Problem p03436 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
#define For(i, a, b) for (int(i) = (int)(a); (i) < (int)(b); ++(i))
#define rFor(i, a, b) for (int(i) = (int)(a)-1; (i) >= (int)(b); --(i))
#define rep(i, n) For((i), 0, (n))
#define rrep(i, n) rFor((i), (n), 0)
#define fi first
#define se second
using namespace std;
typedef long long lint;
typedef unsigned long long ulint;
typedef pair<int, int> pii;
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> T div_floor(T a, T b) {
  if (b < 0)
    a *= -1, b *= -1;
  return a >= 0 ? a / b : (a + 1) / b - 1;
}
template <class T> T div_ceil(T a, T b) {
  if (b < 0)
    a *= -1, b *= -1;
  return a > 0 ? (a - 1) / b + 1 : a / b;
}

constexpr lint mod = 1e9 + 7;
constexpr lint INF = mod * mod;
constexpr int MAX = 200010;

int h, w;
char c[55][55];
int dx[4] = {0, 1, 0, -1};
int dy[4] = {1, 0, -1, 0};

int dfs() {
  using ppi = pair<pii, int>;
  queue<ppi> que;
  que.push({{0, 0}, 0});
  bool used[h][w];
  rep(i, h) rep(j, w) used[i][j] = false;
  while (!que.empty()) {
    int x, y, d;
    tie(x, y) = que.front().fi;
    d = que.front().se;
    if (x == h - 1 && y == w - 1)
      return d;
    used[x][y] = true;
    que.pop();
    rep(i, 4) {
      int nx = x + dx[i], ny = y + dy[i];
      if (0 <= nx && nx < h && 0 <= ny && ny < w && c[nx][ny] != '#' &&
          !used[nx][ny]) {
        que.push({{nx, ny}, d + 1});
      }
    }
  }
  return mod;
}

int main() {
  scanf("%d%d", &h, &w);
  int cnt = 0;
  rep(i, h) rep(j, w) scanf(" %c", &c[i][j]), cnt += (c[i][j] == '.');
  int d = dfs();
  printf("%d\n", d == mod ? -1 : cnt - 1 - d);
}

Fixed submission:
#include <bits/stdc++.h>
#define For(i, a, b) for (int(i) = (int)(a); (i) < (int)(b); ++(i))
#define rFor(i, a, b) for (int(i) = (int)(a)-1; (i) >= (int)(b); --(i))
#define rep(i, n) For((i), 0, (n))
#define rrep(i, n) rFor((i), (n), 0)
#define fi first
#define se second
using namespace std;
typedef long long lint;
typedef unsigned long long ulint;
typedef pair<int, int> pii;
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> T div_floor(T a, T b) {
  if (b < 0)
    a *= -1, b *= -1;
  return a >= 0 ? a / b : (a + 1) / b - 1;
}
template <class T> T div_ceil(T a, T b) {
  if (b < 0)
    a *= -1, b *= -1;
  return a > 0 ? (a - 1) / b + 1 : a / b;
}

constexpr lint mod = 1e9 + 7;
constexpr lint INF = mod * mod;
constexpr int MAX = 200010;

int h, w;
char c[55][55];
int dx[4] = {0, 1, 0, -1};
int dy[4] = {1, 0, -1, 0};

int dfs() {
  using ppi = pair<pii, int>;
  queue<ppi> que;
  que.push({{0, 0}, 0});
  bool used[h][w];
  rep(i, h) rep(j, w) used[i][j] = false;
  while (!que.empty()) {
    int x, y, d;
    tie(x, y) = que.front().fi;
    d = que.front().se;
    if (x == h - 1 && y == w - 1)
      return d;
    used[x][y] = true;
    que.pop();
    rep(i, 4) {
      int nx = x + dx[i], ny = y + dy[i];
      if (0 <= nx && nx < h && 0 <= ny && ny < w && c[nx][ny] != '#' &&
          !used[nx][ny]) {
        used[nx][ny] = true;
        que.push({{nx, ny}, d + 1});
      }
    }
  }
  return mod;
}

int main() {
  scanf("%d%d", &h, &w);
  int cnt = 0;
  rep(i, h) rep(j, w) scanf(" %c", &c[i][j]), cnt += (c[i][j] == '.');
  int d = dfs();
  printf("%d\n", d == mod ? -1 : cnt - 1 - d);
}