Problem p03409 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <complex>
#include <cstdlib>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <unordered_map>
#include <unordered_set>

using namespace std;
using i64 = int_fast64_t;
using ui64 = uint_fast64_t;
using db = long double;
using pii = pair<int, int>;
using pli = pair<int_fast64_t, int>;
using pll = pair<int_fast64_t, int_fast64_t>;
using pdi = pair<double, int>;
template <class T> using vct = vector<T>;
template <class T> using heap = priority_queue<T>;
template <class T> using minheap = priority_queue<T, vector<T>, greater<T>>;
template <class T> constexpr T inf = numeric_limits<T>::max() / 4 - 17;
constexpr int dx[9] = {1, 0, -1, 0, 1, -1, -1, 1, 0};
constexpr int dy[9] = {0, 1, 0, -1, 1, 1, -1, -1, 0};
constexpr long double gold = 1.618033988;
constexpr long double eps = 1e-15;

#define mod 1000000007LL
#define stdout_precision 10
#define stderr_precision 5
#define itr(i, v) for (auto i = begin(v); i != end(v); ++i)
#define ritr(i, v) for (auto i = rbegin(v); i != rend(v); ++i)
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rrep(i, n) for (int i = (n)-1; i >= 0; --i)
#define all(v) begin(v), end(v)
#define rall(v) rbegin(v), rend(v)
#define fir first
#define sec second
#define fro front
#define bac back
#define u_map unordered_map
#define u_set unordered_set
#define l_bnd lower_bound
#define u_bnd upper_bound
#define rsz resize
#define ers erase
#define emp emplace
#define emf emplace_front
#define emb emplace_back
#define pof pop_front
#define pob pop_back
#define mkp make_pair
#define mkt make_tuple
#define popcnt __builtin_popcount

struct setupper {
  setupper() {
    ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    std::cout.tie(nullptr);
    std::cerr.tie(nullptr);
    std::cout << fixed << setprecision(stdout_precision);
    std::cerr << fixed << setprecision(stderr_precision);
#ifdef LOCAL
    freopen("input.txt", "rt", stdin);
    freopen("output.txt", "wt", stdout);
    // std::cerr << "\n---stderr---\n";
    // auto print_atexit = []() {
    //     std::cerr << "Exec time : " << clock() / (double)CLOCKS_PER_SEC *
    //     1000.0 << "ms\n"; std::cerr << "------------\n";
    // };
    // atexit((void(*)())print_atexit);
#endif
  }
} setupper_;

namespace std {
template <class T> void hash_combine(size_t &seed, T const &key) {
  seed ^= hash<T>()(key) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
}
template <class T, class U> struct hash<pair<T, U>> {
  size_t operator()(pair<T, U> const &pr) const {
    size_t seed = 0;
    hash_combine(seed, pr.first);
    hash_combine(seed, pr.second);
    return seed;
  }
};
template <class Tup, size_t index = tuple_size<Tup>::value - 1>
struct hashval_calc {
  static void apply(size_t &seed, Tup const &tup) {
    hashval_calc<Tup, index - 1>::apply(seed, tup);
    hash_combine(seed, get<index>(tup));
  }
};
template <class Tup> struct hashval_calc<Tup, 0> {
  static void apply(size_t &seed, Tup const &tup) {
    hash_combine(seed, get<0>(tup));
  }
};
template <class... T> struct hash<tuple<T...>> {
  size_t operator()(tuple<T...> const &tup) const {
    size_t seed = 0;
    hashval_calc<tuple<T...>>::apply(seed, tup);
    return seed;
  }
};
} // namespace std

