Problem p03053 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <algorithm>
#include <cstdio>
#include <cstring>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <math.h>
#include <queue>
#include <stack>
#include <string>
#include <unordered_map>
#include <utility>
#include <vector>
using namespace std;

/**** Type Define ****/

typedef long long ll;
typedef pair<ll, ll> P;
typedef pair<ll, P> Q;

/**** Const List   ****/

const ll INF = 1LL << 60;
const ll mod = 1000000007;
const ll dx[4] = {1, 0, -1, 0};
const ll dy[4] = {0, -1, 0, 1};

/**** General Functions ****/
ll ketawa(ll n) {
  ll a = 0;
  while (n != 0) {
    a += n % 10;
    n /= 10;
  }
  return a;
}
ll RepeatSquaring(ll N, ll P, ll M) {
  if (P == 0)
    return 1;
  if (P % 2 == 0) {
    ll t = RepeatSquaring(N, P / 2, M);
    return (t % M) * (t % M) % M;
  }
  return (N * RepeatSquaring(N, P - 1, M)) % M;
}
bool IsPrime(ll a) { // order root a
  if (a == 1)
    return false;
  for (int i = 2; i * i <= a; i++) {
    if (a % i == 0 && a != i) {
      return false;
    }
  }
  return true;
}
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }

ll extgcd(ll a, ll b, ll &x, ll &y) {
  if (b == 0) {
    x = 1, y = 0;
    return a;
  }
  ll q = a / b, g = extgcd(b, a - q * b, x, y);
  ll z = x - q * y;
  x = y;
  y = z;
  return g;
}

ll invmod(ll a, ll m) { // a^-1 mod m
  ll x, y;
  extgcd(a, m, x, y);
  x %= m;
  if (x < 0)
    x += m;
  return x;
}

ll nCk(ll n, ll k, ll mod) {
  ll ans = 1;
  for (ll i = n, j = 1; j <= k; i--, j++)
    ans = (((ans * i) % mod) * invmod(j, mod)) % mod;
  return ans;
}

ll lmin(ll a, ll b) { return a > b ? b : a; };

ll lmax(ll a, ll b) { return a > b ? a : b; };

ll lsum(ll a, ll b) { return a + b; };

/**** Zip ****/

template <typename T> class Zip {
  vector<T> d;
  bool flag;

public:
  Zip() { flag = false; }
  void add(T x) {
    d.push_back(x);
    flag = true;
  }
  ll getNum(T x) { // T need to have operator < !!
    if (flag) {
      sort(d.begin(), d.end());
      d.erase(unique(d.begin(), d.end()), d.end());
      flag = false;
    }
    return lower_bound(d.begin(), d.end(), x) - d.begin();
  }
  ll size() {
    if (flag) {
      sort(d.begin(), d.end());
      d.erase(unique(d.begin(), d.end()), d.end());
      flag = false;
    }
    return (ll)d.size();
  }
};

/**** Union Find ****/

class UnionFind {
  vector<ll> par, rank; // par > 0: number, par < 0: -par
public:
  void init(ll n) {
    par.resize(n, 1);
    rank.resize(n, 0);
  }
  ll getSize(ll x) { return par[find(x)]; }
  ll find(ll x) {
    if (par[x] > 0)
      return x;
    return -(par[x] = -find(-par[x]));
  }
  void merge(ll x, ll y) {
    x = find(x);
    y = find(y);
    if (x == y)
      return;
    if (rank[x] < rank[y]) {
      par[y] += par[x];
      par[x] = -y;
    } else {
      par[x] += par[y];
      par[y] = -x;
      if (rank[x] == rank[y])
        rank[x]++;
    }
  }
  bool isSame(ll x, ll y) { return find(x) == find(y); }
};

/**** Segment Tree ****/
class SegmentTree {
public:
  vector<pair<double, double>> node; // node[0]は使用しない
  ll n; // データの個数(nodeの最下層には何個並んでいるか)
  pair<double, double> initial_value; // 初期値
public:
  void Init(ll n_, pair<double, double> initial_value_) {
    n = 1;
    while (n < n_)
      n *= 2;
    node.resize(2 * n);
    for (ll i = 0; i < 2 * n; i++) {
      node[i] = initial_value_;
    }
    initial_value = initial_value_;
  }
  void Update(ll k, pair<double, double> a) {
    // node[k]をaにする
    // それに従って先祖も変わっていく
    k += n;
    node[k] = a;
    while (k > 1) {
      k = k / 2;
      node[k] = pair<double, double>(
          node[k * 2].first * node[k * 2 + 1].first,
          node[k * 2].second * node[k * 2 + 1].first + node[k * 2 + 1].second);
    }
  }
  /*void Watch(){
    for(ll i=0;i<2*n;i++){
      cout<<node[i]<<endl;
    }
  }*/
  double Query() { //[a,b)
    return node[1].first + node[1].second;
  }
};

