Problem p03053 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using Edge = int;
using Graph = vector<vector<Edge>>;
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define SORT(v) sort((v).begin(), (v).end())
#define RSORT(v) sort((v).rbegin(), (v).rend())
const ll MOD = 1000000007;
const ll nmax = 8;
const ll INF = 1e9;
bool graph[nmax][nmax];
struct SegmentTree {
private:
  ll n;
  vector<ll> node;

public:
  SegmentTree(vector<ll> v) {
    ll sz = v.size();
    n = 1;
    while (n < sz) {
      n *= 2;
    }
    node.resize(2 * n - 1, INF);

    for (ll i = 0; i < sz; i++) {
      node[i + n - 1] = v[i];
    }

    for (ll i = n - 2; i >= 0; i--) {
      node[i] = min(node[2 * i + 1], node[2 * i + 2]);
    }
  }
  void update(ll x, ll val) {
    x += (n - 1);
    node[x] = val;
    while (x > 0) {
      x = (x - 1) / 2;
      node[x] = min(node[2 * x + 1], node[2 * x + 2]);
    }
  }

  // findは半開区間で考える
  ll find(ll a, ll b, ll k = 0, ll l = 0, ll r = -1) {
    if (r < 0)
      r = n;

    if (r <= a || b <= l)
      return INF;

    if (a <= l && r <= b)
      return node[k];

    ll vl = find(a, b, 2 * k + 1, l, (l + r) / 2);
    ll vr = find(a, b, 2 * k + 2, (l + r) / 2, r);
    return min(vl, vr);
  }
};

class UnionFind {
public:
  vector<ll> Parent;

  UnionFind(ll N) { Parent = vector<ll>(N, -1); }
  ll find(ll A) {
    if (Parent[A] < 0)
      return A;
    return Parent[A] = find(Parent[A]);
  }

  ll size(ll A) { return -Parent[find(A)]; }

  bool Union(ll A, ll B) {
    A = find(A);
    B = find(B);
    if (A == B) {
      return false;
    }
    if (size(A) < size(B))
      swap(A, B);

    Parent[A] += Parent[B];
    Parent[B] = A;

    return true;
  }
};

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

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

ll mulMod(ll a, ll b) { return (((a % MOD) * (b % MOD)) % MOD); }

ll powMod(ll a, ll p) {
  if (p == 0) {
    return 1;
  } else if (p % 2 == 0) {
    ll half = powMod(a, p / 2);
    return mulMod(half, half);
  } else {
    return mulMod(powMod(a, p - 1), a);
  }
}

ll ceil(ll a, ll b) { return (a + b - 1) / b; }

vector<ll> tsort(Graph G) {
  ll N = G.size();
  vector<ll> in(N);
  for (auto &&edges : G) {
    for (auto &&edge : edges) {
      in[edge]++;
    }
  }

  queue<int> que;

  for (int i = 0; i < N; i++) {
    if (in[i] == 0) {
      que.push(i);
    }
  }
  int cnt = 0;
  vector<ll> ans;
  while (!que.empty()) {
    int v = que.front();
    que.pop();
    ans.push_back(v);
    for (auto &&next : G[v]) {

      in[next]--;
      if (in[next] == 0) {
        que.push(next);
      }
    }
  }
  return ans;
}
Graph G(100);
void treeDFS(int from, int current, int dist, int &maxDist, int &maxVertex) {
  if (dist > maxDist) {
    maxDist = dist;
    maxVertex = current;
  }

  for (auto to : G[current]) {
    if (to == from)
      continue;
    treeDFS(current, to, dist + 1, maxDist, maxVertex);
  }
}

pair<int, int> getTreeDiameter() {
  int start = 0, end = 0, maxDist = 0;
  treeDFS(-1, start, 0, maxDist, end);
  start = end, end = 0, maxDist = 0;
  treeDFS(-1, start, 0, maxDist, end);
  return make_pair(start, end);
}

ll dx[4] = {-1, 0, 0, 1};
ll dy[4] = {0, -1, 1, 0};
void solve(long long H, long long W, std::vector<std::string> A) {
  ll dist[H][W];

  for (int i = 0; i < H; i++) {
    for (int j = 0; j < W; j++) {
      dist[i][j] = INF;
    }
  }
  queue<pair<ll, ll>> que;
  for (int i = 0; i < H; i++) {
    for (int j = 0; j < W; j++) {
      if (A[i][j] == '#') {
        que.push(make_pair(i, j));
        dist[i][j] = 0;
      }
    }
  }

  while (!que.empty()) {
    pair<ll, ll> now = que.front();
    que.pop();

    for (int k = 0; k < 4; k++) {
      ll nx = now.first + dx[k];
      ll ny = now.second + dy[k];

      if (nx < 0 || H <= nx || ny < 0 || W <= ny)
        continue;
      if (dist[nx][ny] != INF || A[nx][ny] == '#')
        continue;

      dist[nx][ny] = min(dist[nx][ny], dist[now.first][now.second] + 1);
      que.push(make_pair(nx, ny));
    }
  }

  ll ans = -1;
  for (int i = 0; i < H; i++) {
    for (int j = 0; j < W; j++) {
      ans = max(ans, dist[i][j]);
    }
  }
  cout << ans << endl;
}

