Problem p04007 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cctype>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <ctime>
#include <deque>
#include <float.h>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits>
#include <map>
#include <math.h>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
#pragma warning(disable : 4996)
using namespace std;
using ll = long long;
unsigned euclidean_gcd(unsigned a, unsigned b) {
  if (a < b)
    return euclidean_gcd(b, a);
  unsigned r;
  while ((r = a % b)) {
    a = b;
    b = r;
  }
  return b;
}

ll ll_gcd(ll a, ll b) {
  if (a < b)
    return ll_gcd(b, a);
  ll r;
  while ((r = a % b)) {
    a = b;
    b = r;
  }
  return b;
}

struct UnionFind {
  vector<ll> par;
  vector<ll> siz;
  UnionFind(ll sz_) : par(sz_), siz(sz_, 1LL) {
    for (ll i = 0; i < sz_; ++i)
      par[i] = i;
  }
  void init(ll sz_) {
    par.resize(sz_);
    siz.assign(sz_, 1LL);
    for (ll i = 0; i < sz_; ++i)
      par[i] = i;
  }
  ll root(ll x) {
    while (par[x] != x) {
      x = par[x] = par[par[x]];
    }
    return x;
  }
  bool merge(ll x, ll y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (siz[x] < siz[y])
      swap(x, y);
    siz[x] += siz[y];
    par[y] = x;
    return true;
  }

  bool issame(ll x, ll y) { return root(x) == root(y); }

  ll size(ll x) { return siz[root(x)]; }
};

long long modpow(long long a, long long n, long long mod) {
  if (n < 0)
    return 0;
  long long res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}

long long modinv(long long a, long long mod) { return modpow(a, mod - 2, mod); }

vector<int> tpsort(vector<vector<int>> &G) {
  int V = G.size();
  vector<int> sorted_vertices;
  queue<int> que;
  vector<int> indegree(V);
  for (int i = 0; i < V; i++) {
    for (int j = 0; j < G[i].size(); j++) {
      indegree[G[i][j]]++;
    }
  }
  for (int i = 0; i < V; i++) {
    if (indegree[i] == 0) {
      que.push(i);
    }
  }
  while (que.empty() == false) {
    int v = que.front();
    que.pop();
    for (int i = 0; i < G[v].size(); i++) {
      int u = G[v][i];
      indegree[u] -= 1;
      if (indegree[u] == 0)
        que.push(u);
    }
    sorted_vertices.push_back(v);
  }
  return sorted_vertices;
}

struct Point {
  double x;
  double y;
};

struct LineSegment {
  Point start;
  Point end;
};

double tenkyori(const LineSegment &line, const Point &point) {
  double x0 = point.x, y0 = point.y;
  double x1 = line.start.x, y1 = line.start.y;
  double x2 = line.end.x, y2 = line.end.y;

  double a = x2 - x1;
  double b = y2 - y1;
  double a2 = a * a;
  double b2 = b * b;
  double r2 = a2 + b2;
  double tt = -(a * (x1 - x0) + b * (y1 - y0));

  if (tt < 0)
    return sqrt((x1 - x0) * (x1 - x0) + (y1 - y0) * (y1 - y0));
  else if (tt > r2)
    return sqrt((x2 - x0) * (x2 - x0) + (y2 - y0) * (y2 - y0));

  double f1 = a * (y1 - y0) - b * (x1 - x0);
  return sqrt((f1 * f1) / r2);
}
void dfs1(vector<vector<ll>> &z, ll k, ll oya, ll &ans, vector<ll> &b) {
  for (auto m : z[k]) {
    if (m != oya)
      dfs1(z, m, k, ans, b);
  }
  vector<ll> s;
  for (auto m : z[k]) {
    if (m != oya)
      s.push_back(b[m]);
  }
  ll m = b.size() - 1;
  for (auto d : s) {
    m -= d;
  }
  b[k] = b.size() - m;
  if (m != 0)
    s.push_back(m);
  ll a = modinv(2, 1000000007);
  for (auto d : s) {
    a += 1000000007 - modinv(modpow(2, b.size() - d, 1000000007), 1000000007);
  }
  a += modinv(modpow(2, b.size(), 1000000007), 1000000007) * (z[k].size() - 1);
  ans += a;
  ans %= 1000000007;
  return;
}

