Problem p02561 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>
using namespace std;
struct fast_ios {
  fast_ios() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  };
} fast_ios_;
#define FOR(i, begin, end) for (int i = (begin); i < (end); i++)
#define REP(i, n) FOR(i, 0, n)
#define IFOR(i, begin, end) for (int i = (end)-1; i >= (begin); i--)
#define IREP(i, n) IFOR(i, 0, n)
#define Sort(v) sort(v.begin(), v.end())
#define Reverse(v) reverse(v.begin(), v.end())
#define all(v) v.begin(), v.end()
#define SZ(v) ((int)v.size())
#define Lower_bound(v, x)                                                      \\
  distance(v.begin(), lower_bound(v.begin(), v.end(), x))
#define Upper_bound(v, x)                                                      \\
  distance(v.begin(), upper_bound(v.begin(), v.end(), x))
#define chmax(a, b) a = max(a, b)
#define chmin(a, b) a = min(a, b)
#define bit(n) (1LL << (n))
#define debug(x) cout << #x << "=" << x << endl;
#define vdebug(v)                                                              \\
  {                                                                            \\
    cout << #v << "=" << endl;                                                 \\
    REP(i_debug, v.size()) { cout << v[i_debug] << ","; }                      \\
    cout << endl;                                                              \\
  }
#define mdebug(m)                                                              \\
  {                                                                            \\
    cout << #m << "=" << endl;                                                 \\
    REP(i_debug, m.size()) {                                                   \\
      REP(j_debug, m[i_debug].size()) { cout << m[i_debug][j_debug] << ","; }  \\
      cout << endl;                                                            \\
    }                                                                          \\
  }
#define pb push_back
#define fi first
#define se second
#define int long long
#define INF 1000000000000000000
template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (auto &x : v)
    is >> x;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, vector<T> &v) {
  for (int i = 0; i < v.size(); i++) {
    cout << v[i];
    if (i != v.size() - 1)
      cout << endl;
  };
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, pair<T1, T2> p) {
  cout << '(' << p.first << ',' << p.second << ')';
  return os;
}
template <typename T> void Out(T x) { cout << x << endl; }
template <typename T1, typename T2> void chOut(bool f, T1 y, T2 n) {
  if (f)
    Out(y);
  else
    Out(n);
}

using vec = vector<int>;
using mat = vector<vec>;
using Pii = pair<int, int>;
using v_bool = vector<bool>;
using v_Pii = vector<Pii>;

// int dx[4] = {1,0,-1,0};
// int dy[4] = {0,1,0,-1};
// char d[4] = {'D','R','U','L'};

const int mod = 1000000007;
// const int mod = 998244353;

struct edge {
  int to, cap, rev, id;
};

class Graph {
public:
  int N;
  vector<vector<edge>> G;
  vector<int> level, iter;

  Graph(int N) : N(N) {
    G = vector<vector<edge>>(N, vector<edge>(0));
    level = vector<int>(N);
    iter = vector<int>(N);
  }

  // 実際の流量を求めたい辺には0以上のidを与える　逆辺のcapを用いる
  void add_Directed_edge(int from, int to, int cap, int id = -1) {
    G[from].push_back((edge){to, cap, (int)G[to].size(), -1});
    G[to].push_back((edge){from, 0, (int)G[from].size() - 1, id});
  }

  void bfs(int s) {
    memset(&level[0], -1, sizeof(level[0]) * level.size());
    queue<int> que;
    level[s] = 0;
    que.push(s);
    while (!que.empty()) {
      int v = que.front();
      que.pop();
      REP(i, G[v].size()) {
        edge &e = G[v][i];
        if (e.cap > 0 && level[e.to] < 0) {
          level[e.to] = level[v] + 1;
          que.push(e.to);
        }
      }
    }
  }

  int dfs(int v, int t, int f) {
    if (v == t)
      return f;
    for (int &i = iter[v]; i < G[v].size(); i++) {
      edge &e = G[v][i];
      if (e.cap > 0 && level[v] < level[e.to]) {
        int d = dfs(e.to, t, min(f, e.cap));
        if (d > 0) {
          e.cap -= d;
          G[e.to][e.rev].cap += d;
          return d;
        }
      }
    }
    return 0;
  }

  // sからtへのmax flowを求める　flow_limitを設定したらそこで打ち切り
  int max_flow(int s, int t, int flow_limit = -1) {
    int flow = 0;
    for (;;) {
      bfs(s);
      if (level[t] < 0)
        return flow;
      memset(&iter[0], 0, sizeof(iter[0]) * iter.size());
      int f;
      while ((f = dfs(s, t, INF)) > 0) {
        flow += f;
        if (flow_limit >= 0 && flow >= flow_limit)
          return flow_limit;
      }
    }
  }

