Problem p02803 (C++)

Buggy submission (status: Runtime Error):
/*Function Template*/
#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef pair<ll, ll> pint;
const int MAX = 510000;
const int MOD = 1000000007;
#define rep(i, n) for (ll i = 0; i < (n); i++)
#define Rep(i, n) for (ll i = 1; i < (n); i++)
#define ALL(a) (a).begin(), (a).end()
#define IOS                                                                    \
  ios::sync_with_stdio(0);                                                     \
  cin.tie(0);                                                                  \
  cout.tie(0);
ll fac[MAX], finv[MAX], inv[MAX];

template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
// テーブルを作る前処理
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (ll i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}
ll Len(ll n) {
  ll s = 0;
  while (n != 0)
    s++, n /= 10;
  return s;
}

ll Sint(ll n) {
  ll m = 0, s = 0, a = n;
  while (a != 0)
    s++, a /= 10;
  for (ll i = s - 1; i >= 0; i--)
    m += n / ((ll)pow(10, i)) - (n / ((ll)pow(10, i + 1))) * 10;
  return m;
}

ll Svec(vector<ll> v) {
  ll n = 0;
  for (ll i = 0; i < v.size(); i++)
    n += v[i];
  return n;
}

ll GCD(ll a, ll b) { return b ? GCD(b, a % b) : a; }

ll LCM(ll a, ll b) { return a / GCD(a, b) * b; }

ll Factorial(ll n) {
  ll m = 1;
  while (n >= 1)
    m *= n, n--;
  return m;
}

void runlength(string s, vector<pair<char, ll>> &p) {
  ll x = 1;
  if (s.size() == 1) {
    p.push_back(pair<char, ll>(s[0], 1));
  }
  for (ll i = 0; i < s.size() - 1; i++) {
    if (s[i] == s[i + 1]) {
      x++;
      if (i == s.size() - 2) {
        p.push_back(pair<char, ll>(s[i], x));
      }
    } else {
      p.push_back(pair<char, ll>(s[i], x));
      x = 1;
      if (i == s.size() - 2) {
        p.push_back(pair<char, ll>(s[s.size() - 1], x));
      }
    }
  }
}

ll COM(ll n, ll k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

const int MAX_N = 100010;
vector<bool> sieve_of_eratosthenes() {
  vector<bool> isPrime(MAX_N + 1, true);
  for (int i = 2; i <= MAX_N; i++) {
    if (isPrime[i]) {
      for (int j = 2 * i; j <= MAX_N; j += i) {
        isPrime[j] = false;
      }
    }
  }
  return isPrime;
}

vector<pint> prime_factorize(ll n) {
  vector<pint> ans;
  for (ll p = 2; p <= sqrt(n); p++) {
    if (n % p != 0)
      continue;
    ll cnt = 0;
    while (n % p == 0) {
      n /= p;
      cnt++;
    }
    ans.push_back(make_pair(p, cnt));
  }
  if (n != 1)
    ans.push_back(make_pair(n, 1));
  return ans;
}

/*↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓*/
int main() {
  IOS;
  ll H, W, ans = 0, dx[4] = {-1, 1, 0, 0}, dy[4] = {0, 0, -1, 1};
  cin >> H >> W;
  vector<string> v(H);
  rep(i, H) cin >> v[i];
  vector<vector<ll>> w(H, vector<ll>(W, -1));
  rep(sy, H) {
    rep(sx, W) {
      rep(gy, H) {
        rep(gx, W) {
          if ((gx == sx && gy == sy) || v[sy][sx] == '#' || v[gy][gx] == '#')
            continue;
          rep(i, H) {
            rep(j, W) {
              if (v[i][j] == '#')
                w[i][j] = -2;
              else
                w[i][j] = -1;
            }
          }
          w[sy][sx] = 0;
          queue<pint> que;
          que.push(pint(sy, sx));
          while (!que.empty()) {
            auto c = que.front();
            que.pop();
            rep(i, 4) {
              ll nx = c.second + dx[i];
              ll ny = c.first + dy[i];
              if (nx < 0 || W <= nx || ny < 0 || H <= ny)
                continue;
              if (w[ny][nx] == -1)
                w[ny][nx] = w[c.second][c.first] + 1, que.push(pint(ny, nx));
            }
          }
          chmax(ans, w[gy][gx]);
        }
      }
    }
  }
  cout << ans << endl;
}

