Problem p02735 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
#define rep(i, n) for (ll i = 0; i < (ll)(n); ++i)
#define rep2(i, s, n) for (ll i = s; i < (ll)(n); i++)
#define repr(i, n) for (ll i = n; i >= 0; i--)
#define pb push_back
#define COUT(x) cout << (x) << "\n"
#define COUTF(x) cout << setprecision(15) << (x) << "\n"
#define ENDL cout << "\n"
#define DF(x) x.erase(x.begin())
#define ALL(x) x.begin(), x.end()
#define SZ(x) (ll) x.size()
#define SORT(x) sort(ALL(x))
#define REVERSE(x) reverse(ALL(x))
#define ANS cout << ans << "\n"
#define init()                                                                 \
  cin.tie(0);                                                                  \
  ios::sync_with_stdio(false)
#define LINE cerr << "[debug] line: " << __LINE__ << "\n";
#define debug(x) cerr << "[debug] " << #x << ": " << x << "\n";
#define debugV(v)                                                              \
  cerr << "[debugV] " << #v << ":";                                            \
  rep(i, v.size()) cerr << " " << v[i];                                        \
  cerr << "\n";
using namespace std;
using ll = long long;
using ld = long double;
using vll = vector<ll>;
using vvll = vector<vector<ll>>;
using mll = map<ll, ll>;
using qll = queue<ll>;
using P = pair<ll, ll>;

constexpr ll INF = 0x3f3f3f3f3f3f3f3f;
constexpr ld PI = 3.141592653589793238462643383279;
ll get_digit(ll x) { return to_string(x).size(); }

ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }

ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }

vector<P> factorize(ll n) {
  vector<P> result;
  for (ll i = 2; i * i <= n; ++i) {
    if (n % i == 0) {
      result.pb({i, 0});
      while (n % i == 0) {
        n /= i;
        result.back().second++;
      }
    }
  }
  if (n != 1) {
    result.pb({n, 1});
  }
  return result;
}

vll divisor(ll n) {
  vll ret;
  for (ll i = 1; i * i <= n; i++) {
    if (n % i == 0) {
      ret.push_back(i);
      if (i * i != n)
        ret.push_back(n / i);
    }
  }
  SORT(ret);
  return (ret);
}

const int mod = 1000000007;
struct mint {
  ll x;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}
  mint operator-() const { return mint(-x); }
  mint &operator+=(const mint a) {
    if ((x += a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= mod;
    return *this;
  }
  mint operator+(const mint a) const { return mint(*this) += a; }
  mint operator-(const mint a) const { return mint(*this) -= a; }
  mint operator*(const mint a) const { return mint(*this) *= a; }
  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime mod
  mint inv() const { return pow(mod - 2); }
  mint &operator/=(const mint a) { return *this *= a.inv(); }
  mint operator/(const mint a) const { return mint(*this) /= a; }
};
istream &operator>>(istream &is, const mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }

struct combination {
  vector<mint> fact, ifact;
  combination(ll n) : fact(n + 1), ifact(n + 1) {
    assert(n < mod);
    fact[0] = 1;
    for (ll i = 1; i <= n; ++i) {
      fact[i] = fact[i - 1] * i;
    }
    ifact[n] = fact[n].inv();
    for (ll i = n; i >= 1; --i) {
      ifact[i - 1] = ifact[i] * i;
    }
  }
  mint operator()(ll n, ll k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
};

struct Z {
  ll x;
  ll y;
  ll count;
  // char before;
};

signed main() {
  init();
  ll H, W;
  cin >> H >> W;
  vector<string> S(H);
  rep(i, H) cin >> S[i];
  vvll ans(H, vll(W, INF));

  queue<Z> q;
  if (S[0][0] == '#') {
    q.push({0, 0, 1});
  } else {
    q.push({0, 0, 0});
  }

  while (!q.empty()) {
    Z z = q.front();
    q.pop();
    if (ans[z.x][z.y] < z.count)
      continue;
    ans[z.x][z.y] = min(ans[z.x][z.y], z.count);

    if (S[z.x][z.y] == '#') {
      if (z.y != W - 1) {
        q.push({z.x, z.y + 1, z.count});
      }
      if (z.x != H - 1) {
        q.push({z.x + 1, z.y, z.count});
      }
      continue;
    }

    if (z.y != W - 1) {
      if (S[z.x][z.y] == S[z.x][z.y + 1]) {
        q.push({z.x, z.y + 1, z.count});
      } else {
        q.push({z.x, z.y + 1, z.count + 1});
      }
    }

    if (z.x != H - 1) {
      if (S[z.x][z.y] == S[z.x + 1][z.y]) {
        q.push({z.x + 1, z.y, z.count});
      } else {
        q.push({z.x + 1, z.y, z.count + 1});
      }
    }
  }