/**** LIS ****/
ll lis(ll *a, ll n, ll *dp) {
  fill(dp, dp + n, INF); // INFを代入
  for (ll i = 0; i < n; i++)
    *lower_bound(dp, dp + n, a[i]) = a[i];
  return (ll)(lower_bound(dp, dp + n, INF) - dp);
}

/**** main function ****/
ll h, w;
string a[1001];
queue<P> kouho;
ll kita[1001][1001];
ll ans[1001][1001];
int main() {
  cin >> h >> w;
  for (ll i = 0; i < h; i++) {
    cin >> a[i];
  }
  for (ll i = 0; i < h; i++) {
    for (ll j = 0; j < w; j++) {
      if (a[i][j] == '#') {
        kouho.push(P(i, j));
      } else {
        ans[i][j] = 999999999;
      }
    }
  }
  while (!kouho.empty()) {
    ll x = kouho.front().first;
    ll y = kouho.front().second;
    kouho.pop();
    kita[x][y] = 1;
    for (ll i = 0; i < 4; i++) {
      if (x + dx[i] < 0 || x + dx[i] >= h || y + dy[i] < 0 || y + dy[i] >= w)
        continue;
      ans[x + dx[i]][y + dy[i]] =
          lmin(ans[x + dx[i]][y + dy[i]], ans[x][y] + 1);
      if (kita[x + dx[i]][y + dy[i]] == 0) {
        kouho.push(P(x + dx[i], y + dy[i]));
      }
    }
  }
  ll ans2 = 0;
  for (ll i = 0; i < h; i++) {
    for (ll j = 0; j < w; j++) {
      ans2 = lmax(ans2, ans[i][j]);
    }
  }
  cout << ans2 << endl;
}


Fixed submission:
#include <algorithm>
#include <cstdio>
#include <cstring>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <math.h>
#include <queue>
#include <stack>
#include <string>
#include <unordered_map>
#include <utility>
#include <vector>
using namespace std;

/**** Type Define ****/

typedef long long ll;
typedef pair<ll, ll> P;
typedef pair<ll, P> Q;

/**** Const List   ****/

const ll INF = 1LL << 60;
const ll mod = 1000000007;
const ll dx[4] = {1, 0, -1, 0};
const ll dy[4] = {0, -1, 0, 1};

/**** General Functions ****/
ll ketawa(ll n) {
  ll a = 0;
  while (n != 0) {
    a += n % 10;
    n /= 10;
  }
  return a;
}
ll RepeatSquaring(ll N, ll P, ll M) {
  if (P == 0)
    return 1;
  if (P % 2 == 0) {
    ll t = RepeatSquaring(N, P / 2, M);
    return (t % M) * (t % M) % M;
  }
  return (N * RepeatSquaring(N, P - 1, M)) % M;
}
bool IsPrime(ll a) { // order root a
  if (a == 1)
    return false;
  for (int i = 2; i * i <= a; i++) {
    if (a % i == 0 && a != i) {
      return false;
    }
  }
  return true;
}
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }

ll extgcd(ll a, ll b, ll &x, ll &y) {
  if (b == 0) {
    x = 1, y = 0;
    return a;
  }
  ll q = a / b, g = extgcd(b, a - q * b, x, y);
  ll z = x - q * y;
  x = y;
  y = z;
  return g;
}

ll invmod(ll a, ll m) { // a^-1 mod m
  ll x, y;
  extgcd(a, m, x, y);
  x %= m;
  if (x < 0)
    x += m;
  return x;
}

ll nCk(ll n, ll k, ll mod) {
  ll ans = 1;
  for (ll i = n, j = 1; j <= k; i--, j++)
    ans = (((ans * i) % mod) * invmod(j, mod)) % mod;
  return ans;
}

ll lmin(ll a, ll b) { return a > b ? b : a; };

ll lmax(ll a, ll b) { return a > b ? a : b; };

ll lsum(ll a, ll b) { return a + b; };