template <class T, class U> istream &operator>>(istream &s, pair<T, U> &p) {
  return s >> p.first >> p.second;
}
template <class T, class U>
ostream &operator<<(ostream &s, const pair<T, U> p) {
  return s << p.first << " " << p.second;
}
template <class T> ostream &operator<<(ostream &s, const vector<T> &v) {
  for (size_t i = 0; i < v.size(); ++i)
    s << (i ? " " : "") << v[i];
  return s;
}
#define dump(...)                                                              \
  cerr << " [ " << __LINE__ << " : " << __FUNCTION__ << " ] " << #__VA_ARGS__  \
       << " : ";                                                               \
  dump_func(__VA_ARGS__)
template <class T> void dump_func(T x) { cerr << x << '\n'; }
template <class T, class... Rest> void dump_func(T x, Rest... rest) {
  cerr << x << ",";
  dump_func(rest...);
}
template <class T = int> T read() {
  T x;
  return cin >> x, x;
}
template <class T> void write(T x) { cout << x << '\n'; }
template <class T, class... Rest> void write(T x, Rest... rest) {
  cout << x << ' ';
  write(rest...);
}
void writeln() {}
template <class T, class... Rest> void writeln(T x, Rest... rest) {
  cout << x << '\n';
  writeln(rest...);
}
#define esc(...) writeln(__VA_ARGS__), exit(0)

namespace updater {
template <class T> static void add(T &x, const T &y) { x += y; }
template <class T> static void ext_add(T &x, const T &y, size_t w) {
  x += y * w;
}
template <class T> static void mul(T &x, const T &y) { x *= y; }
template <class T> static void ext_mul(T &x, const T &y, size_t w) {
  x *= (T)pow(y, w);
}
template <class T, class U> static bool chmax(T &x, const U &y) {
  return x < y ? x = y, true : false;
}
template <class T, class U> static bool chmin(T &x, const U &y) {
  return x > y ? x = y, true : false;
}
}; // namespace updater
using updater::chmax;
using updater::chmin;

template <class T> T minf(const T &x, const T &y) { return min(x, y); }
template <class T> T mixf(const T &x, const T &y) { return max(x, y); }
bool bit(i64 n, uint8_t e) { return (n >> e) & 1; }
i64 mask(i64 n, uint8_t e) { return n & ((1 << e) - 1); }
int ilog(uint64_t x, uint64_t b = 2) { return x ? 1 + ilog(x / b, b) : -1; }
template <class F> i64 binry(i64 ok, i64 ng, const F &fn) {
  while (abs(ok - ng) > 1) {
    i64 mid = (ok + ng) / 2;
    (fn(mid) ? ok : ng) = mid;
  }
  return ok;
}
template <class A, size_t N, class T> void init(A (&array)[N], const T &val) {
  fill((T *)array, (T *)(array + N), val);
}
template <class A> void cmprs(A ary[], size_t n) {
  vector<A> tmp(ary, ary + n);
  tmp.erase(unique(begin(tmp), end(tmp)), end(tmp));
  for (A *i = ary; i != ary + n; ++i)
    *i = l_bnd(all(tmp), *i) - begin(tmp);
}
template <class T> void cmprs(vector<T> &v) {
  vector<T> tmp = v;
  sort(begin(tmp), end(tmp));
  tmp.erase(unique(begin(tmp), end(tmp)), end(tmp));
  for (auto i = begin(v); i != end(v); ++i)
    *i = l_bnd(all(tmp), *i) - begin(tmp);
}
template <class F> void for_subset(uint_fast64_t s, const F &fn) {
  uint_fast64_t tmp = s;
  do {
    fn(tmp);
  } while ((--tmp &= s) != s);
}

/* The main code follows. */

template <class cost_t = int, class cap_t = int> struct Flow {
  struct edge {
    int to;
    cost_t cost;
    cap_t cap;
    edge *rev;
  };
  size_t V, E;
  vector<vector<edge>> adj;

  Flow(size_t V_) : V(V_), E() {
    adj.resize(V);
    for (size_t v = 0; v != V; ++v)
      adj.reserve(V);
  }

  void add_edge(int from, int to, cost_t cost, cap_t cap) {
    adj[from].emplace_back((edge){to, cost, cap, nullptr});
    adj[to].emplace_back((edge){from, -cost, 0, nullptr});
    adj[from].back().rev = &(adj[to].back());
    adj[to].back().rev = &(adj[from].back());
    ++E;
  }

  vector<cost_t> Dijkstra(int s) {
    vector<cost_t> dist(V, inf<cost_t>);
    priority_queue<pair<cost_t, int>, vector<pair<cost_t, int>>,
                   greater<pair<cost_t, int>>>
        que;
    que.emplace(dist[s] = 0, s);
    while (!que.empty()) {
      auto p = que.top();
      que.pop();
      int v = p.second;
      if (dist[v] < p.first)
        continue;
      for (edge &e : adj[v]) {
        if (e.cap > 0 && dist[v] + e.cost < dist[e.to])
          que.emplace(dist[e.to] = dist[v] + e.cost, e.to);
      }
    }
    return dist;
  }

  struct Dinic_impl {
    vector<int> level, itr;
    Flow &F;

