Problem p02735 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
#include <math.h>
#define _GLIBCXX_DEBUG
#define _LIBCPP_DEBUG 0

using namespace std;
#define ll long long
#define rep(i, n) for (int i = 0; i < n; i++)
#define rrep(i, n) for (int i = n - 1; i >= 0; i--)
#define MOD (1000000007)
#define vi vector<int>
#define vl vector<ll>
#define vb vector<bool>
#define vvi vector<vi>
#define vvl vector<vl>
#define vvb vector<vb>
#define pii pair<int, int>
#define pli pair<ll, int>
#define pll pair<ll, ll>
#define pb push_back
#define sz(s) (int)(s.size())

#define mp make_pair
#define all(a) (a).begin(), (a).end()

template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}

ll keta(ll n) {
  string s = to_string(n);
  ll num = s.size();
  return num;
}

const ll INF = 1LL << 60;

struct Edge {
  int to;
  int weight;
  Edge(int t, int w) : to(t), weight(w) {}
};
using Graph = vector<vector<Edge>>;
using P = pair<ll, int>;

class UnionFind {
public:
  vi Parent, diff_weight;
  UnionFind(int n) {
    Parent = vi(n, -1);
    diff_weight = vi(n, 0);
  }

  int root(int a) {
    if (Parent[a] < 0)
      return a;
    int r = root(Parent[a]);
    diff_weight[a] += diff_weight[Parent[a]];
    return Parent[a] = r;
  }

  int weight(int a) {
    root(a);
    return diff_weight[a];
  }

  bool issame(int a, int b) { return root(a) == root(b); }

  int size(int a) { return -Parent[root(a)]; }

  bool merge(int a, int b, int w = 0) {
    w += weight(a);
    w -= weight(b);

    a = root(a);
    b = root(b);

    if (a == b)
      return false;

    if (size(a) < size(b))
      swap(a, b), w = -w;
    Parent[a] += Parent[b];
    Parent[b] = a;
    diff_weight[b] = w;
    return true;
  }

  int diff(int a, int b) { return weight(b) - weight(a); }
};

vi MP(string s) {
  vi A(s.size() + 1);
  A[0] = -1;
  int j = -1;
  rep(i, s.size()) {
    while (j >= 0 && s[i] != s[j])
      j = A[j];
    j++;
    A[i + 1] = j;
  }
  return A;
}

vi Manacher(string s) {
  vi R(s.size());
  int i = 0, j = 0;
  while (i < s.size()) {
    while (i - j >= 0 && i + j < s.size() && s[i - j] == s[i + j])
      ++j;
    R[i] = j;
    int k = 1;
    while (i - k >= 0 && i + k < s.size() && k + R[i - k] < j)
      R[i + k] = R[i - k], k++;
    i += k;
    j -= k;
  }
  return R;
}

vi Z_algorithm(string &s) {
  vi Z(s.size());
  Z[0] = s.size();
  int i = 1, j = 0;
  while (i < s.size()) {
    while (i + j < s.size() && s[j] == s[i + j])
      j++;
    Z[i] = j;
    if (j == 0) {
      ++i;
      continue;
    }
    int k = 1;
    while (i + k < s.size() && k + Z[k] < j)
      Z[i + k] = Z[k], ++k;
    i += k;
    j -= k;
  }
  return Z;
}

const int MAX = 1e6 + 1;

long long fac[MAX], finv[MAX], inv[MAX];

// テーブルを作る前処理
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

// 二項係数計算
long long COM(ll n, ll k) {
  if (n >= MAX) {
    ll tmp = 1;
    rep(i, k) {
      tmp *= (n - i);
      tmp %= MOD;
    }
    return tmp * finv[k] % MOD;
  }

  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

ll POW(ll x, ll n) {
  ll ret = 1;
  if (n < 0) { // n < 0 にも対応
    n *= -1;
    x = inv[x];
  }
  while (0 < n) {
    if (n % 2 == 0) {
      x = x * x % MOD;
      n /= 2;
    } else {
      ret = ret * x % MOD;
      n--;
    }
  }
  return ret;
}

const int dh[2] = {1, 0};
const int dw[2] = {0, 1};

int main() {
  int H, W;
  cin >> H >> W;
  vector<string> s(H);
  rep(h, H) cin >> s[h];

  int dist[H][W][2];
  rep(h, H) rep(w, W) rep(i, 2) dist[h][w][i] = 1e9;
  rep(i, 2) dist[0][0][i] = 0;
  if (s[0][0] == '#') {
    rep(i, 2) dist[0][0][i]++;
  }

  queue<pii> q;
  q.push(mp(0, 0));

  while (!q.empty()) {
    pii v = q.front();
    q.pop();
    int h = v.first;
    int w = v.second;
    rep(i, 2) {
      // 状態 dist[h][w][i] を配る
      rep(dir, 2) {
        int nh = h + dh[dir];
        int nw = w + dw[dir];
        if (nh < 0 || nh == H || nw < 0 || nw == W)
          continue;
        if (s[h][w] == '.' && s[nh][nw] == '#')
          chmin(dist[nh][nw][dir], dist[h][w][i] + 1);
        else if (s[h][w] == '#' && s[nh][nw] == '#' && i != dir) {
          chmin(dist[nh][nw][dir], dist[h][w][i] + 1);
        } else
          chmin(dist[nh][nw][dir], dist[h][w][i]);
        q.push(mp(nh, nw));
      }
    }
  }
  cout << min(dist[H - 1][W - 1][0], dist[H - 1][W - 1][1]) << endl;
}


