Problem p02803 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
using namespace std;
typedef long long ll;
typedef pair<ll, ll> PLL;
typedef vector<int> VI;
typedef vector<char> VC;
typedef vector<double> VD;
typedef vector<string> VS;
typedef vector<ll> VLL;
typedef vector<PLL> VP;
const static int INF = 1000000000;
const static int MOD = 1000000007;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
#define rep(i, n) for (ll i = 0; i < (ll)(n); i++)
#define repd(i, n) for (ll i = n - 1; i >= 0; i--)
#define rept(i, m, n) for (ll i = m; i < n; i++)
#define stl_rep(itr, x) for (auto itr = x.begin(); itr != x.end(); ++itr)
#define all(x) (x).begin(), (x).end()
#define F first
#define S second
#define PF push_front
#define PB push_back
#define SORT(V) sort((V).begin(), (V).end())
#define RVERSE(V) reverse((V).begin(), (V).end())
#define paired make_pair
#define PRINT(V)                                                               \
  for (auto v : (V))                                                           \
  cout << v << " "
// charを整数に
int ctoi(char c) {
  if (c >= '0' && c <= '9') {
    return c - '0';
  }
  return 0;
}
// 累積和 for (int i = 0; i < N; ++i) s[i+1] = s[i] + a[i];
void cum_sum(int N, vector<double> a, vector<double> &s) {
  for (int i = 0; i < N; i++) {
    s[i + 1] = s[i] + a[i];
  }
}
// ユークリッドの控除法
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
// 最小公倍数
ll lcm(ll a, ll b) {
  ll g = gcd(a, b);
  return a / g * b; // Be careful not to overflow
}
// 素数判定
bool is_prime(long long n) {
  if (n <= 1)
    return false;
  for (long long p = 2; p * p <= n; ++p) {
    if (n % p == 0)
      return false;
  }
  return true;
}
int getdigit(ll num) {
  unsigned digit = 0;
  while (num != 0) {
    num /= 10;
    digit++;
  }
  return digit;
}
// 空白文字も入力 getline(cin, S);
// 桁数指定 setprecision

int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int H, W;
  int maxi = 0;
  cin >> H >> W;
  vector<string> S(H);
  rep(i, H) { cin >> S[i]; }
  for (int i = 0; i < H; i++) {
    for (int j = 0; j < S[i].size(); j++) {
      int sx, sy;
      if (S[i][j] == '.') {
        sx = i;
        sy = j;
      }
      vector<vector<int>> dist(H, vector<int>(W, -1));
      queue<pair<int, int>> que;
      dist[sx][sy] = 0;
      que.push(make_pair(sx, sy));
      while (!que.empty()) {
        auto v = que.front();
        que.pop();
        int vx = v.F;
        int vy = v.S;
        for (int k = 0; k < 4; k++) {
          int nx = vx + dx[k];
          int ny = vy + dy[k];
          if (nx < 0 || ny < 0 || nx >= H || ny >= W)
            continue;
          if (S[nx][ny] == '#')
            continue;
          if (dist[nx][ny] != -1)
            continue;
          dist[nx][ny] = dist[vx][vy] + 1;
          que.push(make_pair(nx, ny));
        }
      }
      rep(l, H) {
        rep(m, S[l].size()) {
          if (S[l][m] != '#') {
            maxi = max(maxi, dist[l][m]);
          }
        }
      }
    }
  }
  cout << maxi << endl;
}

Fixed submission:
#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
using namespace std;
typedef long long ll;
typedef pair<ll, ll> PLL;
typedef vector<int> VI;
typedef vector<char> VC;
typedef vector<double> VD;
typedef vector<string> VS;
typedef vector<ll> VLL;
typedef vector<PLL> VP;
const static int INF = 1000000000;
const static int MOD = 1000000007;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
#define rep(i, n) for (ll i = 0; i < (ll)(n); i++)
#define repd(i, n) for (ll i = n - 1; i >= 0; i--)
#define rept(i, m, n) for (ll i = m; i < n; i++)
#define stl_rep(itr, x) for (auto itr = x.begin(); itr != x.end(); ++itr)
#define all(x) (x).begin(), (x).end()
#define F first
#define S second
#define PF push_front
#define PB push_back
#define SORT(V) sort((V).begin(), (V).end())
#define RVERSE(V) reverse((V).begin(), (V).end())
#define paired make_pair
#define PRINT(V)                                                               \
  for (auto v : (V))                                                           \
  cout << v << " "
// charを整数に
int ctoi(char c) {
  if (c >= '0' && c <= '9') {
    return c - '0';
  }
  return 0;
}
// 累積和 for (int i = 0; i < N; ++i) s[i+1] = s[i] + a[i];
void cum_sum(int N, vector<double> a, vector<double> &s) {
  for (int i = 0; i < N; i++) {
    s[i + 1] = s[i] + a[i];
  }
}
// ユークリッドの控除法
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
// 最小公倍数
ll lcm(ll a, ll b) {
  ll g = gcd(a, b);
  return a / g * b; // Be careful not to overflow
}
// 素数判定
bool is_prime(long long n) {
  if (n <= 1)
    return false;
  for (long long p = 2; p * p <= n; ++p) {
    if (n % p == 0)
      return false;
  }
  return true;
}
int getdigit(ll num) {
  unsigned digit = 0;
  while (num != 0) {
    num /= 10;
    digit++;
  }
  return digit;
}
// 空白文字も入力 getline(cin, S);
// 桁数指定 setprecision

int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int H, W;
  int maxi = 0;
  cin >> H >> W;
  vector<string> S(H);
  rep(i, H) { cin >> S[i]; }
  for (int i = 0; i < H; i++) {
    for (int j = 0; j < S[i].size(); j++) {
      int sx, sy;
      if (S[i][j] == '.') {
        sx = i;
        sy = j;
      } else {
        continue;
      }
      vector<vector<int>> dist(H, vector<int>(W, -1));
      queue<pair<int, int>> que;
      dist[sx][sy] = 0;
      que.push(make_pair(sx, sy));
      while (!que.empty()) {
        auto v = que.front();
        que.pop();
        int vx = v.F;
        int vy = v.S;
        for (int k = 0; k < 4; k++) {
          int nx = vx + dx[k];
          int ny = vy + dy[k];
          if (nx < 0 || ny < 0 || nx >= H || ny >= W)
            continue;
          if (S[nx][ny] == '#')
            continue;
          if (dist[nx][ny] != -1)
            continue;
          dist[nx][ny] = dist[vx][vy] + 1;
          que.push(make_pair(nx, ny));
        }
      }
      rep(l, H) {
        rep(m, S[l].size()) {
          if (S[l][m] != '#') {
            maxi = max(maxi, dist[l][m]);
          }
        }
      }
    }
  }
  cout << maxi << endl;
}