 0;
}

Fixed submission:
#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using datas = pair<ll, ll>;
using ddatas = pair<double, double>;
using tdata = pair<ll, datas>;
using vec = vector<ll>;
using mat = vector<vec>;
using pvec = vector<datas>;
using pmat = vector<pvec>;
#define For(i, a, b) for (i = a; i < b; i++)
#define bFor(i, a, b) for (i = a; i >= b; i--)
#define rep(i, N) For(i, 0, N)
#define rep1(i, N) For(i, 1, N)
#define brep(i, N) bFor(i, N - 1, 0)
#define all(v) (v).begin(), (v).end()
#define allr(v) (v).rbegin(), (v).rend()
#define vsort(v) sort(all(v))
#define vrsort(v) sort(allr(v))
#define endl "\n"
#define pb push_back
#define output(v)                                                              \
  do {                                                                         \
    bool f = 0;                                                                \
    for (auto outi : v) {                                                      \
      cout << (f ? " " : "") << outi;                                          \
      f = 1;                                                                   \
    }                                                                          \
    cout << endl;                                                              \
  } while (0)
const ll mod = 1000000007;
const ll inf = 1LL << 60;
template <class T> inline bool chmax(T &a, T b) {
  bool x = a < b;
  if (x)
    a = b;
  return x;
}
template <class T> inline bool chmin(T &a, T b) {
  bool x = a > b;
  if (x)
    a = b;
  return x;
}

double distance(ddatas &x, ddatas &y) {
  double a = x.first - y.first, b = x.second - y.second;
  return sqrt(a * a + b * b);
}

ll modinv(ll a) {
  ll b = mod, u = 1, v = 0, t;
  while (b) {
    t = a / b;
    a -= t * b;
    swap(a, b);
    u -= t * v;
    swap(u, v);
  }
  return (u + mod) % mod;
}

ll moddevide(ll a, ll b) { return (a * modinv(b)) % mod; }

vec modncrlistp, modncrlistm;

ll modncr(ll n, ll r) {
  ll i, size = modncrlistp.size();
  if (size <= n) {
    modncrlistp.resize(n + 1);
    modncrlistm.resize(n + 1);
    if (!size) {
      modncrlistp[0] = modncrlistm[0] = 1;
      size++;
    }
    For(i, size, n + 1) {
      modncrlistp[i] = modncrlistp[i - 1] * i % mod;
      modncrlistm[i] = modinv(modncrlistp[i]);
    }
  }
  return modncrlistp[n] * modncrlistm[r] % mod * modncrlistm[n - r] % mod;
}

ll modpow(ll a, ll n) {
  ll res = 1;
  while (n) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}

ll gcd(ll a, ll b) {
  if (!b)
    return a;
  return (a % b == 0) ? b : gcd(b, a % b);
}
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }

ll countdigits(ll n) {
  ll ans = 0;
  while (n) {
    n /= 10;
    ans++;
  }
  return ans;
}

ll sumdigits(ll n) {
  ll ans = 0;
  while (n) {
    ans += n % 10;
    n /= 10;
  }
  return ans;
}

int main() {
  ll i, j, k, n, ans = 0;
  cin >> n;
  string s;
  mat G(n), g(n, vec(n));
  vec color(n, -1);
  rep(i, n) {
    cin >> s;
    rep(j, n) {
      g[i][j] = (s[j] == '1') ? 1 : inf;
      if (g[i][j] == 1) {
        G[i].pb(j);
      }
    }
    g[i][i] = 0;
  }
  color[0] = 0;
  queue<datas> que;
  for (auto i : G[0]) {
    que.push(datas(i, 1));
  }
  while (!que.empty()) {
    auto x = que.front();
    que.pop();
    if (color[x.first] != -1) {
      if (color[x.first] != x.second) {
        cout << -1 << endl;
        return 0;
      }
      continue;
    }
    color[x.first] = x.second;
    for (auto i : G[x.first]) {
      if (color[i] != -1) {
        if (color[i] == x.second) {
          cout << -1 << endl;
          return 0;
        }
      } else {
        que.push(datas(i, 1 - x.second));
      }
    }
  }
  rep(k, n) {
    rep(i, n) {
      rep(j, n) { g[i][j] = min(g[i][j], g[i][k] + g[k][j]); }
    }
  }
  rep(i, n) {
    ll res = *max_element(all(g[i]));
    ans = max(res, ans);
  }
  cout << ans + 1 << endl;
  return 0;
}