Problem p02579 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
#define rep(i, n) for (long long int i = 0; i < n; i++)
#define _rep(i, m, n) for (long long int i = m; i < n; i++)
using namespace std;
typedef long long ll;
typedef pair<int, int> P;
const ll mod = 1000000007;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};

ll gcd(ll A, ll B) {
  if (B == 0)
    return A;
  return gcd(B, A % B);
}
ll lcm(ll A, ll B) { return A * B / gcd(A, B); }
template <typename T> T pow(T a, ll n, T e = 1) {
  T ret = e;
  while (n) {
    if (n & 1)
      ret *= a;
    a *= a;
    n >>= 1;
  }
  return ret;
}

template <int mod> struct ModInt {
  int x;
  ModInt() : x(0) {}
  ModInt(long long x_) {
    if ((x = x_ % mod + mod) >= mod)
      x -= mod;
  }
  ModInt &operator+=(ModInt rhs) {
    if ((x += rhs.x) >= mod)
      x -= mod;
    return *this;
  }
  ModInt &operator-=(ModInt rhs) {
    if ((x -= rhs.x) < 0)
      x += mod;
    return *this;
  }
  ModInt &operator*=(ModInt rhs) {
    x = (unsigned long long)x * rhs.x % mod;
    return *this;
  }
  ModInt &operator/=(ModInt rhs) {
    x = (unsigned long long)x * rhs.inv().x % mod;
    return *this;
  }

  ModInt operator-() const { return -x < 0 ? mod - x : -x; }
  ModInt operator+(ModInt rhs) const { return ModInt(*this) += rhs; }
  ModInt operator-(ModInt rhs) const { return ModInt(*this) -= rhs; }
  ModInt operator*(ModInt rhs) const { return ModInt(*this) *= rhs; }
  ModInt operator/(ModInt rhs) const { return ModInt(*this) /= rhs; }
  bool operator==(ModInt rhs) const { return x == rhs.x; }
  bool operator!=(ModInt rhs) const { return x != rhs.x; }
  ModInt inv() const { return pow(*this, mod - 2); }

  friend ostream &operator<<(ostream &s, ModInt<mod> a) {
    s << a.x;
    return s;
  }
  friend istream &operator>>(istream &s, ModInt<mod> &a) {
    s >> a.x;
    return s;
  }
};

using mint = ModInt<1000000007>;
using Graph = vector<vector<int>>;
Graph G;
/*------------------------------------------------------------------*/
int h, w;
char s[1010][1010];

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  cin >> h >> w;
  int ch, cw, dh, dw;
  cin >> ch >> cw >> dh >> dw;
  ch--, cw--, dh--, dw--;
  rep(i, h) rep(j, w) cin >> s[i][j];

  int INF = 1e8;

  deque<pair<int, int>> dq;
  dq.push_back({ch, cw});
  vector<vector<int>> dist(h, vector<int>(w, INF));
  dist[ch][cw] = 0;

  while (!dq.empty()) {
    int x = dq.front().first;
    int y = dq.front().second;
    dq.pop_front();

    for (int i = -2; i <= 2; i++) {
      for (int j = -2; j <= 2; j++) {
        int nx = x + i;
        int ny = y + j;
        if (nx < 0 or nx >= h or ny < 0 or ny >= w)
          continue;
        if (s[nx][ny] == '#')
          continue;
        if (i == 0 and j == 0)
          continue;
        if (abs(i) + abs(j) <= 1 and dist[x][y] < dist[nx][ny]) {
          dist[nx][ny] = dist[x][y];
          dq.push_back({nx, ny});
        }
        if (dist[x][y] + 1 < dist[nx][ny]) {
          dist[nx][ny] = dist[x][y] + 1;
          dq.push_back({nx, ny});
        }
      }
    }
  }
  if (dist[dh][dw] < INF)
    cout << dist[dh][dw] << endl;
  else
    cout << -1 << endl;
}