int main() {
  long long H;
  scanf("%lld", &H);
  long long W;
  scanf("%lld", &W);
  std::vector<std::string> A(W);
  for (int i = 0; i < W; i++) {
    std::cin >> A[i];
  }
  solve(H, W, std::move(A));
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using Edge = int;
using Graph = vector<vector<Edge>>;
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define SORT(v) sort((v).begin(), (v).end())
#define RSORT(v) sort((v).rbegin(), (v).rend())
const ll MOD = 1000000007;
const ll nmax = 8;
const ll INF = 1e9;
bool graph[nmax][nmax];
struct SegmentTree {
private:
  ll n;
  vector<ll> node;

public:
  SegmentTree(vector<ll> v) {
    ll sz = v.size();
    n = 1;
    while (n < sz) {
      n *= 2;
    }
    node.resize(2 * n - 1, INF);

    for (ll i = 0; i < sz; i++) {
      node[i + n - 1] = v[i];
    }

    for (ll i = n - 2; i >= 0; i--) {
      node[i] = min(node[2 * i + 1], node[2 * i + 2]);
    }
  }
  void update(ll x, ll val) {
    x += (n - 1);
    node[x] = val;
    while (x > 0) {
      x = (x - 1) / 2;
      node[x] = min(node[2 * x + 1], node[2 * x + 2]);
    }
  }

  // findは半開区間で考える
  ll find(ll a, ll b, ll k = 0, ll l = 0, ll r = -1) {
    if (r < 0)
      r = n;

    if (r <= a || b <= l)
      return INF;

    if (a <= l && r <= b)
      return node[k];

    ll vl = find(a, b, 2 * k + 1, l, (l + r) / 2);
    ll vr = find(a, b, 2 * k + 2, (l + r) / 2, r);
    return min(vl, vr);
  }
};

class UnionFind {
public:
  vector<ll> Parent;

  UnionFind(ll N) { Parent = vector<ll>(N, -1); }
  ll find(ll A) {
    if (Parent[A] < 0)
      return A;
    return Parent[A] = find(Parent[A]);
  }

  ll size(ll A) { return -Parent[find(A)]; }

  bool Union(ll A, ll B) {
    A = find(A);
    B = find(B);
    if (A == B) {
      return false;
    }
    if (size(A) < size(B))
      swap(A, B);

    Parent[A] += Parent[B];
    Parent[B] = A;

    return true;
  }
};

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

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

ll mulMod(ll a, ll b) { return (((a % MOD) * (b % MOD)) % MOD); }

ll powMod(ll a, ll p) {
  if (p == 0) {
    return 1;
  } else if (p % 2 == 0) {
    ll half = powMod(a, p / 2);
    return mulMod(half, half);
  } else {
    return mulMod(powMod(a, p - 1), a);
  }
}

ll ceil(ll a, ll b) { return (a + b - 1) / b; }

vector<ll> tsort(Graph G) {
  ll N = G.size();
  vector<ll> in(N);
  for (auto &&edges : G) {
    for (auto &&edge : edges) {
      in[edge]++;
    }
  }

  queue<int> que;

  for (int i = 0; i < N; i++) {
    if (in[i] == 0) {
      que.push(i);
    }
  }
  int cnt = 0;
  vector<ll> ans;
  while (!que.empty()) {
    int v = que.front();
    que.pop();
    ans.push_back(v);
    for (auto &&next : G[v]) {

      in[next]--;
      if (in[next] == 0) {
        que.push(next);
      }
    }
  }
  return ans;
}
Graph G(100);
void treeDFS(int from, int current, int dist, int &maxDist, int &maxVertex) {
  if (dist > maxDist) {
    maxDist = dist;
    maxVertex = current;
  }

  for (auto to : G[current]) {
    if (to == from)
      continue;
    treeDFS(current, to, dist + 1, maxDist, maxVertex);
  }
}

pair<int, int> getTreeDiameter() {
  int start = 0, end = 0, maxDist = 0;
  treeDFS(-1, start, 0, maxDist, end);
  start = end, end = 0, maxDist = 0;
  treeDFS(-1, start, 0, maxDist, end);
  return make_pair(start, end);
}

ll dx[4] = {-1, 0, 0, 1};
ll dy[4] = {0, -1, 1, 0};
void solve(long long H, long long W, std::vector<std::string> A) {
  ll dist[H][W];

  for (int i = 0; i < H; i++) {
    for (int j = 0; j < W; j++) {
      dist[i][j] = INF;
    }
  }
  queue<pair<ll, ll>> que;
  for (int i = 0; i < H; i++) {
    for (int j = 0; j < W; j++) {
      if (A[i][j] == '#') {
        que.push(make_pair(i, j));
        dist[i][j] = 0;
      }
    }
  }

  while (!que.empty()) {
    pair<ll, ll> now = que.front();
    que.pop();

    for (int k = 0; k < 4; k++) {
      ll nx = now.first + dx[k];
      ll ny = now.second + dy[k];

      if (nx < 0 || H <= nx || ny < 0 || W <= ny)
        continue;
      if (dist[nx][ny] != INF || A[nx][ny] == '#')
        continue;

      dist[nx][ny] = min(dist[nx][ny], dist[now.first][now.second] + 1);
      que.push(make_pair(nx, ny));
    }
  }

  ll ans = -1;
  for (int i = 0; i < H; i++) {
    for (int j = 0; j < W; j++) {
      ans = max(ans, dist[i][j]);
    }
  }
  cout << ans << endl;
}

int main() {
  long long H;
  scanf("%lld", &H);
  long long W;
  scanf("%lld", &W);
  std::vector<std::string> A(H);
  for (int i = 0; i < H; i++) {
    std::cin >> A[i];
  }
  solve(H, W, std::move(A));
  return 0;
}