Problem p03165 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;

#define REP(i, s) for (int i = 0; i < s; ++i)
#define ALL(v) (v).begin(), (v).end()
#define COUT(x) cout << #x << " = " << (x) << " (L" << __LINE__ << ")" << endl
#define EACH(i, s)                                                             \
  for (__typeof__((s).begin()) i = (s).begin(); i != (s).end(); ++i)
#define DEBUG
#define int long long
#define INF 1e9
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T1, class T2> ostream &operator<<(ostream &s, pair<T1, T2> P) {
  return s << '<' << P.first << ", " << P.second << '>';
}
template <class T> ostream &operator<<(ostream &s, vector<T> P) {
  for (int i = 0; i < P.size(); ++i) {
    if (i > 0) {
      s << " ";
    }
    s << P[i];
  }
  return s;
}
template <class T> ostream &operator<<(ostream &s, vector<vector<T>> P) {
  for (int i = 0; i < P.size(); ++i) {
    s << endl << P[i];
  }
  return s << endl;
}
template <class T> ostream &operator<<(ostream &s, set<T> P) {
  EACH(it, P) { s << "<" << *it << "> "; }
  return s << endl;
}
template <class T1, class T2> ostream &operator<<(ostream &s, map<T1, T2> P) {
  EACH(it, P) { s << "<" << it->first << "->" << it->second << "> "; }
  return s << endl;
}
template <class T> void show(vector<T> v) {
  for (int i = 0; i < v.size(); i++) {
    cerr << v[i] << " ";
  }
  cerr << "\n";
}
typedef long long ll;
int dp[3010][3010];
signed main() {
  string s, t;
  cin >> s >> t;
  dp[0][0] = 0;
  string ans = "";
  for (int i = 1; i <= s.size(); i++) {
    for (int j = 1; j <= t.size(); j++) {
      if (s[i - 1] == t[j - 1]) {
        dp[i][j] = dp[i - 1][j - 1] + 1;

      } else {
        dp[i][j] = max(dp[i - 1][j], dp[i][j - 1]);
      }
    }
  }
  int i = s.size(), j = t.size();
  while (i > 0 && j > 0) {
    if (dp[i][j] == dp[i - 1][j])
      i--;
    else if (dp[i][j] == dp[i][j - 1])
      j--;
    else {
      ans += s[i - 1] + ans;
      i--, j--;
    }
  }
  cout << ans << endl;
  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;

#define REP(i, s) for (int i = 0; i < s; ++i)
#define ALL(v) (v).begin(), (v).end()
#define COUT(x) cout << #x << " = " << (x) << " (L" << __LINE__ << ")" << endl
#define EACH(i, s)                                                             \
  for (__typeof__((s).begin()) i = (s).begin(); i != (s).end(); ++i)
#define DEBUG
#define int long long
#define INF 1e9
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T1, class T2> ostream &operator<<(ostream &s, pair<T1, T2> P) {
  return s << '<' << P.first << ", " << P.second << '>';
}
template <class T> ostream &operator<<(ostream &s, vector<T> P) {
  for (int i = 0; i < P.size(); ++i) {
    if (i > 0) {
      s << " ";
    }
    s << P[i];
  }
  return s;
}
template <class T> ostream &operator<<(ostream &s, vector<vector<T>> P) {
  for (int i = 0; i < P.size(); ++i) {
    s << endl << P[i];
  }
  return s << endl;
}
template <class T> ostream &operator<<(ostream &s, set<T> P) {
  EACH(it, P) { s << "<" << *it << "> "; }
  return s << endl;
}
template <class T1, class T2> ostream &operator<<(ostream &s, map<T1, T2> P) {
  EACH(it, P) { s << "<" << it->first << "->" << it->second << "> "; }
  return s << endl;
}
template <class T> void show(vector<T> v) {
  for (int i = 0; i < v.size(); i++) {
    cerr << v[i] << " ";
  }
  cerr << "\n";
}
typedef long long ll;
int dp[3010][3010];
signed main() {
  string s, t;
  cin >> s >> t;
  dp[0][0] = 0;
  string ans = "";
  for (int i = 1; i <= s.size(); i++) {
    for (int j = 1; j <= t.size(); j++) {
      if (s[i - 1] == t[j - 1]) {
        dp[i][j] = dp[i - 1][j - 1] + 1;

      } else {
        dp[i][j] = max(dp[i - 1][j], dp[i][j - 1]);
      }
    }
  }
  int i = s.size(), j = t.size();
  while (i > 0 && j > 0) {
    if (dp[i][j] == dp[i - 1][j])
      i--;
    else if (dp[i][j] == dp[i][j - 1])
      j--;
    else {
      ans = s[i - 1] + ans;
      i--, j--;
    }
  }
  cout << ans << endl;
  return 0;
}