Problem p01825 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<ll, ll> P;

#define EACH(i, a) for (auto &i : a)
#define FOR(i, a, b) for (ll i = (a); i < (b); i++)
#define RFOR(i, a, b) for (ll i = (b)-1; i >= (a); i--)
#define REP(i, n) for (ll i = 0; i < (n); i++)
#define RREP(i, n) for (ll i = (n)-1; i >= 0; i--)
#define debug(x) cout << #x << ": " << x << endl
#define pb push_back
#define ALL(a) (a).begin(), (a).end()

const ll linf = 1e18;
const int inf = 1e9;
const double eps = 1e-12;
const double pi = acos(-1);

template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  EACH(x, vec) is >> x;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, vector<T> &vec) {
  REP(i, vec.size()) {
    if (i)
      os << " ";
    os << vec[i];
  }
  return os;
}
template <typename T> ostream &operator<<(ostream &os, vector<vector<T>> &vec) {
  REP(i, vec.size()) {
    if (i)
      os << endl;
    os << vec[i];
  }
  return os;
}

typedef double Weight;

struct Edge {
  ll to, cap;
  Weight cost;
  ll rev;
};

vector<vector<Edge>> G;

void add_edge(ll from, ll to, ll cap, Weight cost) {
  if (from < 0 || to < 0)
    return;
  G[from].push_back({to, cap, cost, (ll)G[to].size()});
  G[to].push_back({from, 0, -cost, (ll)G[from].size() - 1});
}

Weight dist[21000], h[21000] = {0};
ll prevV[21000], prevE[21000];
double min_cost_flow(ll s, ll t, ll f) {
  typedef pair<Weight, ll> _P;
  Weight res = 0;
  while (f > 0) {
    fill_n(dist, 21000, inf);
    dist[s] = 0;
    priority_queue<_P, vector<_P>, greater<_P>> Q;
    Q.push({0, s});
    while (!Q.empty()) {
      _P p = Q.top();
      Q.pop();
      ll v = p.second;
      if (p.first > dist[v])
        continue;
      REP(i, G[v].size()) {
        Edge &e = G[v][i];
        if (e.cap > 0 && dist[v] + e.cost + h[v] - h[e.to] < dist[e.to]) {
          dist[e.to] = dist[v] + e.cost + h[v] - h[e.to];
          prevV[e.to] = v;
          prevE[e.to] = i;
          Q.push({dist[e.to], e.to});
        }
      }
    }
    REP(i, G.size()) h[i] += dist[i];

    if (abs(dist[t] - inf) < eps) {
      cout << "ERROR" << endl;
      exit(1);
    }

    ll d = f;
    for (ll v = t; v != s; v = prevV[v]) {
      d = min(d, G[prevV[v]][prevE[v]].cap);
    }
    f -= d;
    res += d * h[t];
    for (ll v = t; v != s; v = prevV[v]) {
      Edge &e = G[prevV[v]][prevE[v]];
      e.cap -= d;
      G[e.to][e.rev].cap += d;
    }
  }
  return res;
}

typedef complex<double> Point;
template <typename T> istream &operator>>(istream &is, complex<T> &p) {
  T x, y;
  is >> x >> y;
  p = {x, y};
  return is;
}

int main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(0);
  int n;
  cin >> n;
  Point sp;
  cin >> sp;
  vector<pair<Point, Point>> v(n);
  REP(i, n) cin >> v[i].first >> v[i].second;
  double ans = 0;
  REP(i, n) ans += abs(v[i].second - v[i].first);
  int lid = 0;
  vector<int> fid(n), tid(n);
  REP(i, n) fid[i] = lid++;
  REP(i, n) tid[i] = lid++;
  int s = lid++, t = lid++;
  G.clear();
  G.resize(lid);
  REP(i, n) REP(j, n) {
    double cost = abs(v[i].second - v[j].first);
    add_edge(fid[i], tid[j], 1, cost);
  }
  REP(i, n) add_edge(s, fid[i], 1, 0);
  REP(i, n) add_edge(tid[i], t, 1, 0);
  ans += min_cost_flow(s, t, n);
  cout << fixed << setprecision(10) << ans << endl;
}