Fixed submission:
#include <bits/stdc++.h>
#define rep(i, n) for (long long int i = 0; i < n; i++)
#define _rep(i, m, n) for (long long int i = m; i < n; i++)
using namespace std;
typedef long long ll;
typedef pair<int, int> P;
const ll mod = 1000000007;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};

ll gcd(ll A, ll B) {
  if (B == 0)
    return A;
  return gcd(B, A % B);
}
ll lcm(ll A, ll B) { return A * B / gcd(A, B); }
template <typename T> T pow(T a, ll n, T e = 1) {
  T ret = e;
  while (n) {
    if (n & 1)
      ret *= a;
    a *= a;
    n >>= 1;
  }
  return ret;
}

template <int mod> struct ModInt {
  int x;
  ModInt() : x(0) {}
  ModInt(long long x_) {
    if ((x = x_ % mod + mod) >= mod)
      x -= mod;
  }
  ModInt &operator+=(ModInt rhs) {
    if ((x += rhs.x) >= mod)
      x -= mod;
    return *this;
  }
  ModInt &operator-=(ModInt rhs) {
    if ((x -= rhs.x) < 0)
      x += mod;
    return *this;
  }
  ModInt &operator*=(ModInt rhs) {
    x = (unsigned long long)x * rhs.x % mod;
    return *this;
  }
  ModInt &operator/=(ModInt rhs) {
    x = (unsigned long long)x * rhs.inv().x % mod;
    return *this;
  }

  ModInt operator-() const { return -x < 0 ? mod - x : -x; }
  ModInt operator+(ModInt rhs) const { return ModInt(*this) += rhs; }
  ModInt operator-(ModInt rhs) const { return ModInt(*this) -= rhs; }
  ModInt operator*(ModInt rhs) const { return ModInt(*this) *= rhs; }
  ModInt operator/(ModInt rhs) const { return ModInt(*this) /= rhs; }
  bool operator==(ModInt rhs) const { return x == rhs.x; }
  bool operator!=(ModInt rhs) const { return x != rhs.x; }
  ModInt inv() const { return pow(*this, mod - 2); }

  friend ostream &operator<<(ostream &s, ModInt<mod> a) {
    s << a.x;
    return s;
  }
  friend istream &operator>>(istream &s, ModInt<mod> &a) {
    s >> a.x;
    return s;
  }
};

using mint = ModInt<1000000007>;
using Graph = vector<vector<int>>;
Graph G;
/*------------------------------------------------------------------*/
int h, w;
char s[1010][1010];

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  cin >> h >> w;
  int ch, cw, dh, dw;
  cin >> ch >> cw >> dh >> dw;
  ch--, cw--, dh--, dw--;
  rep(i, h) rep(j, w) cin >> s[i][j];

  int INF = 1e8;

  deque<pair<int, int>> dq;
  dq.push_back({ch, cw});
  vector<vector<int>> dist(h, vector<int>(w, INF));
  dist[ch][cw] = 0;

  while (!dq.empty()) {
    int x = dq.front().first;
    int y = dq.front().second;
    dq.pop_front();

    for (int i = -2; i <= 2; i++) {
      for (int j = -2; j <= 2; j++) {
        int nx = x + i;
        int ny = y + j;
        if (nx < 0 or nx >= h or ny < 0 or ny >= w)
          continue;
        if (s[nx][ny] == '#')
          continue;
        if (i == 0 and j == 0)
          continue;
        if (abs(i) + abs(j) <= 1 and dist[x][y] < dist[nx][ny]) {
          dist[nx][ny] = dist[x][y];
          dq.push_front({nx, ny});
        }
        if (dist[x][y] + 1 < dist[nx][ny]) {
          dist[nx][ny] = dist[x][y] + 1;
          dq.push_back({nx, ny});
        }
      }
    }
  }
  if (dist[dh][dw] < INF)
    cout << dist[dh][dw] << endl;
  else
    cout << -1 << endl;
}