Problem p01410 (C++) — buggy submission, verdict: Time Limit Exceeded

#include <bits/stdc++.h>
using namespace std;
#define MAX_V 3500
#define INF 1 << 28
typedef pair<int, int> P;
struct edge {
  int to, cap, cost, rev;
  edge() {}
  edge(int to, int cap, int cost, int rev)
      : to(to), cap(cap), cost(cost), rev(rev) {}
};

int V;
vector<edge> G[MAX_V];
int h[MAX_V];
int dist[MAX_V];
int prevv[MAX_V], preve[MAX_V];

void add_edge(int from, int to, int cap, int cost) {
  G[from].push_back(edge(to, cap, cost, G[to].size()));
  G[to].push_back(edge(from, 0, -cost, G[from].size() - 1));
}
bool ff;
int min_cost_flow(int s, int t, int f) {
  int res = 0;
  fill(h, h + V, 0);
  while (f > 0) {
    priority_queue<P, vector<P>, greater<P>> que;
    fill(dist, dist + V, INF);
    dist[s] = 0;
    que.push(P(0, s));
    while (!que.empty()) {
      P p = que.top();
      que.pop();
      int v = p.second;
      if (dist[v] < p.first)
        continue;
      for (int i = 0; i < (int)G[v].size(); i++) {
        edge &e = G[v][i];
        if (e.cap > 0 && dist[e.to] > dist[v] + e.cost + h[v] - h[e.to]) {
          dist[e.to] = dist[v] + e.cost + h[v] - h[e.to];
          prevv[e.to] = v;
          preve[e.to] = i;
          que.push(P(dist[e.to], e.to));
        }
      }
    }
    if (dist[t] == INF) {
      return res;
    }
    for (int v = 0; v < V; v++)
      h[v] += dist[v];

    int d = f;
    for (int v = t; v != s; v = prevv[v]) {
      d = min(d, G[prevv[v]][preve[v]].cap);
    }
    f -= d;
    res += d * h[t];
    for (int v = t; v != s; v = prevv[v]) {
      edge &e = G[prevv[v]][preve[v]];
      e.cap -= d;
      G[v][e.rev].cap += d;
    }
  }
  return res;
}

int main() {
  int n;
  cin >> n;
  vector<P> ab(n);
  vector<int> v(n * 2);
  for (int i = 0; i < n; i++) {
    cin >> ab[i].first >> ab[i].second;
    v[i * 2] = ab[i].first;
    v[i * 2 + 1] = ab[i].second;
  }
  sort(v.begin(), v.end());
  v.erase(unique(v.begin(), v.end()), v.end());
  int m = v.size();
  map<int, int> r;
  for (int i = 0; i < m; i++)
    r[v[i]] = i;
  V = n + m + 2;
  int s = n + m, t = n + m + 1;
  for (int i = 0; i < n; i++)
    add_edge(s, i, 1, 0), add_edge(i, t, 1, 0);
  for (int i = 0; i < m; i++)
    add_edge(n + i, t, 1, 0);
  for (int i = 0; i < n; i++) {
    add_edge(i, n + r[ab[i].first], 1, -ab[i].second);
    add_edge(i, n + r[ab[i].second], 1, -ab[i].first);
  }
  cout << -min_cost_flow(s, t, n) << endl;
  return 0;
}

Error: TLE

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
#define MAX_V 3500
#define INF 1 << 28
typedef pair<int, int> P;
struct edge {
  int to, cap, cost, rev;
  edge() {}
  edge(int to, int cap, int cost, int rev)
      : to(to), cap(cap), cost(cost), rev(rev) {}
};

int V;
vector<edge> G[MAX_V];
int h[MAX_V];
int dist[MAX_V];
int prevv[MAX_V], preve[MAX_V];

void add_edge(int from, int to, int cap, int cost) {
  G[from].push_back(edge(to, cap, cost, G[to].size()));
  G[to].push_back(edge(from, 0, -cost, G[from].size() - 1));
}
bool ff;
int min_cost_flow(int s, int t, int f) {
  int res = 0;
  fill(h, h + V, 0);
  while (f > 0) {
    priority_queue<P, vector<P>, greater<P>> que;
    fill(dist, dist + V, INF);
    dist[s] = 0;
    que.push(P(0, s));
    while (!que.empty()) {
      P p = que.top();
      que.pop();
      int v = p.second;
      if (dist[v] < p.first)
        continue;
      for (int i = 0; i < (int)G[v].size(); i++) {
        edge &e = G[v][i];
        if (e.cap > 0 && dist[e.to] > dist[v] + e.cost + h[v] - h[e.to]) {
          dist[e.to] = dist[v] + e.cost + h[v] - h[e.to];
          prevv[e.to] = v;
          preve[e.to] = i;
          que.push(P(dist[e.to], e.to));
        }
      }
    }
    if (dist[t] == INF) {
      ff = 0;
      return -1;
    }
    for (int v = 0; v < V; v++)
      h[v] += dist[v];

    int d = f;
    for (int v = t; v != s; v = prevv[v]) {
      d = min(d, G[prevv[v]][preve[v]].cap);
    }
    f -= d;
    res += d * h[t];
    for (int v = t; v != s; v = prevv[v]) {
      edge &e = G[prevv[v]][preve[v]];
      e.cap -= d;
      G[v][e.rev].cap += d;
    }
  }
  return res;
}

int main() {
  int n;
  cin >> n;
  vector<P> ab(n);
  vector<int> v(n * 2);
  for (int i = 0; i < n; i++) {
    cin >> ab[i].first >> ab[i].second;
    v[i * 2] = ab[i].first;
    v[i * 2 + 1] = ab[i].second;
  }
  sort(v.begin(), v.end());
  v.erase(unique(v.begin(), v.end()), v.end());
  int m = v.size();
  map<int, int> r;
  for (int i = 0; i < m; i++)
    r[v[i]] = i;
  V = n + m + 2;
  int s = n + m, t = n + m + 1;
  for (int i = 0; i < n; i++)
    add_edge(s, i, 1, 0);
  for (int i = 0; i < m; i++)
    add_edge(n + i, t, 1, 0);
  for (int i = 0; i < n; i++) {
    add_edge(i, n + r[ab[i].first], 1, -ab[i].second);
    add_edge(i, n + r[ab[i].second], 1, -ab[i].first);
  }
  int ans = 0, res = 0, tmp, k = 0;
  ff = 1;
  while ((tmp = min_cost_flow(s, t, 1)), ff) {
    res += tmp;
    ans = min(ans, res);
    k++;
    if (k == n)
      break;
  }
  cout << -ans << endl;
  return 0;
}