Problem p00895 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <cctype>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <vector>

using namespace std;

inline int toInt(string s) {
  int v;
  istringstream sin(s);
  sin >> v;
  return v;
}
template <class T> inline string toString(T x) {
  ostringstream sout;
  sout << x;
  return sout.str();
}
template <class T> inline T sqr(T x) { return x * x; }

typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<string> vs;
typedef pair<int, int> pii;
typedef long long ll;

#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define pb push_back
#define mp make_pair
#define each(i, c)                                                             \
  for (typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define exist(s, e) ((s).find(e) != (s).end())
#define range(i, a, b) for (int i = (a); i < (b); ++i)
#define rep(i, n) range(i, 0, n)
#define clr(a, b) memset((a), (b), sizeof(a))
#define dump(x) cerr << #x << " = " << (x) << endl;
#define debug(x)                                                               \
  cerr << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \
       << " " << __FILE__ << endl;

#define x first
#define y second

const double eps = 1e-10;
const double pi = acos(-1.0);
const ll INF = 1LL << 62;
const int inf = 1 << 29;

int dx[] = {1, 1, 0, -1, -1, -1, 0, 1};
int dy[] = {0, 1, 1, 1, 0, -1, -1, -1};

int main(void) {
  for (int w, h; cin >> h >> w, h;) {
    vs strs(h);
    rep(y, h) { cin >> strs[y]; }

    set<string> s;
    string res = "0";
    rep(y, h) {
      rep(x, w) {
        rep(i, 8) {
          string cur;
          pii next = mp(x, y);

          do {
            cur.pb(strs[next.y][next.x]);
            if (1 < cur.size() && !s.insert(cur).second) {
              if ((res.size() < cur.size()) ||
                  (res.size() == cur.size() && cur < res)) {
                res = cur;
              }
            }

            next.x = (next.x + dx[i] + w) % w;
            next.y = (next.y + dy[i] + h) % h;
            debug(cur);

          } while (next != mp(x, y));
        }
      }
    }

    cout << res << endl;
  }

  return 0;
}

Fixed submission:
#include <algorithm>
#include <bitset>
#include <cctype>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <vector>

using namespace std;

inline int toInt(string s) {
  int v;
  istringstream sin(s);
  sin >> v;
  return v;
}
template <class T> inline string toString(T x) {
  ostringstream sout;
  sout << x;
  return sout.str();
}
template <class T> inline T sqr(T x) { return x * x; }

typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<string> vs;
typedef pair<int, int> pii;
typedef long long ll;

#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define pb push_back
#define mp make_pair
#define each(i, c)                                                             \
  for (typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define exist(s, e) ((s).find(e) != (s).end())
#define range(i, a, b) for (int i = (a); i < (b); ++i)
#define rep(i, n) range(i, 0, n)
#define clr(a, b) memset((a), (b), sizeof(a))
#define dump(x) cerr << #x << " = " << (x) << endl;
#define debug(x)                                                               \
  cerr << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \
       << " " << __FILE__ << endl;

#define x first
#define y second

const double eps = 1e-10;
const double pi = acos(-1.0);
const ll INF = 1LL << 62;
const int inf = 1 << 29;

int dx[] = {1, 1, 0, -1, -1, -1, 0, 1};
int dy[] = {0, 1, 1, 1, 0, -1, -1, -1};

int main(void) {
  for (int w, h; cin >> h >> w, h;) {
    vs strs(h);
    rep(y, h) { cin >> strs[y]; }

    set<string> s;
    string res = "0";
    rep(y, h) {
      rep(x, w) {
        rep(i, 8) {
          string cur;
          pii next = mp(x, y);

          do {
            cur.pb(strs[next.y][next.x]);
            if (1 < cur.size() && !s.insert(cur).second) {
              if ((res.size() < cur.size()) ||
                  (res.size() == cur.size() && cur < res)) {
                res = cur;
              }
            }

            next.x = (next.x + dx[i] + w) % w;
            next.y = (next.y + dy[i] + h) % h;

          } while (next != mp(x, y));
        }
      }
    }

    cout << res << endl;
  }

  return 0;
}