Problem p01187 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>
#define r(i, n) for (int i = 0; i < n; i++)
using namespace std;
struct Dinic {
  const int INF = 1 << 28;

  struct edge {
    int to, cap, rev;
    edge() {}
    edge(int to, int cap, int rev) : to(to), cap(cap), rev(rev) {}
  };

  int n;
  vector<vector<edge>> G;
  vector<map<int, int>> M;
  vector<int> level, iter;

  Dinic() {}
  Dinic(int sz) : n(sz), G(n), M(n), level(n), iter(n) {}

  void add_edge(int from, int to, int cap) {
    M[from][to] = G[from].size();
    M[to][from] = G[to].size();
    G[from].push_back(edge(to, cap, G[to].size()));
    // undirected
    // G[to].push_back(edge(from,cap,G[from].size()-1));
    // directed
    G[to].push_back(edge(from, 0, G[from].size() - 1));
  }

  void bfs(int s) {
    fill(level.begin(), level.end(), -1);
    queue<int> que;
    level[s] = 0;
    que.push(s);
    while (!que.empty()) {
      int v = que.front();
      que.pop();
      for (int i = 0; i < (int)G[v].size(); i++) {
        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 < (int)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;
  }

  int flow(int s, int t, int lim) {
    int fl = 0;
    for (;;) {
      bfs(s);
      if (level[t] < 0 || lim == 0)
        return fl;
      fill(iter.begin(), iter.end(), 0);
      int f;
      while ((f = dfs(s, t, lim)) > 0) {
        fl += f;
        lim -= f;
      }
    }
  }

  int flow(int s, int t) { return flow(s, t, INF); }

  // cap==1 only
  bool back_edge(int s, int t, int from, int to) {
    for (int i = 0; i < (int)G[from].size(); i++) {
      edge &e = G[from][i];
      if (e.to == to) {
        if (e.cap == 0 && flow(from, to, 1) == 0) {
          flow(from, s, 1);
          flow(t, to, 1);
          return 1;
        }
      }
    }
    return 0;
  }
};
int main() {
  int n, m, x, N;
  while (cin >> n, n) {
    map<int, int> M;
    cin >> N;
    r(i, N) {
      cin >> x;
      M[x] = i;
    }
    Dinic d(N + n + 2);
    r(i, N) d.add_edge(N + n, i, 1);
    r(i, n) d.add_edge(N + i, N + n + 1, 1);
    r(i, n) {
      cin >> m;
      r(j, m) {
        cin >> x;
        if (M.count(x))
          d.add_edge(M[x], N + i, 1);
      }
    }
    cout << d.flow(N + n, N + n + 1, 1e8) << endl;
  }
}


Error: 0

Fixed submission:

#include <bits/stdc++.h>
#define r(i, n) for (int i = 0; i < n; i++)
using namespace std;
struct Dinic {
  const int INF = 1 << 28;

  struct edge {
    int to, cap, rev;
    edge() {}
    edge(int to, int cap, int rev) : to(to), cap(cap), rev(rev) {}
  };

  int n;
  vector<vector<edge>> G;
  vector<map<int, int>> M;
  vector<int> level, iter;

  Dinic() {}
  Dinic(int sz) : n(sz), G(n), M(n), level(n), iter(n) {}

  void add_edge(int from, int to, int cap) {
    M[from][to] = G[from].size();
    M[to][from] = G[to].size();
    G[from].push_back(edge(to, cap, G[to].size()));
    // undirected
    // G[to].push_back(edge(from,cap,G[from].size()-1));
    // directed
    G[to].push_back(edge(from, 0, G[from].size() - 1));
  }

  void bfs(int s) {
    fill(level.begin(), level.end(), -1);
    queue<int> que;
    level[s] = 0;
    que.push(s);
    while (!que.empty()) {
      int v = que.front();
      que.pop();
      for (int i = 0; i < (int)G[v].size(); i++) {
        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 < (int)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;
  }

  int flow(int s, int t, int lim) {
    int fl = 0;
    for (;;) {
      bfs(s);
      if (level[t] < 0 || lim == 0)
        return fl;
      fill(iter.begin(), iter.end(), 0);
      int f;
      while ((f = dfs(s, t, lim)) > 0) {
        fl += f;
        lim -= f;
      }
    }
  }

  int flow(int s, int t) { return flow(s, t, INF); }

  // cap==1 only
  bool back_edge(int s, int t, int from, int to) {
    for (int i = 0; i < (int)G[from].size(); i++) {
      edge &e = G[from][i];
      if (e.to == to) {
        if (e.cap == 0 && flow(from, to, 1) == 0) {
          flow(from, s, 1);
          flow(t, to, 1);
          return 1;
        }
      }
    }
    return 0;
  }
};
int main() {
  int n, m, x, N;
  while (cin >> n, n) {
    map<int, int> M;
    cin >> N;
    r(i, N) {
      cin >> x;
      M[x] = i;
    }
    Dinic d(N + n + 2);
    r(i, N) d.add_edge(N + n, i, 1);
    r(i, n) d.add_edge(N + i, N + n + 1, 1);
    r(i, n) {
      cin >> m;
      r(j, m) {
        cin >> x;
        if (M.count(x))
          d.add_edge(M[x], n + i, 1);
      }
    }
    cout << d.flow(N + n, N + n + 1, 1e8) << endl;
  }
}