Fixed submission:
#include <bits/stdc++.h>
#include <math.h>
#define _GLIBCXX_DEBUG
#define _LIBCPP_DEBUG 0

using namespace std;
#define ll long long
#define rep(i, n) for (int i = 0; i < n; i++)
#define rrep(i, n) for (int i = n - 1; i >= 0; i--)
#define MOD (1000000007)
#define vi vector<int>
#define vl vector<ll>
#define vb vector<bool>
#define vvi vector<vi>
#define vvl vector<vl>
#define vvb vector<vb>
#define pii pair<int, int>
#define pli pair<ll, int>
#define pll pair<ll, ll>
#define pb push_back
#define sz(s) (int)(s.size())

#define mp make_pair
#define all(a) (a).begin(), (a).end()

template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}

ll keta(ll n) {
  string s = to_string(n);
  ll num = s.size();
  return num;
}

const ll INF = 1LL << 60;

struct Edge {
  int to;
  int weight;
  Edge(int t, int w) : to(t), weight(w) {}
};
using Graph = vector<vector<Edge>>;
using P = pair<ll, int>;

class UnionFind {
public:
  vi Parent, diff_weight;
  UnionFind(int n) {
    Parent = vi(n, -1);
    diff_weight = vi(n, 0);
  }

  int root(int a) {
    if (Parent[a] < 0)
      return a;
    int r = root(Parent[a]);
    diff_weight[a] += diff_weight[Parent[a]];
    return Parent[a] = r;
  }

  int weight(int a) {
    root(a);
    return diff_weight[a];
  }

  bool issame(int a, int b) { return root(a) == root(b); }

  int size(int a) { return -Parent[root(a)]; }

  bool merge(int a, int b, int w = 0) {
    w += weight(a);
    w -= weight(b);

    a = root(a);
    b = root(b);

    if (a == b)
      return false;

    if (size(a) < size(b))
      swap(a, b), w = -w;
    Parent[a] += Parent[b];
    Parent[b] = a;
    diff_weight[b] = w;
    return true;
  }

  int diff(int a, int b) { return weight(b) - weight(a); }
};

vi MP(string s) {
  vi A(s.size() + 1);
  A[0] = -1;
  int j = -1;
  rep(i, s.size()) {
    while (j >= 0 && s[i] != s[j])
      j = A[j];
    j++;
    A[i + 1] = j;
  }
  return A;
}

vi Manacher(string s) {
  vi R(s.size());
  int i = 0, j = 0;
  while (i < s.size()) {
    while (i - j >= 0 && i + j < s.size() && s[i - j] == s[i + j])
      ++j;
    R[i] = j;
    int k = 1;
    while (i - k >= 0 && i + k < s.size() && k + R[i - k] < j)
      R[i + k] = R[i - k], k++;
    i += k;
    j -= k;
  }
  return R;
}

vi Z_algorithm(string &s) {
  vi Z(s.size());
  Z[0] = s.size();
  int i = 1, j = 0;
  while (i < s.size()) {
    while (i + j < s.size() && s[j] == s[i + j])
      j++;
    Z[i] = j;
    if (j == 0) {
      ++i;
      continue;
    }
    int k = 1;
    while (i + k < s.size() && k + Z[k] < j)
      Z[i + k] = Z[k], ++k;
    i += k;
    j -= k;
  }
  return Z;
}

const int MAX = 1e6 + 1;

long long fac[MAX], finv[MAX], inv[MAX];

// テーブルを作る前処理
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

// 二項係数計算
long long COM(ll n, ll k) {
  if (n >= MAX) {
    ll tmp = 1;
    rep(i, k) {
      tmp *= (n - i);
      tmp %= MOD;
    }
    return tmp * finv[k] % MOD;
  }

  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

ll POW(ll x, ll n) {
  ll ret = 1;
  if (n < 0) { // n < 0 にも対応
    n *= -1;
    x = inv[x];
  }
  while (0 < n) {
    if (n % 2 == 0) {
      x = x * x % MOD;
      n /= 2;
    } else {
      ret = ret * x % MOD;
      n--;
    }
  }
  return ret;
}

const int dh[2] = {1, 0};
const int dw[2] = {0, 1};

int main() {
  int H, W;
  cin >> H >> W;
  vector<string> s(H);
  rep(h, H) cin >> s[h];
  vvi dp(H, vi(W, 1e9));
  dp[0][0] = 0;
  if (s[0][0] == '#')
    dp[0][0]++;
  rep(h, H) rep(w, W) {
    // dp[h][w] を dp[nh][nw] に配る
    rep(dir, 2) {
      int nh = h + dh[dir];
      int nw = w + dw[dir];
      // printf("nh:%d nw:%d\n", nh, nw);
      if (nh == H || nw == W)
        continue;
      if (s[h][w] == '.' && s[nh][nw] == '#')
        chmin(dp[nh][nw], dp[h][w] + 1);
      else
        chmin(dp[nh][nw], dp[h][w]);
    }
  }

  // rep(h,H) rep(w,W) {
  //   if(w != W-1) printf("%d ", dp[h][w]);
  //   else printf("%d\n", dp[h][w]);
  // }

  cout << dp[H - 1][W - 1] << endl;
}