/**** Zip ****/

template <typename T> class Zip {
  vector<T> d;
  bool flag;

public:
  Zip() { flag = false; }
  void add(T x) {
    d.push_back(x);
    flag = true;
  }
  ll getNum(T x) { // T need to have operator < !!
    if (flag) {
      sort(d.begin(), d.end());
      d.erase(unique(d.begin(), d.end()), d.end());
      flag = false;
    }
    return lower_bound(d.begin(), d.end(), x) - d.begin();
  }
  ll size() {
    if (flag) {
      sort(d.begin(), d.end());
      d.erase(unique(d.begin(), d.end()), d.end());
      flag = false;
    }
    return (ll)d.size();
  }
};

/**** Union Find ****/

class UnionFind {
  vector<ll> par, rank; // par > 0: number, par < 0: -par
public:
  void init(ll n) {
    par.resize(n, 1);
    rank.resize(n, 0);
  }
  ll getSize(ll x) { return par[find(x)]; }
  ll find(ll x) {
    if (par[x] > 0)
      return x;
    return -(par[x] = -find(-par[x]));
  }
  void merge(ll x, ll y) {
    x = find(x);
    y = find(y);
    if (x == y)
      return;
    if (rank[x] < rank[y]) {
      par[y] += par[x];
      par[x] = -y;
    } else {
      par[x] += par[y];
      par[y] = -x;
      if (rank[x] == rank[y])
        rank[x]++;
    }
  }
  bool isSame(ll x, ll y) { return find(x) == find(y); }
};

/**** Segment Tree ****/
class SegmentTree {
public:
  vector<pair<double, double>> node; // node[0]は使用しない
  ll n; // データの個数(nodeの最下層には何個並んでいるか)
  pair<double, double> initial_value; // 初期値
public:
  void Init(ll n_, pair<double, double> initial_value_) {
    n = 1;
    while (n < n_)
      n *= 2;
    node.resize(2 * n);
    for (ll i = 0; i < 2 * n; i++) {
      node[i] = initial_value_;
    }
    initial_value = initial_value_;
  }
  void Update(ll k, pair<double, double> a) {
    // node[k]をaにする
    // それに従って先祖も変わっていく
    k += n;
    node[k] = a;
    while (k > 1) {
      k = k / 2;
      node[k] = pair<double, double>(
          node[k * 2].first * node[k * 2 + 1].first,
          node[k * 2].second * node[k * 2 + 1].first + node[k * 2 + 1].second);
    }
  }
  /*void Watch(){
    for(ll i=0;i<2*n;i++){
      cout<<node[i]<<endl;
    }
  }*/
  double Query() { //[a,b)
    return node[1].first + node[1].second;
  }
};

/**** LIS ****/
ll lis(ll *a, ll n, ll *dp) {
  fill(dp, dp + n, INF); // INFを代入
  for (ll i = 0; i < n; i++)
    *lower_bound(dp, dp + n, a[i]) = a[i];
  return (ll)(lower_bound(dp, dp + n, INF) - dp);
}

/**** main function ****/
ll h, w;
string a[1001];
queue<P> kouho;
ll kita[1001][1001];
ll ans[1001][1001];
int main() {
  cin >> h >> w;
  for (ll i = 0; i < h; i++) {
    cin >> a[i];
  }
  for (ll i = 0; i < h; i++) {
    for (ll j = 0; j < w; j++) {
      if (a[i][j] == '#') {
        kouho.push(P(i, j));
      } else {
        ans[i][j] = 999999999;
      }
    }
  }
  while (!kouho.empty()) {
    ll x = kouho.front().first;
    ll y = kouho.front().second;
    kouho.pop();
    kita[x][y] = 1;
    for (ll i = 0; i < 4; i++) {
      if (x + dx[i] < 0 || x + dx[i] >= h || y + dy[i] < 0 || y + dy[i] >= w)
        continue;
      ans[x + dx[i]][y + dy[i]] =
          lmin(ans[x + dx[i]][y + dy[i]], ans[x][y] + 1);
      if (kita[x + dx[i]][y + dy[i]] == 0) {
        kouho.push(P(x + dx[i], y + dy[i]));
        kita[x + dx[i]][y + dy[i]] = 1;
      }
    }
  }
  ll ans2 = 0;
  for (ll i = 0; i < h; i++) {
    for (ll j = 0; j < w; j++) {
      ans2 = lmax(ans2, ans[i][j]);
    }
  }
  cout << ans2 << endl;
}