  COUT(ans[H - 1][W - 1]);
  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
#define rep(i, n) for (ll i = 0; i < (ll)(n); ++i)
#define rep2(i, s, n) for (ll i = s; i < (ll)(n); i++)
#define repr(i, n) for (ll i = n; i >= 0; i--)
#define pb push_back
#define COUT(x) cout << (x) << "\n"
#define COUTF(x) cout << setprecision(15) << (x) << "\n"
#define ENDL cout << "\n"
#define DF(x) x.erase(x.begin())
#define ALL(x) x.begin(), x.end()
#define SZ(x) (ll) x.size()
#define SORT(x) sort(ALL(x))
#define REVERSE(x) reverse(ALL(x))
#define ANS cout << ans << "\n"
#define init()                                                                 \
  cin.tie(0);                                                                  \
  ios::sync_with_stdio(false)
#define LINE cerr << "[debug] line: " << __LINE__ << "\n";
#define debug(x) cerr << "[debug] " << #x << ": " << x << "\n";
#define debugV(v)                                                              \
  cerr << "[debugV] " << #v << ":";                                            \
  rep(i, v.size()) cerr << " " << v[i];                                        \
  cerr << "\n";
using namespace std;
using ll = long long;
using ld = long double;
using vll = vector<ll>;
using vvll = vector<vector<ll>>;
using mll = map<ll, ll>;
using qll = queue<ll>;
using P = pair<ll, ll>;

constexpr ll INF = 0x3f3f3f3f3f3f3f3f;
constexpr ld PI = 3.141592653589793238462643383279;
ll get_digit(ll x) { return to_string(x).size(); }

ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }

ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }

vector<P> factorize(ll n) {
  vector<P> result;
  for (ll i = 2; i * i <= n; ++i) {
    if (n % i == 0) {
      result.pb({i, 0});
      while (n % i == 0) {
        n /= i;
        result.back().second++;
      }
    }
  }
  if (n != 1) {
    result.pb({n, 1});
  }
  return result;
}

vll divisor(ll n) {
  vll ret;
  for (ll i = 1; i * i <= n; i++) {
    if (n % i == 0) {
      ret.push_back(i);
      if (i * i != n)
        ret.push_back(n / i);
    }
  }
  SORT(ret);
  return (ret);
}

const int mod = 1000000007;
struct mint {
  ll x;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}
  mint operator-() const { return mint(-x); }
  mint &operator+=(const mint a) {
    if ((x += a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= mod;
    return *this;
  }
  mint operator+(const mint a) const { return mint(*this) += a; }
  mint operator-(const mint a) const { return mint(*this) -= a; }
  mint operator*(const mint a) const { return mint(*this) *= a; }
  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime mod
  mint inv() const { return pow(mod - 2); }
  mint &operator/=(const mint a) { return *this *= a.inv(); }
  mint operator/(const mint a) const { return mint(*this) /= a; }
};
istream &operator>>(istream &is, const mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }

struct combination {
  vector<mint> fact, ifact;
  combination(ll n) : fact(n + 1), ifact(n + 1) {
    assert(n < mod);
    fact[0] = 1;
    for (ll i = 1; i <= n; ++i) {
      fact[i] = fact[i - 1] * i;
    }
    ifact[n] = fact[n].inv();
    for (ll i = n; i >= 1; --i) {
      ifact[i - 1] = ifact[i] * i;
    }
  }
  mint operator()(ll n, ll k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
};

struct Z {
  ll x;
  ll y;
  ll count;
  // char before;
};

signed main() {
  init();
  ll H, W;
  cin >> H >> W;
  vector<string> S(H);
  rep(i, H) cin >> S[i];
  vvll ans(H, vll(W, INF));

  queue<Z> q;
  if (S[0][0] == '#') {
    q.push({0, 0, 1});
  } else {
    q.push({0, 0, 0});
  }

  while (!q.empty()) {
    Z z = q.front();
    q.pop();
    if (ans[z.x][z.y] <= z.count)
      continue;
    ans[z.x][z.y] = min(ans[z.x][z.y], z.count);

    if (S[z.x][z.y] == '#') {
      if (z.y != W - 1) {
        q.push({z.x, z.y + 1, z.count});
      }
      if (z.x != H - 1) {
        q.push({z.x + 1, z.y, z.count});
      }
      continue;
    }

    if (z.y != W - 1) {
      if (S[z.x][z.y] == S[z.x][z.y + 1]) {
        q.push({z.x, z.y + 1, z.count});
      } else {
        q.push({z.x, z.y + 1, z.count + 1});
      }
    }

    if (z.x != H - 1) {
      if (S[z.x][z.y] == S[z.x + 1][z.y]) {
        q.push({z.x + 1, z.y, z.count});
      } else {
        q.push({z.x + 1, z.y, z.count + 1});
      }
    }
  }

  COUT(ans[H - 1][W - 1]);
  return 0;
}