Problem p03409 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
#define MOD 1000000007
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep1(i, n) for (int i = 1; i <= (int)(n); i++)
#define showmap(is, js, x)                                                     \
  {                                                                            \
    rep(i, is) {                                                               \
      rep(j, js) { cout << x[i][j] << " "; }                                   \
      cout << endl;                                                            \
    }                                                                          \
  }
#define show(x)                                                                \
  {                                                                            \
    for (auto i : x) {                                                         \
      cout << i << " ";                                                        \
    }                                                                          \
    cout << endl;                                                              \
  }
#define showm(m)                                                               \
  {                                                                            \
    for (auto i : m) {                                                         \
      cout << m.x << " ";                                                      \
    }                                                                          \
    cout << endl;                                                              \
  }
typedef long long ll;
typedef pair<int, int> P;
typedef pair<ll, ll> llP;
ll gcd(int x, int y) { return y ? gcd(y, x % y) : x; }
ll lcm(ll x, ll y) { return (x * y) / gcd(x, y); }

typedef int FLOW;           // フローを表す型、今回は int 型
const int MAX_V = 100;      // グラフの最大ノード数
const FLOW INF = 100000000; // 十分大きい値

// グラフの辺の構造体
struct Edge {
  int rev, from, to;
  FLOW cap, icap;
  Edge(int r, int f, int t, FLOW c) : rev(r), from(f), to(t), cap(c), icap(c) {}
  friend ostream &operator<<(ostream &s, const Edge &E) {
    if (E.cap > 0)
      return s << E.from << "->" << E.to << '(' << E.cap << ')';
    else
      return s;
  }
};

// グラフ構造体
struct Graph {
  int V;
  vector<Edge> list[MAX_V];

  Graph(int n = 0) : V(n) {
    for (int i = 0; i < MAX_V; ++i)
      list[i].clear();
  }
  void init(int n = 0) {
    V = n;
    for (int i = 0; i < MAX_V; ++i)
      list[i].clear();
  }
  void resize(int n = 0) { V = n; }
  void reset() {
    for (int i = 0; i < V; ++i)
      for (int j = 0; j < list[i].size(); ++j)
        list[i][j].cap = list[i][j].icap;
  }
  inline vector<Edge> &operator[](int i) { return list[i]; }

  Edge &redge(Edge e) {
    if (e.from != e.to)
      return list[e.to][e.rev];
    else
      return list[e.to][e.rev + 1];
  }

  void addedge(int from, int to, FLOW cap) {
    list[from].push_back(Edge((int)list[to].size(), from, to, cap));
    list[to].push_back(Edge((int)list[from].size() - 1, to, from, 0));
  }
};

// 最大流を求めるサブルーチンたち
static int level[MAX_V];
static int iter[MAX_V];

void dibfs(Graph &G, int s) {
  for (int i = 0; i < MAX_V; ++i)
    level[i] = -1;
  level[s] = 0;
  queue<int> que;
  que.push(s);
  while (!que.empty()) {
    int v = que.front();
    que.pop();
    for (int i = 0; i < G[v].size(); ++i) {
      Edge &e = G[v][i];
      if (level[e.to] < 0 && e.cap > 0) {
        level[e.to] = level[v] + 1;
        que.push(e.to);
      }
    }
  }
}

FLOW didfs(Graph &G, int v, int t, FLOW f) {
  if (v == t)
    return f;
  for (int &i = iter[v]; i < G[v].size(); ++i) {
    Edge &e = G[v][i], &re = G.redge(e);
    if (level[v] < level[e.to] && e.cap > 0) {
      FLOW d = didfs(G, e.to, t, min(f, e.cap));
      if (d > 0) {
        e.cap -= d;
        re.cap += d;
        return d;
      }
    }
  }
  return 0;
}

// 最大流を求めるメイン関数
FLOW Dinic(Graph &G, int s, int t) {
  FLOW res = 0;
  while (true) {
    dibfs(G, s);
    if (level[t] < 0)
      return res;
    memset(iter, 0, sizeof(iter));
    FLOW flow;
    while ((flow = didfs(G, s, t, INF)) > 0) {
      res += flow;
    }
  }
}

int main() {
  int n;

  cin >> n;
  vector<P> rp(n), bp(n);

  rep(i, n) { cin >> rp[i].first >> rp[i].second; }
  rep(i, n) { cin >> bp[i].first >> bp[i].second; }

  Graph G(n + n + 2);

  int s_node = n + n;
  int t_node = n + n + 1;

  rep(i, n) {
    rep(j, n) {
      if (rp[i].first <= bp[j].first && rp[i].second <= bp[j].second) {
        G.addedge(i, j + n, 1);
      }
    }
  }

  rep(i, n) G.addedge(s_node, i, 1);
  rep(i, n) G.addedge(i + n, t_node, 1);

