Problem p01136 (C++) — buggy submission, verdict: Runtime Error

#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
int main() {
  int n, m, a;
  cin >> n;
  while (n != 0) {
    int i, j, k, l;
    int v[51][31] = {{}};
    for (i = 0; i < n; i++) {
      cin >> m;
      for (j = 0; j < m; j++) {
        cin >> a;
        v[i][a] = 1;
      }
    }
    vector<int> g[51];
    vector<int> p[51];
    for (i = 0; i < n; i++)
      g[i].push_back(i);
    bool f = false, x;
    for (i = 1; i < 31; i++) {
      // cout << "day" << i << endl;
      for (k = 0; k < n; k++) {
        sort(g[k].begin(), g[k].end());
        g[k].erase(unique(g[k].begin(), g[k].end()), g[k].end());
        p[k].clear();
        for (l = 0; l < g[k].size(); l++)
          p[k].push_back(g[k][l]);
        // for(l=0;l<p[k].size();l++) cout << p[k][l] << " ";
        // cout << endl;
      }
      for (j = 0; j < n; j++) {
        if (v[j][i] == 0)
          continue;
        for (k = j + 1; k < n; k++) {
          if (v[k][i] == 0)
            continue;
          // cout << i << ":" << j << ":" << k << endl;
          for (l = 0; l < p[j].size(); l++)
            g[k].push_back(p[j][l]);
          for (l = 0; l < p[k].size(); l++)
            g[j].push_back(p[k][l]);
        }
      }
      bool li[51];
      for (j = 0; j < n; j++) {
        for (k = 0; k < n; k++)
          li[k] = false;
        for (k = 0; k < g[j].size(); k++) {
          // cout << g[j][k] << " ";
          li[g[j][k]] = true;
        }
        // cout << endl;
        x = true;
        for (k = 0; k < n; k++)
          if (!li[k])
            x = false;
        if (x) {
          f = true;
          break;
        }
      }
      if (f) {
        cout << i << endl;
        break;
      }
    }
    if (!f)
      cout << -1 << endl;
    cin >> n;
  }
}

Error: 0

Fixed submission:

#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
int main() {
  int n, m, a;
  cin >> n;
  while (n != 0) {
    int i, j, k, l;
    int v[51][31] = {{}};
    for (i = 0; i < n; i++) {
      cin >> m;
      for (j = 0; j < m; j++) {
        cin >> a;
        v[i][a] = 1;
      }
    }
    vector<int> g[51];
    vector<int> p[51];
    for (i = 0; i < n; i++)
      g[i].push_back(i);
    bool f = false, x;
    for (i = 1; i < 31; i++) {
      // cout << "day" << i << endl;
      for (k = 0; k < n; k++) {
        sort(g[i].begin(), g[i].end());
        g[i].erase(unique(g[i].begin(), g[i].end()), g[i].end());
        p[k].clear();
        for (l = 0; l < g[k].size(); l++)
          p[k].push_back(g[k][l]);
        // for(l=0;l<p[k].size();l++) cout << p[k][l] << " ";
        // cout << endl;
      }
      for (j = 0; j < n; j++) {
        if (v[j][i] == 0)
          continue;
        for (k = j + 1; k < n; k++) {
          if (v[k][i] == 0)
            continue;
          // cout << i << ":" << j << ":" << k << endl;
          for (l = 0; l < p[j].size(); l++)
            g[k].push_back(p[j][l]);
          for (l = 0; l < p[k].size(); l++)
            g[j].push_back(p[k][l]);
        }
      }
      bool li[51];
      for (j = 0; j < n; j++) {
        for (k = 0; k < n; k++)
          li[k] = false;
        for (k = 0; k < g[j].size(); k++) {
          // cout << g[j][k] << " ";
          li[g[j][k]] = true;
        }
        // cout << endl;
        x = true;
        for (k = 0; k < n; k++)
          if (!li[k])
            x = false;
        if (x) {
          f = true;
          break;
        }
      }
      if (f) {
        cout << i << endl;
        break;
      }
    }
    if (!f)
      cout << -1 << endl;
    cin >> n;
  }
}