Problem p02803 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cctype>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <time.h>
#include <tuple>
#include <utility>
#include <vector>
#define ll long long
#define itn int
#define endl '\n'
#define co(ans) cout << ans << endl
#define COYE cout << "YES" << endl
#define COYe cout << "Yes" << endl
#define COye cout << "yes" << endl
#define CONO cout << "NO" << endl
#define CONo cout << "No" << endl
#define COno cout << "no" << endl
#define FORE(i, a) for (auto &i : a)
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define FFOR(i, a, b) for (int i = (a); i <= (b); ++i)
#define REP(i, n) FOR(i, 0, n)
#define RREP(i, n) FFOR(i, 1, n)
#define PB push_back
#define MP make_pair
#define ALL(V) (V).begin(), (V).end()
#define SORT(V) sort((V).begin(), (V).end())
#define REVERSE(V) reverse((V).begin(), (V).end())
#define EACH(V, i)                                                             \
  for (typeof((V).begin()) i = (V).begin(); i != (V).end(); ++i)
#define equals(a, b) (fabs((a) - (b)) < EPS)
#define INF ((1LL << 62) - (1LL << 31))
#define EPS 1e-10
#define PI 3.141592653589793238
#define MOD 1000000007
#define MAX 5100000
using namespace std;
using Edge = pair<ll, ll>;
using Graph = vector<vector<int>>;
inline int toInt(string s) {
  int v;
  istringstream sin(s);
  sin >> v;
  return v;
}
template <class T> inline string toString(T x) {
  ostringstream sout;
  sout << x;
  return sout.str();
}
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;
const int dx[4] = {0, 1, 0, -1}, dy[4] = {1, 0, -1, 0};
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; }

int main() {
  ll H, W, ans = 0;
  cin >> H >> W;
  char S[H][W];
  REP(i, H) {
    REP(j, W) { cin >> S[i][j]; }
  }

  REP(i, H) {
    REP(j, W) {
      if (S[i][j] == '#')
        continue;
      ll DIS[H][W];
      bool CAN[H][W];
      REP(k, H) {
        REP(l, W) {
          DIS[k][l] = MAX;
          CAN[k][l] = false;
        }
      }
      DIS[i][j] = 0;

      queue<pair<ll, ll>> Q;
      Q.push(MP(i, j));
      while (!Q.empty()) {
        auto it = Q.front();
        Q.pop();
        ll vx = it.first, vy = it.second;
        CAN[vx][vy] = true;
        REP(k, 4) {
          ll nx = vx + dx[k], ny = vy + dy[k];
          if (nx < 0 || H <= nx || ny < 0 || W <= ny)
            continue;
          if (S[nx][ny] == '#' || CAN[nx][ny])
            continue;
          chmin(DIS[nx][ny], DIS[vx][vy] + 1);
          Q.push(MP(nx, ny));
        }
      }

      REP(k, H) {
        REP(l, W) {
          if (DIS[k][l] != MAX)
            chmax(ans, DIS[k][l]);
        }
      }
    }
  }

  co(ans);
  return 0;
}

Fixed submission:
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cctype>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <time.h>
#include <tuple>
#include <utility>
#include <vector>
#define ll long long
#define itn int
#define endl '\n'
#define co(ans) cout << ans << endl
#define COYE cout << "YES" << endl
#define COYe cout << "Yes" << endl
#define COye cout << "yes" << endl
#define CONO cout << "NO" << endl
#define CONo cout << "No" << endl
#define COno cout << "no" << endl
#define FORE(i, a) for (auto &i : a)
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define FFOR(i, a, b) for (int i = (a); i <= (b); ++i)
#define REP(i, n) FOR(i, 0, n)
#define RREP(i, n) FFOR(i, 1, n)
#define PB push_back
#define MP make_pair
#define ALL(V) (V).begin(), (V).end()
#define SORT(V) sort((V).begin(), (V).end())
#define REVERSE(V) reverse((V).begin(), (V).end())
#define EACH(V, i)                                                             \
  for (typeof((V).begin()) i = (V).begin(); i != (V).end(); ++i)
#define equals(a, b) (fabs((a) - (b)) < EPS)
#define INF ((1LL << 62) - (1LL << 31))
#define EPS 1e-10
#define PI 3.141592653589793238
#define MOD 1000000007
#define MAX 5100000
using namespace std;
using Edge = pair<ll, ll>;
using Graph = vector<vector<int>>;
inline int toInt(string s) {
  int v;
  istringstream sin(s);
  sin >> v;
  return v;
}
template <class T> inline string toString(T x) {
  ostringstream sout;
  sout << x;
  return sout.str();
}
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;
const int dx[4] = {0, 1, 0, -1}, dy[4] = {1, 0, -1, 0};
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; }

int main() {
  ll H, W, ans = 0;
  cin >> H >> W;
  char S[H][W];
  REP(i, H) {
    REP(j, W) { cin >> S[i][j]; }
  }

  REP(i, H) {
    REP(j, W) {
      if (S[i][j] == '#')
        continue;
      ll DIS[H][W];
      bool CAN[H][W];
      REP(k, H) {
        REP(l, W) {
          DIS[k][l] = MAX;
          CAN[k][l] = false;
        }
      }
      DIS[i][j] = 0;

      queue<pair<ll, ll>> Q;
      Q.push(MP(i, j));
      while (!Q.empty()) {
        auto it = Q.front();
        Q.pop();
        ll vx = it.first, vy = it.second;
        CAN[vx][vy] = true;
        REP(k, 4) {
          ll nx = vx + dx[k], ny = vy + dy[k];
          if (nx < 0 || H <= nx || ny < 0 || W <= ny)
            continue;
          if (S[nx][ny] == '#' || CAN[nx][ny])
            continue;
          chmin(DIS[nx][ny], DIS[vx][vy] + 1);
          Q.push(MP(nx, ny));
          CAN[nx][ny] = true;
        }
      }

      REP(k, H) {
        REP(l, W) {
          if (DIS[k][l] != MAX)
            chmax(ans, DIS[k][l]);
        }
      }
    }
  }

  co(ans);
  return 0;
}