    Dinic_impl(Flow &F_) : F(F_) { level.resize(F.V), itr.resize(F.V); }

    void bfs(int s) {
      fill(begin(level), end(level), -1);
      queue<int> que;
      level[s] = 0;
      que.emplace(s);
      while (!que.empty()) {
        int v = que.front();
        que.pop();
        for (edge &e : F.adj[v]) {
          if (e.cap > 0 && level[e.to] < 0) {
            level[e.to] = level[v] + 1;
            que.emplace(e.to);
          }
        }
      }
    }

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

    cap_t max_flow(int s, int t) {
      cap_t flow = 0, f;
      while (true) {
        bfs(s);
        if (level[t] < 0)
          return flow;
        fill(begin(itr), end(itr), 0);
        while ((f = dfs(s, t, inf<cap_t>)) > 0)
          flow += f;
      }
    }
  };

  cap_t Dinic(int s, int t) { return Dinic_impl(*this).max_flow(s, t); }
};

int n;
vector<pii> red;
vector<pii> blue;

signed main() {
  cin >> n;
  rep(i, n) {
    int a, b;
    cin >> a >> b;
    red.emplace_back(a, b);
  }
  rep(i, n) {
    int a, b;
    cin >> a >> b;
    blue.emplace_back(a, b);
  }
  Flow<> flow(n * 2 + 2);
  rep(i, n) {
    flow.add_edge(0, i + 1, 1, 1);
    flow.add_edge(i + n + 1, n * 2 + 1, 1, 1);
  }
  rep(i, n) {
    rep(j, n) {
      if (red[i].fir < blue[j].fir && red[i].sec < blue[j].sec) {
        flow.add_edge(i + 1, j + n + 1, 1, 1);
      }
    }
  }
  esc(flow.Dinic(0, 2 * n + 1));
}

Fixed submission:
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <complex>
#include <cstdlib>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <unordered_map>
#include <unordered_set>

using namespace std;
using i64 = int_fast64_t;
using ui64 = uint_fast64_t;
using db = long double;
using pii = pair<int, int>;
using pli = pair<int_fast64_t, int>;
using pll = pair<int_fast64_t, int_fast64_t>;
using pdi = pair<double, int>;
template <class T> using vct = vector<T>;
template <class T> using heap = priority_queue<T>;
template <class T> using minheap = priority_queue<T, vector<T>, greater<T>>;
template <class T> constexpr T inf = numeric_limits<T>::max() / 4 - 17;
constexpr int dx[9] = {1, 0, -1, 0, 1, -1, -1, 1, 0};
constexpr int dy[9] = {0, 1, 0, -1, 1, 1, -1, -1, 0};
constexpr long double gold = 1.618033988;
constexpr long double eps = 1e-15;

#define mod 1000000007LL
#define stdout_precision 10
#define stderr_precision 5
#define itr(i, v) for (auto i = begin(v); i != end(v); ++i)
#define ritr(i, v) for (auto i = rbegin(v); i != rend(v); ++i)
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rrep(i, n) for (int i = (n)-1; i >= 0; --i)
#define all(v) begin(v), end(v)
#define rall(v) rbegin(v), rend(v)
#define fir first
#define sec second
#define fro front
#define bac back
#define u_map unordered_map
#define u_set unordered_set
#define l_bnd lower_bound
#define u_bnd upper_bound
#define rsz resize
#define ers erase
#define emp emplace
#define emf emplace_front
#define emb emplace_back
#define pof pop_front
#define pob pop_back
#define mkp make_pair
#define mkt make_tuple
#define popcnt __builtin_popcount