Fixed submission:
/*Function Template*/
#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef pair<ll, ll> pint;
const int MAX = 510000;
const int MOD = 1000000007;
#define rep(i, n) for (ll i = 0; i < (n); i++)
#define Rep(i, n) for (ll i = 1; i < (n); i++)
#define ALL(a) (a).begin(), (a).end()
#define IOS                                                                    \
  ios::sync_with_stdio(0);                                                     \
  cin.tie(0);                                                                  \
  cout.tie(0);
ll fac[MAX], finv[MAX], inv[MAX];

template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
// テーブルを作る前処理
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (ll i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}
ll Len(ll n) {
  ll s = 0;
  while (n != 0)
    s++, n /= 10;
  return s;
}

ll Sint(ll n) {
  ll m = 0, s = 0, a = n;
  while (a != 0)
    s++, a /= 10;
  for (ll i = s - 1; i >= 0; i--)
    m += n / ((ll)pow(10, i)) - (n / ((ll)pow(10, i + 1))) * 10;
  return m;
}

ll Svec(vector<ll> v) {
  ll n = 0;
  for (ll i = 0; i < v.size(); i++)
    n += v[i];
  return n;
}

ll GCD(ll a, ll b) { return b ? GCD(b, a % b) : a; }

ll LCM(ll a, ll b) { return a / GCD(a, b) * b; }

ll Factorial(ll n) {
  ll m = 1;
  while (n >= 1)
    m *= n, n--;
  return m;
}

void runlength(string s, vector<pair<char, ll>> &p) {
  ll x = 1;
  if (s.size() == 1) {
    p.push_back(pair<char, ll>(s[0], 1));
  }
  for (ll i = 0; i < s.size() - 1; i++) {
    if (s[i] == s[i + 1]) {
      x++;
      if (i == s.size() - 2) {
        p.push_back(pair<char, ll>(s[i], x));
      }
    } else {
      p.push_back(pair<char, ll>(s[i], x));
      x = 1;
      if (i == s.size() - 2) {
        p.push_back(pair<char, ll>(s[s.size() - 1], x));
      }
    }
  }
}

ll COM(ll n, ll k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

const int MAX_N = 100010;
vector<bool> sieve_of_eratosthenes() {
  vector<bool> isPrime(MAX_N + 1, true);
  for (int i = 2; i <= MAX_N; i++) {
    if (isPrime[i]) {
      for (int j = 2 * i; j <= MAX_N; j += i) {
        isPrime[j] = false;
      }
    }
  }
  return isPrime;
}

vector<pint> prime_factorize(ll n) {
  vector<pint> ans;
  for (ll p = 2; p <= sqrt(n); p++) {
    if (n % p != 0)
      continue;
    ll cnt = 0;
    while (n % p == 0) {
      n /= p;
      cnt++;
    }
    ans.push_back(make_pair(p, cnt));
  }
  if (n != 1)
    ans.push_back(make_pair(n, 1));
  return ans;
}

/*↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓*/
int main() {
  IOS;
  ll H, W, ans = 0, dx[4] = {-1, 1, 0, 0}, dy[4] = {0, 0, -1, 1};
  cin >> H >> W;
  vector<string> v(H);
  rep(i, H) cin >> v[i];
  vector<vector<ll>> w(H, vector<ll>(W, -1));
  rep(sy, H) {
    rep(sx, W) {
      rep(gy, H) {
        rep(gx, W) {
          if ((gx == sx && gy == sy) || v[sy][sx] == '#' || v[gy][gx] == '#')
            continue;
          rep(i, H) {
            rep(j, W) {
              if (v[i][j] == '#')
                w[i][j] = -2;
              else
                w[i][j] = -1;
            }
          }
          w[sy][sx] = 0;
          queue<pint> que;
          que.push(pint(sy, sx));
          while (!que.empty()) {
            auto c = que.front();
            que.pop();
            rep(i, 4) {
              ll nx = c.second + dx[i];
              ll ny = c.first + dy[i];
              if (nx < 0 || W <= nx || ny < 0 || H <= ny)
                continue;
              if (w[ny][nx] == -1)
                w[ny][nx] = w[c.first][c.second] + 1, que.push(pint(ny, nx));
            }
          }
          chmax(ans, w[gy][gx]);
        }
      }
    }
  }
  cout << ans << endl;
}