ll merge_cnt(vector<int> &a) {
  int n = a.size();
  if (n <= 1) {
    return 0;
  }

  ll cnt = 0;
  vector<int> b(a.begin(), a.begin() + n / 2);
  vector<int> c(a.begin() + n / 2, a.end());

  cnt += merge_cnt(b);
  cnt += merge_cnt(c);

  int ai = 0, bi = 0, ci = 0;
  while (ai < n) {
    if (bi < b.size() && (ci == c.size() || b[bi] <= c[ci])) {
      a[ai++] = b[bi++];
    } else {
      cnt += n / 2 - bi;
      a[ai++] = c[ci++];
    }
  }
  return cnt;
}

int main() {
  ll n, m;
  cin >> n >> m;
  vector<string> z(n);
  vector<vector<char>> a(n, vector<char>(m, '.'));
  vector<vector<char>> b(n, vector<char>(m, '.'));
  for (int i = 0; i < n; i++) {
    cin >> z[i];
  }
  for (int i = 0; i < m; i++) {
    a[0][i] = '#';
    b[m - 1][i] = '#';
  }
  for (int i = 1; i < n - 1; i++) {
    for (int j = 0; j < m; j++) {
      if (j % 2 == 0)
        a[i][j] = '#';
      else
        b[i][j] = '#';
      if (z[i][j] == '#') {
        a[i][j] = '#';
        b[i][j] = '#';
      }
    }
  }
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < m; j++) {
      cout << a[i][j];
    }
    cout << endl;
  }
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < m; j++) {
      cout << b[i][j];
    }
    cout << endl;
  }
}

Fixed submission:
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cctype>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <ctime>
#include <deque>
#include <float.h>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits>
#include <map>
#include <math.h>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
#pragma warning(disable : 4996)
using namespace std;
using ll = long long;
unsigned euclidean_gcd(unsigned a, unsigned b) {
  if (a < b)
    return euclidean_gcd(b, a);
  unsigned r;
  while ((r = a % b)) {
    a = b;
    b = r;
  }
  return b;
}

ll ll_gcd(ll a, ll b) {
  if (a < b)
    return ll_gcd(b, a);
  ll r;
  while ((r = a % b)) {
    a = b;
    b = r;
  }
  return b;
}

struct UnionFind {
  vector<ll> par;
  vector<ll> siz;
  UnionFind(ll sz_) : par(sz_), siz(sz_, 1LL) {
    for (ll i = 0; i < sz_; ++i)
      par[i] = i;
  }
  void init(ll sz_) {
    par.resize(sz_);
    siz.assign(sz_, 1LL);
    for (ll i = 0; i < sz_; ++i)
      par[i] = i;
  }
  ll root(ll x) {
    while (par[x] != x) {
      x = par[x] = par[par[x]];
    }
    return x;
  }
  bool merge(ll x, ll y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (siz[x] < siz[y])
      swap(x, y);
    siz[x] += siz[y];
    par[y] = x;
    return true;
  }

  bool issame(ll x, ll y) { return root(x) == root(y); }

  ll size(ll x) { return siz[root(x)]; }
};

long long modpow(long long a, long long n, long long mod) {
  if (n < 0)
    return 0;
  long long res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}