  FLOW ans = Dinic(G, s_node, t_node);
  cout << ans << endl;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define MOD 1000000007
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep1(i, n) for (int i = 1; i <= (int)(n); i++)
#define showmap(is, js, x)                                                     \
  {                                                                            \
    rep(i, is) {                                                               \
      rep(j, js) { cout << x[i][j] << " "; }                                   \
      cout << endl;                                                            \
    }                                                                          \
  }
#define show(x)                                                                \
  {                                                                            \
    for (auto i : x) {                                                         \
      cout << i << " ";                                                        \
    }                                                                          \
    cout << endl;                                                              \
  }
#define showm(m)                                                               \
  {                                                                            \
    for (auto i : m) {                                                         \
      cout << m.x << " ";                                                      \
    }                                                                          \
    cout << endl;                                                              \
  }
typedef long long ll;
typedef pair<int, int> P;
typedef pair<ll, ll> llP;
ll gcd(int x, int y) { return y ? gcd(y, x % y) : x; }
ll lcm(ll x, ll y) { return (x * y) / gcd(x, y); }

typedef int FLOW;           // フローを表す型、今回は int 型
const int MAX_V = 210;      // グラフの最大ノード数
const FLOW INF = 100000000; // 十分大きい値

// グラフの辺の構造体
struct Edge {
  int rev, from, to;
  FLOW cap, icap;
  Edge(int r, int f, int t, FLOW c) : rev(r), from(f), to(t), cap(c), icap(c) {}
  friend ostream &operator<<(ostream &s, const Edge &E) {
    if (E.cap > 0)
      return s << E.from << "->" << E.to << '(' << E.cap << ')';
    else
      return s;
  }
};

// グラフ構造体
struct Graph {
  int V;
  vector<Edge> list[MAX_V];

  Graph(int n = 0) : V(n) {
    for (int i = 0; i < MAX_V; ++i)
      list[i].clear();
  }
  void init(int n = 0) {
    V = n;
    for (int i = 0; i < MAX_V; ++i)
      list[i].clear();
  }
  void resize(int n = 0) { V = n; }
  void reset() {
    for (int i = 0; i < V; ++i)
      for (int j = 0; j < list[i].size(); ++j)
        list[i][j].cap = list[i][j].icap;
  }
  inline vector<Edge> &operator[](int i) { return list[i]; }

  Edge &redge(Edge e) {
    if (e.from != e.to)
      return list[e.to][e.rev];
    else
      return list[e.to][e.rev + 1];
  }

  void addedge(int from, int to, FLOW cap) {
    list[from].push_back(Edge((int)list[to].size(), from, to, cap));
    list[to].push_back(Edge((int)list[from].size() - 1, to, from, 0));
  }
};

// 最大流を求めるサブルーチンたち
static int level[MAX_V];
static int iter[MAX_V];

void dibfs(Graph &G, int s) {
  for (int i = 0; i < MAX_V; ++i)
    level[i] = -1;
  level[s] = 0;
  queue<int> que;
  que.push(s);
  while (!que.empty()) {
    int v = que.front();
    que.pop();
    for (int i = 0; i < G[v].size(); ++i) {
      Edge &e = G[v][i];
      if (level[e.to] < 0 && e.cap > 0) {
        level[e.to] = level[v] + 1;
        que.push(e.to);
      }
    }
  }
}

FLOW didfs(Graph &G, int v, int t, FLOW f) {
  if (v == t)
    return f;
  for (int &i = iter[v]; i < G[v].size(); ++i) {
    Edge &e = G[v][i], &re = G.redge(e);
    if (level[v] < level[e.to] && e.cap > 0) {
      FLOW d = didfs(G, e.to, t, min(f, e.cap));
      if (d > 0) {
        e.cap -= d;
        re.cap += d;
        return d;
      }
    }
  }
  return 0;
}

// 最大流を求めるメイン関数
FLOW Dinic(Graph &G, int s, int t) {
  FLOW res = 0;
  while (true) {
    dibfs(G, s);
    if (level[t] < 0)
      return res;
    memset(iter, 0, sizeof(iter));
    FLOW flow;
    while ((flow = didfs(G, s, t, INF)) > 0) {
      res += flow;
    }
  }
}

int main() {
  int n;

  cin >> n;
  vector<P> rp(n), bp(n);

  rep(i, n) { cin >> rp[i].first >> rp[i].second; }
  rep(i, n) { cin >> bp[i].first >> bp[i].second; }

  Graph G(n + n + 2);

  int s_node = n + n;
  int t_node = n + n + 1;

  rep(i, n) {
    rep(j, n) {
      if (rp[i].first <= bp[j].first && rp[i].second <= bp[j].second) {
        G.addedge(i, j + n, 1);
      }
    }
  }

  rep(i, n) G.addedge(s_node, i, 1);
  rep(i, n) G.addedge(i + n, t_node, 1);

  FLOW ans = Dinic(G, s_node, t_node);
  cout << ans << endl;
}