struct setupper {
  setupper() {
    ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    std::cout.tie(nullptr);
    std::cerr.tie(nullptr);
    std::cout << fixed << setprecision(stdout_precision);
    std::cerr << fixed << setprecision(stderr_precision);
#ifdef LOCAL
    freopen("input.txt", "rt", stdin);
    freopen("output.txt", "wt", stdout);
    // std::cerr << "\n---stderr---\n";
    // auto print_atexit = []() {
    //     std::cerr << "Exec time : " << clock() / (double)CLOCKS_PER_SEC *
    //     1000.0 << "ms\n"; std::cerr << "------------\n";
    // };
    // atexit((void(*)())print_atexit);
#endif
  }
} setupper_;

namespace std {
template <class T> void hash_combine(size_t &seed, T const &key) {
  seed ^= hash<T>()(key) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
}
template <class T, class U> struct hash<pair<T, U>> {
  size_t operator()(pair<T, U> const &pr) const {
    size_t seed = 0;
    hash_combine(seed, pr.first);
    hash_combine(seed, pr.second);
    return seed;
  }
};
template <class Tup, size_t index = tuple_size<Tup>::value - 1>
struct hashval_calc {
  static void apply(size_t &seed, Tup const &tup) {
    hashval_calc<Tup, index - 1>::apply(seed, tup);
    hash_combine(seed, get<index>(tup));
  }
};
template <class Tup> struct hashval_calc<Tup, 0> {
  static void apply(size_t &seed, Tup const &tup) {
    hash_combine(seed, get<0>(tup));
  }
};
template <class... T> struct hash<tuple<T...>> {
  size_t operator()(tuple<T...> const &tup) const {
    size_t seed = 0;
    hashval_calc<tuple<T...>>::apply(seed, tup);
    return seed;
  }
};
} // namespace std

template <class T, class U> istream &operator>>(istream &s, pair<T, U> &p) {
  return s >> p.first >> p.second;
}
template <class T, class U>
ostream &operator<<(ostream &s, const pair<T, U> p) {
  return s << p.first << " " << p.second;
}
template <class T> ostream &operator<<(ostream &s, const vector<T> &v) {
  for (size_t i = 0; i < v.size(); ++i)
    s << (i ? " " : "") << v[i];
  return s;
}
#define dump(...)                                                              \
  cerr << " [ " << __LINE__ << " : " << __FUNCTION__ << " ] " << #__VA_ARGS__  \
       << " : ";                                                               \
  dump_func(__VA_ARGS__)
template <class T> void dump_func(T x) { cerr << x << '\n'; }
template <class T, class... Rest> void dump_func(T x, Rest... rest) {
  cerr << x << ",";
  dump_func(rest...);
}
template <class T = int> T read() {
  T x;
  return cin >> x, x;
}
template <class T> void write(T x) { cout << x << '\n'; }
template <class T, class... Rest> void write(T x, Rest... rest) {
  cout << x << ' ';
  write(rest...);
}
void writeln() {}
template <class T, class... Rest> void writeln(T x, Rest... rest) {
  cout << x << '\n';
  writeln(rest...);
}
#define esc(...) writeln(__VA_ARGS__), exit(0)

namespace updater {
template <class T> static void add(T &x, const T &y) { x += y; }
template <class T> static void ext_add(T &x, const T &y, size_t w) {
  x += y * w;
}
template <class T> static void mul(T &x, const T &y) { x *= y; }
template <class T> static void ext_mul(T &x, const T &y, size_t w) {
  x *= (T)pow(y, w);
}
template <class T, class U> static bool chmax(T &x, const U &y) {
  return x < y ? x = y, true : false;
}
template <class T, class U> static bool chmin(T &x, const U &y) {
  return x > y ? x = y, true : false;
}
}; // namespace updater
using updater::chmax;
using updater::chmin;