long long modinv(long long a, long long mod) { return modpow(a, mod - 2, mod); }

vector<int> tpsort(vector<vector<int>> &G) {
  int V = G.size();
  vector<int> sorted_vertices;
  queue<int> que;
  vector<int> indegree(V);
  for (int i = 0; i < V; i++) {
    for (int j = 0; j < G[i].size(); j++) {
      indegree[G[i][j]]++;
    }
  }
  for (int i = 0; i < V; i++) {
    if (indegree[i] == 0) {
      que.push(i);
    }
  }
  while (que.empty() == false) {
    int v = que.front();
    que.pop();
    for (int i = 0; i < G[v].size(); i++) {
      int u = G[v][i];
      indegree[u] -= 1;
      if (indegree[u] == 0)
        que.push(u);
    }
    sorted_vertices.push_back(v);
  }
  return sorted_vertices;
}

struct Point {
  double x;
  double y;
};

struct LineSegment {
  Point start;
  Point end;
};

double tenkyori(const LineSegment &line, const Point &point) {
  double x0 = point.x, y0 = point.y;
  double x1 = line.start.x, y1 = line.start.y;
  double x2 = line.end.x, y2 = line.end.y;

  double a = x2 - x1;
  double b = y2 - y1;
  double a2 = a * a;
  double b2 = b * b;
  double r2 = a2 + b2;
  double tt = -(a * (x1 - x0) + b * (y1 - y0));

  if (tt < 0)
    return sqrt((x1 - x0) * (x1 - x0) + (y1 - y0) * (y1 - y0));
  else if (tt > r2)
    return sqrt((x2 - x0) * (x2 - x0) + (y2 - y0) * (y2 - y0));

  double f1 = a * (y1 - y0) - b * (x1 - x0);
  return sqrt((f1 * f1) / r2);
}
void dfs1(vector<vector<ll>> &z, ll k, ll oya, ll &ans, vector<ll> &b) {
  for (auto m : z[k]) {
    if (m != oya)
      dfs1(z, m, k, ans, b);
  }
  vector<ll> s;
  for (auto m : z[k]) {
    if (m != oya)
      s.push_back(b[m]);
  }
  ll m = b.size() - 1;
  for (auto d : s) {
    m -= d;
  }
  b[k] = b.size() - m;
  if (m != 0)
    s.push_back(m);
  ll a = modinv(2, 1000000007);
  for (auto d : s) {
    a += 1000000007 - modinv(modpow(2, b.size() - d, 1000000007), 1000000007);
  }
  a += modinv(modpow(2, b.size(), 1000000007), 1000000007) * (z[k].size() - 1);
  ans += a;
  ans %= 1000000007;
  return;
}

ll merge_cnt(vector<int> &a) {
  int n = a.size();
  if (n <= 1) {
    return 0;
  }

  ll cnt = 0;
  vector<int> b(a.begin(), a.begin() + n / 2);
  vector<int> c(a.begin() + n / 2, a.end());

  cnt += merge_cnt(b);
  cnt += merge_cnt(c);

  int ai = 0, bi = 0, ci = 0;
  while (ai < n) {
    if (bi < b.size() && (ci == c.size() || b[bi] <= c[ci])) {
      a[ai++] = b[bi++];
    } else {
      cnt += n / 2 - bi;
      a[ai++] = c[ci++];
    }
  }
  return cnt;
}

int main() {
  ll n, m;
  cin >> n >> m;
  vector<string> z(n);
  vector<vector<char>> a(n, vector<char>(m, '.'));
  vector<vector<char>> b(n, vector<char>(m, '.'));
  for (int i = 0; i < n; i++) {
    cin >> z[i];
  }
  for (int i = 0; i < m; i++) {
    a[0][i] = '#';
    b[n - 1][i] = '#';
  }
  for (int i = 1; i < n - 1; i++) {
    for (int j = 0; j < m; j++) {
      if (j % 2 == 0)
        a[i][j] = '#';
      else
        b[i][j] = '#';
      if (z[i][j] == '#') {
        a[i][j] = '#';
        b[i][j] = '#';
      }
    }
  }
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < m; j++) {
      cout << a[i][j];
    }
    cout << endl;
  }
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < m; j++) {
      cout << b[i][j];
    }
    cout << endl;
  }
}