  // 実際の流量を求める、max_flow(s, t)を使ってから
  vector<int> calc_flow(int num_id) {
    vector<int> flow(num_id, 0);
    REP(v, N) for (auto e : G[v]) if (e.id >= 0) flow[e.id] = e.cap;
    return flow;
  }

  void dfs_for_calc_min_cut(int v, vector<bool> &belong2s) {
    belong2s[v] = true;
    for (edge e : G[v])
      if (e.cap > 0 && !belong2s[e.to])
        dfs_for_calc_min_cut(e.to, belong2s);
  }

  // 残余グラフでsから到達可能か求める、max_flow(s, t)を使ってから
  vector<bool> min_cut(int s) {
    vector<bool> belong2s(N);
    dfs_for_calc_min_cut(s, belong2s);
    return belong2s;
  }
};

signed main() {

  int N, M;
  cin >> N >> M;
  vector<string> S(N);
  cin >> S;

  Graph G(N * M + 2);
  int s = N * M, t = s + 1;

  REP(i, N) REP(j, M) if (S[i][j] == '.') {
    if ((i + j) % 2 == 0)
      G.add_Directed_edge(s, i * M + j, 1);
    else
      G.add_Directed_edge(i * M + j, t, 1);
  }

  int n = 0;
  v_Pii p;
  v_bool h;
  REP(i, N) REP(j, M) {
    if (i + 1 < N) {
      if ((i + j) % 2 == 0)
        G.add_Directed_edge(i * M + j, (i + 1) * M + j, 1, n);
      else
        G.add_Directed_edge((i + 1) * M + j, i * M + j, 1, n);
      p.pb(Pii(i, j));
      h.pb(true);
      n++;
    }
    if (j + 1 < M) {
      if ((i + j) % 2 == 0)
        G.add_Directed_edge(i * M + j, i * M + j + 1, 1, n);
      else
        G.add_Directed_edge(i * M + j + 1, i * M + j, 1, n);
      p.pb(Pii(i, j));
      h.pb(false);
      n++;
    }
  }

  int f = G.max_flow(s, t);
  Out(f);

  vec F = G.calc_flow(n);
  REP(k, n) if (F[k] > 0) {
    int i = p[k].fi, j = p[k].se;
    if (h[k]) {
      S[i][j] = 'v';
      S[i + 1][j] = '^';
    } else {
      S[i][j] = '>';
      S[i][j + 1] = '<';
    }
  }
  Out(S);

  return 0;
}


Error: -11

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
struct fast_ios {
  fast_ios() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  };
} fast_ios_;
#define FOR(i, begin, end) for (int i = (begin); i < (end); i++)
#define REP(i, n) FOR(i, 0, n)
#define IFOR(i, begin, end) for (int i = (end)-1; i >= (begin); i--)
#define IREP(i, n) IFOR(i, 0, n)
#define Sort(v) sort(v.begin(), v.end())
#define Reverse(v) reverse(v.begin(), v.end())
#define all(v) v.begin(), v.end()
#define SZ(v) ((int)v.size())
#define Lower_bound(v, x)                                                      \\
  distance(v.begin(), lower_bound(v.begin(), v.end(), x))
#define Upper_bound(v, x)                                                      \\
  distance(v.begin(), upper_bound(v.begin(), v.end(), x))
#define chmax(a, b) a = max(a, b)
#define chmin(a, b) a = min(a, b)
#define bit(n) (1LL << (n))
#define debug(x) cout << #x << "=" << x << endl;
#define vdebug(v)                                                              \\
  {                                                                            \\
    cout << #v << "=" << endl;                                                 \\
    REP(i_debug, v.size()) { cout << v[i_debug] << ","; }                      \\
    cout << endl;                                                              \\
  }
#define mdebug(m)                                                              \\
  {                                                                            \\
    cout << #m << "=" << endl;                                                 \\
    REP(i_debug, m.size()) {                                                   \\
      REP(j_debug, m[i_debug].size()) { cout << m[i_debug][j_debug] << ","; }  \\
      cout << endl;                                                            \\
    }                                                                          \\
  }
#define pb push_back
#define fi first
#define se second
#define int long long
#define INF 1000000000000000000
template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (auto &x : v)
    is >> x;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, vector<T> &v) {
  for (int i = 0; i < v.size(); i++) {
    cout << v[i];
    if (i != v.size() - 1)
      cout << endl;
  };
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, pair<T1, T2> p) {
  cout << '(' << p.first << ',' << p.second << ')';
  return os;
}
template <typename T> void Out(T x) { cout << x << endl; }
template <typename T1, typename T2> void chOut(bool f, T1 y, T2 n) {
  if (f)
    Out(y);
  else
    Out(n);
}

using vec = vector<int>;
using mat = vector<vec>;
using Pii = pair<int, int>;
using v_bool = vector<bool>;
using v_Pii = vector<Pii>;