template <class T> T minf(const T &x, const T &y) { return min(x, y); }
template <class T> T mixf(const T &x, const T &y) { return max(x, y); }
bool bit(i64 n, uint8_t e) { return (n >> e) & 1; }
i64 mask(i64 n, uint8_t e) { return n & ((1 << e) - 1); }
int ilog(uint64_t x, uint64_t b = 2) { return x ? 1 + ilog(x / b, b) : -1; }
template <class F> i64 binry(i64 ok, i64 ng, const F &fn) {
  while (abs(ok - ng) > 1) {
    i64 mid = (ok + ng) / 2;
    (fn(mid) ? ok : ng) = mid;
  }
  return ok;
}
template <class A, size_t N, class T> void init(A (&array)[N], const T &val) {
  fill((T *)array, (T *)(array + N), val);
}
template <class A> void cmprs(A ary[], size_t n) {
  vector<A> tmp(ary, ary + n);
  tmp.erase(unique(begin(tmp), end(tmp)), end(tmp));
  for (A *i = ary; i != ary + n; ++i)
    *i = l_bnd(all(tmp), *i) - begin(tmp);
}
template <class T> void cmprs(vector<T> &v) {
  vector<T> tmp = v;
  sort(begin(tmp), end(tmp));
  tmp.erase(unique(begin(tmp), end(tmp)), end(tmp));
  for (auto i = begin(v); i != end(v); ++i)
    *i = l_bnd(all(tmp), *i) - begin(tmp);
}
template <class F> void for_subset(uint_fast64_t s, const F &fn) {
  uint_fast64_t tmp = s;
  do {
    fn(tmp);
  } while ((--tmp &= s) != s);
}

/* The main code follows. */

template <class cost_t = int, class cap_t = int> struct Flow {
  struct edge {
    int to;
    cost_t cost;
    cap_t cap;
    edge *rev;
  };
  size_t V, E;
  vector<vector<edge>> adj;

  Flow(size_t V_) : V(V_), E() {
    adj.resize(V);
    for (size_t v = 0; v != V; ++v)
      adj[v].reserve(V);
  }

  void add_edge(int from, int to, cost_t cost, cap_t cap) {
    adj[from].emplace_back((edge){to, cost, cap, nullptr});
    adj[to].emplace_back((edge){from, -cost, 0, nullptr});
    adj[from].back().rev = &(adj[to].back());
    adj[to].back().rev = &(adj[from].back());
    ++E;
  }

  vector<cost_t> Dijkstra(int s) {
    vector<cost_t> dist(V, inf<cost_t>);
    priority_queue<pair<cost_t, int>, vector<pair<cost_t, int>>,
                   greater<pair<cost_t, int>>>
        que;
    que.emplace(dist[s] = 0, s);
    while (!que.empty()) {
      auto p = que.top();
      que.pop();
      int v = p.second;
      if (dist[v] < p.first)
        continue;
      for (edge &e : adj[v]) {
        if (e.cap > 0 && dist[v] + e.cost < dist[e.to])
          que.emplace(dist[e.to] = dist[v] + e.cost, e.to);
      }
    }
    return dist;
  }

  struct Dinic_impl {
    vector<int> level, itr;
    Flow &F;

    Dinic_impl(Flow &F_) : F(F_) { level.resize(F.V), itr.resize(F.V); }

    void bfs(int s) {
      fill(begin(level), end(level), -1);
      queue<int> que;
      level[s] = 0;
      que.emplace(s);
      while (!que.empty()) {
        int v = que.front();
        que.pop();
        for (edge &e : F.adj[v]) {
          if (e.cap > 0 && level[e.to] < 0) {
            level[e.to] = level[v] + 1;
            que.emplace(e.to);
          }
        }
      }
    }

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

    cap_t max_flow(int s, int t) {
      cap_t flow = 0, f;
      while (true) {
        bfs(s);
        if (level[t] < 0)
          return flow;
        fill(begin(itr), end(itr), 0);
        while ((f = dfs(s, t, inf<cap_t>)) > 0)
          flow += f;
      }
    }
  };

  cap_t Dinic(int s, int t) { return Dinic_impl(*this).max_flow(s, t); }
};

int n;
vector<pii> red;
vector<pii> blue;

signed main() {
  cin >> n;
  rep(i, n) {
    int a, b;
    cin >> a >> b;
    red.emplace_back(a, b);
  }
  rep(i, n) {
    int a, b;
    cin >> a >> b;
    blue.emplace_back(a, b);
  }
  Flow<> flow(n * 2 + 2);
  rep(i, n) {
    flow.add_edge(0, i + 1, 1, 1);
    flow.add_edge(i + n + 1, n * 2 + 1, 1, 1);
  }
  rep(i, n) {
    rep(j, n) {
      if (red[i].fir < blue[j].fir && red[i].sec < blue[j].sec) {
        flow.add_edge(i + 1, j + n + 1, 1, 1);
      }
    }
  }
  esc(flow.Dinic(0, 2 * n + 1));
}