Problem p02803 (C++)

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

typedef long long ll;

const long long MOD = 1000000007;
long long modpow(long long a, long long n, long long m) {
  long long ans = 1;
  while (n) {
    if (n & 1) {
      ans = (ans * a) % m;
    }
    a = (a * a) % m;
    n >>= 1;
  }
  return ans;
}
long long combi(long long n, long long a) {
  long long ans = 1, ans1 = 1;
  for (long long i = n - a + 1; i <= n; i++) {
    ans *= i % MOD;
    ans %= MOD;
  }

  for (long long i = 2; i <= a; i++)
    ans1 = (ans1 * i) % MOD;
  ans1 = modpow(ans1, MOD - 2, MOD);
  return ((ans % MOD) * ans1) % MOD;
}

void dfs(string s, char mx, ll N) {
  if (s.length() == N) {
    cout << s.c_str() << endl;
  } else {
    for (char c = 'a'; c <= mx; c++) {
      dfs(s + c, ((c == mx) ? (char)(mx + 1) : mx), N);
    }
  }
}

int bfs(int i, int j, int h, int w, char tmp[][21]) {
  int ans[21][21] = {0};
  char c[21][21] = {0};
  queue<pair<int, int>> s;

  for (int i = 0; i <= h; i++) {
    for (int j = 0; j <= w; j++) {
      c[i][j] = tmp[i][j];
    }
  }

  s.push(make_pair(i, j));

  while (s.size() > 0) {

    pair<int, int> tmp = s.front();
    s.pop();
    c[tmp.first][tmp.second] = '#';
    for (int i = -1; i <= 1; i++) {
      for (int j = -1; j <= 1; j++) {
        if (tmp.first + i < 1 || tmp.first + i > h) {
          continue;
        }
        if (tmp.second + j < 1 || tmp.second + j > w) {
          continue;
        }
        if (i != 0 && j != 0) {
          continue;
        }
        if (i == 0 && j == 0) {
          continue;
        }
        if (c[tmp.first + i][tmp.second + j] == '#') {
          continue;
        }

        if (ans[tmp.first + i][tmp.second + j] == 0) {
          ans[tmp.first + i][tmp.second + j] = ans[tmp.first][tmp.second] + 1;
        } else {
          ans[tmp.first + i][tmp.second + j] =
              min(ans[tmp.first + i][tmp.second + j],
                  ans[tmp.first][tmp.second] + 1);
        }
        s.push(make_pair(tmp.first + i, tmp.second + j));
      }
    }
  }

  int a = 0;

  for (int i = 1; i <= h; i++) {
    for (int j = 1; j <= w; j++) {
      a = max(a, ans[i][j]);
    }
  }

  return a;
}

int main() {
  ll h, w;
  cin >> h >> w;

  char c[21][21] = {0};

  for (int i = 1; i <= h; i++) {
    for (int j = 1; j <= w; j++) {
      cin >> c[i][j];
    }
  }

  int a = 0;
  for (int i = 1; i <= h; i++) {
    for (int j = 1; j <= w; j++) {
      if (c[i][j] == '#') {
        continue;
      }

      a = max(a, bfs(i, j, h, w, c));
    }
  }
  cout << a << endl;
  return 0;
}


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

typedef long long ll;

const long long MOD = 1000000007;
long long modpow(long long a, long long n, long long m) {
  long long ans = 1;
  while (n) {
    if (n & 1) {
      ans = (ans * a) % m;
    }
    a = (a * a) % m;
    n >>= 1;
  }
  return ans;
}
long long combi(long long n, long long a) {
  long long ans = 1, ans1 = 1;
  for (long long i = n - a + 1; i <= n; i++) {
    ans *= i % MOD;
    ans %= MOD;
  }

  for (long long i = 2; i <= a; i++)
    ans1 = (ans1 * i) % MOD;
  ans1 = modpow(ans1, MOD - 2, MOD);
  return ((ans % MOD) * ans1) % MOD;
}

void dfs(string s, char mx, ll N) {
  if (s.length() == N) {
    cout << s.c_str() << endl;
  } else {
    for (char c = 'a'; c <= mx; c++) {
      dfs(s + c, ((c == mx) ? (char)(mx + 1) : mx), N);
    }
  }
}

int bfs(int i, int j, int h, int w, char tmp[][21]) {
  int ans[21][21] = {0};
  char c[21][21] = {0};
  queue<pair<int, int>> s;

  for (int i = 0; i <= h; i++) {
    for (int j = 0; j <= w; j++) {
      c[i][j] = tmp[i][j];
    }
  }

  s.push(make_pair(i, j));

  while (s.size() > 0) {

    pair<int, int> tmp = s.front();
    s.pop();
    c[tmp.first][tmp.second] = '#';
    for (int i = -1; i <= 1; i++) {
      for (int j = -1; j <= 1; j++) {
        if (tmp.first + i < 1 || tmp.first + i > h) {
          continue;
        }
        if (tmp.second + j < 1 || tmp.second + j > w) {
          continue;
        }
        if (i != 0 && j != 0) {
          continue;
        }
        if (i == 0 && j == 0) {
          continue;
        }
        if (c[tmp.first + i][tmp.second + j] == '#') {
          continue;
        }
        c[tmp.first + i][tmp.second + j] = '#';
        if (ans[tmp.first + i][tmp.second + j] == 0) {
          ans[tmp.first + i][tmp.second + j] = ans[tmp.first][tmp.second] + 1;
        } else {
          ans[tmp.first + i][tmp.second + j] =
              min(ans[tmp.first + i][tmp.second + j],
                  ans[tmp.first][tmp.second] + 1);
        }
        s.push(make_pair(tmp.first + i, tmp.second + j));
      }
    }
  }

  int a = 0;

  for (int i = 1; i <= h; i++) {
    for (int j = 1; j <= w; j++) {
      a = max(a, ans[i][j]);
    }
  }

  return a;
}

int main() {
  ll h, w;
  cin >> h >> w;

  char c[21][21] = {0};

  for (int i = 1; i <= h; i++) {
    for (int j = 1; j <= w; j++) {
      cin >> c[i][j];
    }
  }

  int a = 0;
  for (int i = 1; i <= h; i++) {
    for (int j = 1; j <= w; j++) {
      if (c[i][j] == '#') {
        continue;
      }

      a = max(a, bfs(i, j, h, w, c));
    }
  }
  cout << a << endl;
  return 0;
}