Problem p03436 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
template <class T> using vt = vector<T>;
template <class T> using vvt = vector<vt<T>>;
template <class T> using ttt = tuple<T, T>;
using tii = tuple<int, int>;
using tiii = tuple<int, int, int>;
using vi = vector<int>;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define pb push_back
#define mt make_tuple
#define ALL(a) (a).begin(), (a).end()
#define FST first
#define SEC second
#define DEB cerr << "!" << endl
#define SHOW(a, b) cerr << (a) << " " << (b) << endl
#define DIV int(1e9 + 7)
const int INF = (INT_MAX / 2);
const ll LLINF = (LLONG_MAX / 2);
const double eps = 1e-8;
// const double PI = M_PI;
inline ll pow(ll x, ll n, ll m) {
  ll r = 1;
  while (n > 0) {
    if ((n & 1) == 1)
      r = r * x % m;
    x = x * x % m;
    n >>= 1;
  }
  return r % m;
}
inline ll lcm(ll d1, ll d2) { return d1 / __gcd(d1, d2) * d2; }

/*Coding Space*/
unordered_map<int, unordered_map<int, char>> m;
const int v[5] = {0, 1, 0, -1, 0};
int main() {
  int h, w;
  cin >> h >> w;
  rep(i, h) rep(j, w) cin >> m[i][j];
  int w_cnt = 0;
  rep(i, h) rep(j, w) if (m[i][j] == '.') w_cnt++;

  queue<tiii> q;
  q.push(tiii{0, 0, 0});

  while (!q.empty()) {
    int x, y, s;
    tie(y, x, s) = q.front();
    q.pop();
    if (x == w - 1 && y == h - 1) {
      cout << w_cnt - s - 1 << endl;
      return 0;
    }
    rep(i, 4) if (m[y + v[i]][x + v[i + 1]] == '.') {
      q.push(tiii{y + v[i], x + v[i + 1], s + 1});
    }
    m[y][x] = 'w';
  }
  cout << -1 << endl;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
template <class T> using vt = vector<T>;
template <class T> using vvt = vector<vt<T>>;
template <class T> using ttt = tuple<T, T>;
using tii = tuple<int, int>;
using tiii = tuple<int, int, int>;
using vi = vector<int>;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define pb push_back
#define mt make_tuple
#define ALL(a) (a).begin(), (a).end()
#define FST first
#define SEC second
#define DEB cerr << "!" << endl
#define SHOW(a, b) cerr << (a) << " " << (b) << endl
#define DIV int(1e9 + 7)
const int INF = (INT_MAX / 2);
const ll LLINF = (LLONG_MAX / 2);
const double eps = 1e-8;
// const double PI = M_PI;
inline ll pow(ll x, ll n, ll m) {
  ll r = 1;
  while (n > 0) {
    if ((n & 1) == 1)
      r = r * x % m;
    x = x * x % m;
    n >>= 1;
  }
  return r % m;
}
inline ll lcm(ll d1, ll d2) { return d1 / __gcd(d1, d2) * d2; }

/*Coding Space*/
unordered_map<int, unordered_map<int, char>> m;
const int v[5] = {0, 1, 0, -1, 0};
int main() {
  int h, w;
  cin >> h >> w;
  rep(i, h) rep(j, w) cin >> m[i][j];
  int w_cnt = 0;
  rep(i, h) rep(j, w) if (m[i][j] == '.') w_cnt++;

  queue<tiii> q;
  q.push(tiii{0, 0, 0});

  while (!q.empty()) {
    int x, y, s;
    tie(y, x, s) = q.front();
    q.pop();
    if (x == w - 1 && y == h - 1) {
      cout << w_cnt - s - 1 << endl;
      return 0;
    }
    rep(i, 4) if (m[y + v[i]][x + v[i + 1]] == '.') {
      q.push(tiii{y + v[i], x + v[i + 1], s + 1});
      m[y + v[i]][x + v[i + 1]] = 'w';
    }
    m[y][x] = 'w';
  }
  cout << -1 << endl;
}