Fixed submission:
#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<ll, ll> P;

#define EACH(i, a) for (auto &i : a)
#define FOR(i, a, b) for (ll i = (a); i < (b); i++)
#define RFOR(i, a, b) for (ll i = (b)-1; i >= (a); i--)
#define REP(i, n) for (ll i = 0; i < (n); i++)
#define RREP(i, n) for (ll i = (n)-1; i >= 0; i--)
#define debug(x) cout << #x << ": " << x << endl
#define pb push_back
#define ALL(a) (a).begin(), (a).end()

const ll linf = 1e18;
const int inf = 1e9;
const double eps = 1e-12;
const double pi = acos(-1);

template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  EACH(x, vec) is >> x;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, vector<T> &vec) {
  REP(i, vec.size()) {
    if (i)
      os << " ";
    os << vec[i];
  }
  return os;
}
template <typename T> ostream &operator<<(ostream &os, vector<vector<T>> &vec) {
  REP(i, vec.size()) {
    if (i)
      os << endl;
    os << vec[i];
  }
  return os;
}

typedef double Weight;

struct Edge {
  ll to, cap;
  Weight cost;
  ll rev;
};

vector<vector<Edge>> G;

void add_edge(ll from, ll to, ll cap, Weight cost) {
  if (from < 0 || to < 0)
    return;
  G[from].push_back({to, cap, cost, (ll)G[to].size()});
  G[to].push_back({from, 0, -cost, (ll)G[from].size() - 1});
}

Weight dist[21000], h[21000] = {0};
ll prevV[21000], prevE[21000];
double min_cost_flow(ll s, ll t, ll f) {
  typedef pair<Weight, ll> _P;
  Weight res = 0;
  while (f > 0) {
    fill_n(dist, 21000, inf);
    dist[s] = 0;
    priority_queue<_P, vector<_P>, greater<_P>> Q;
    Q.push({0, s});
    while (!Q.empty()) {
      _P p = Q.top();
      Q.pop();
      ll v = p.second;
      if (p.first > dist[v])
        continue;
      REP(i, G[v].size()) {
        Edge &e = G[v][i];
        if (e.cap > eps &&
            dist[v] + e.cost + h[v] - h[e.to] + eps < dist[e.to]) {
          dist[e.to] = dist[v] + e.cost + h[v] - h[e.to];
          prevV[e.to] = v;
          prevE[e.to] = i;
          Q.push({dist[e.to], e.to});
        }
      }
    }
    REP(i, G.size()) h[i] += dist[i];

    if (abs(dist[t] - inf) < eps) {
      cout << "ERROR" << endl;
      exit(1);
    }

    ll d = f;
    for (ll v = t; v != s; v = prevV[v]) {
      d = min(d, G[prevV[v]][prevE[v]].cap);
    }
    f -= d;
    res += d * h[t];
    for (ll v = t; v != s; v = prevV[v]) {
      Edge &e = G[prevV[v]][prevE[v]];
      e.cap -= d;
      G[e.to][e.rev].cap += d;
    }
  }
  return res;
}

typedef complex<double> Point;
template <typename T> istream &operator>>(istream &is, complex<T> &p) {
  T x, y;
  is >> x >> y;
  p = {x, y};
  return is;
}

int main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(0);
  int n;
  cin >> n;
  Point sp;
  cin >> sp;
  vector<pair<Point, Point>> v(n);
  REP(i, n) cin >> v[i].first >> v[i].second;
  double ans = 0;
  REP(i, n) ans += abs(v[i].second - v[i].first);
  int lid = 0;
  vector<int> fid(n), tid(n);
  REP(i, n) fid[i] = lid++;
  REP(i, n) tid[i] = lid++;
  int s = lid++, t = lid++;
  G.clear();
  G.resize(lid);
  REP(i, n) REP(j, n) {
    double cost = abs(v[i].second - v[j].first);
    add_edge(fid[i], tid[j], 1, cost);
  }
  REP(i, n) add_edge(s, fid[i], 1, 0);
  REP(i, n) add_edge(tid[i], t, 1, 0);
  ans += min_cost_flow(s, t, n);
  cout << fixed << setprecision(10) << ans << endl;
}