// int dx[4] = {1,0,-1,0};
// int dy[4] = {0,1,0,-1};
// char d[4] = {'D','R','U','L'};

const int mod = 1000000007;
// const int mod = 998244353;

struct edge {
  int to, cap, rev, id;
};

class Graph {
public:
  int N;
  vector<vector<edge>> G;
  vector<int> level, iter;

  Graph(int N) : N(N) {
    G = vector<vector<edge>>(N, vector<edge>(0));
    level = vector<int>(N);
    iter = vector<int>(N);
  }

  // 実際の流量を求めたい辺には0以上のidを与える　逆辺のcapを用いる
  void add_Directed_edge(int from, int to, int cap, int id = -1) {
    G[from].push_back((edge){to, cap, (int)G[to].size(), -1});
    G[to].push_back((edge){from, 0, (int)G[from].size() - 1, id});
  }

  void bfs(int s) {
    memset(&level[0], -1, sizeof(level[0]) * level.size());
    queue<int> que;
    level[s] = 0;
    que.push(s);
    while (!que.empty()) {
      int v = que.front();
      que.pop();
      REP(i, G[v].size()) {
        edge &e = G[v][i];
        if (e.cap > 0 && level[e.to] < 0) {
          level[e.to] = level[v] + 1;
          que.push(e.to);
        }
      }
    }
  }

  int dfs(int v, int t, int f) {
    if (v == t)
      return f;
    for (int &i = iter[v]; i < G[v].size(); i++) {
      edge &e = G[v][i];
      if (e.cap > 0 && level[v] < level[e.to]) {
        int d = dfs(e.to, t, min(f, e.cap));
        if (d > 0) {
          e.cap -= d;
          G[e.to][e.rev].cap += d;
          return d;
        }
      }
    }
    return 0;
  }

  // sからtへのmax flowを求める　flow_limitを設定したらそこで打ち切り
  int max_flow(int s, int t, int flow_limit = -1) {
    int flow = 0;
    for (;;) {
      bfs(s);
      if (level[t] < 0)
        return flow;
      memset(&iter[0], 0, sizeof(iter[0]) * iter.size());
      int f;
      while ((f = dfs(s, t, INF)) > 0) {
        flow += f;
        if (flow_limit >= 0 && flow >= flow_limit)
          return flow_limit;
      }
    }
  }

  // 実際の流量を求める、max_flow(s, t)を使ってから
  vector<int> calc_flow(int num_id) {
    vector<int> flow(num_id, 0);
    REP(v, N) for (auto e : G[v]) if (e.id >= 0) flow[e.id] = e.cap;
    return flow;
  }

  void dfs_for_calc_min_cut(int v, vector<bool> &belong2s) {
    belong2s[v] = true;
    for (edge e : G[v])
      if (e.cap > 0 && !belong2s[e.to])
        dfs_for_calc_min_cut(e.to, belong2s);
  }

  // 残余グラフでsから到達可能か求める、max_flow(s, t)を使ってから
  vector<bool> min_cut(int s) {
    vector<bool> belong2s(N);
    dfs_for_calc_min_cut(s, belong2s);
    return belong2s;
  }
};

signed main() {

  int N, M;
  cin >> N >> M;
  vector<string> S(N);
  cin >> S;

  Graph G(N * M + 2);
  int s = N * M, t = s + 1;

  REP(i, N) REP(j, M) if (S[i][j] == '.') {
    if ((i + j) % 2 == 0)
      G.add_Directed_edge(s, i * M + j, 1);
    else
      G.add_Directed_edge(i * M + j, t, 1);
  }

  int n = 0;
  v_Pii p;
  v_bool h;
  REP(i, N) REP(j, M) {
    if (i + 1 < N) {
      if ((i + j) % 2 == 0)
        G.add_Directed_edge(i * M + j, (i + 1) * M + j, 1, n);
      else
        G.add_Directed_edge((i + 1) * M + j, i * M + j, 1, n);
      p.pb(Pii(i, j));
      h.pb(true);
      n++;
    }
    if (j + 1 < M) {
      if ((i + j) % 2 == 0)
        G.add_Directed_edge(i * M + j, i * M + j + 1, 1, n);
      else
        G.add_Directed_edge(i * M + j + 1, i * M + j, 1, n);
      p.pb(Pii(i, j));
      h.pb(false);
      n++;
    }
  }

  int f = G.max_flow(s, t);
  Out(f);

  vec F = G.calc_flow(n);
  REP(k, n) if (F[k] > 0) {
    int i = p[k].fi, j = p[k].se;
    if (h[k]) {
      S[i][j] = '>';
      S[i][j + 1] = '<';
    } else {
      S[i][j] = 'v';
      S[i + 1][j] = '^';
    }
  }
  Out(S);

  return 0;
}