Problem p03357 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (int)n; i++)
#define all(c) (c).begin(), (c).end()
#define pb push_back
#define dbg(...)                                                               \
  do {                                                                         \
    cerr << __LINE__ << ": ";                                                  \
    dbgprint(#__VA_ARGS__, __VA_ARGS__);                                       \
  } while (0);

using namespace std;

namespace std {
template <class S, class T> struct hash<pair<S, T>> {
  size_t operator()(const pair<S, T> &p) const {
    return ((size_t)1e9 + 7) * hash<S>()(p.first) + hash<T>()(p.second);
  }
};
template <class T> struct hash<vector<T>> {
  size_t operator()(const vector<T> &v) const {
    size_t h = 0;
    for (auto i : v)
      h = h * ((size_t)1e9 + 7) + hash<T>()(i) + 1;
    return h;
  }
};
} // namespace std
template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << "[ ";
  rep(i, v.size()) os << v[i] << (i == v.size() - 1 ? " ]" : ", ");
  return os;
}
template <class T> ostream &operator<<(ostream &os, const set<T> &v) {
  os << "{ ";
  for (const auto &i : v)
    os << i << ", ";
  return os << "}";
}
template <class T, class U>
ostream &operator<<(ostream &os, const map<T, U> &v) {
  os << "{";
  for (const auto &i : v)
    os << " " << i.first << ": " << i.second << ",";
  return os << "}";
}
template <class T, class U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  return os << "(" << p.first << ", " << p.second << ")";
}
void dbgprint(const string &fmt) { cerr << endl; }
template <class H, class... T>
void dbgprint(const string &fmt, const H &h, const T &...r) {
  cerr << fmt.substr(0, fmt.find(",")) << "= " << h << " ";
  dbgprint(fmt.substr(fmt.find(",") + 1), r...);
}
typedef long long ll;
typedef vector<int> vi;
typedef pair<int, int> pi;
const int inf = (int)1e9;
const double INF = 1e12, EPS = 1e-9;

int cntw[2001][2001], cntb[2001][2001];
int dp[2001][2001];

int main() {
  cin.tie(0);
  cin.sync_with_stdio(0);
  int n;
  cin >> n;
  vector<pi> in;
  vi pos(2 * n);
  rep(i, 2 * n) {
    char c;
    int p;
    cin >> c >> p;
    p--;
    in.emplace_back(c == 'B', p);
    pos[p + (c == 'B' ? n : 0)] = i;

    if (i) {
      rep(j, 2 * n) cntw[i][j] = cntw[i - 1][j];
      rep(j, 2 * n) cntb[i][j] = cntb[i - 1][j];
    }
    if (c == 'B')
      for (int j = p; j < 2 * n; j++)
        cntb[i][j]++;
    if (c == 'W')
      for (int j = p; j < 2 * n; j++)
        cntw[i][j]++;
  }

  // rep(i, 2 * n) rep(j, 2 * n) cerr<<cntw[i][j]<<(j==2*n-1?"\n":" ");

  rep(i, n + 1) rep(j, n + 1) dp[i][j] = inf;
  dp[0][0] = 0;
  rep(i, n + 1) rep(j, n + 1) {
    if (i < n) {
      int p = pos[i], w = cntw[p][2 * n - 1] - cntw[p][i],
          b = cntb[p][2 * n - 1] - (j ? cntb[p][j - 1] : 0);
      dp[i + 1][j] = min(dp[i + 1][j], dp[i][j] + w + b);
    }
    if (j < n) {
      int p = pos[j + n], w = cntw[p][2 * n - 1] - (i ? cntw[p][i - 1] : 0),
          b = cntb[p][2 * n - 1] - cntb[p][j];
      dp[i][j + 1] = min(dp[i][j + 1], dp[i][j] + w + b);
    }
  }
  // dbg(pos[0], cntw[pos[0]][2 * n - 1]);
  // rep(i, n + 1) rep(j, n + 1) cerr<<dp[i][j]<<(j==n?"\n":" ");
  cout << dp[n][n] << endl;

  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (int)n; i++)
#define all(c) (c).begin(), (c).end()
#define pb push_back
#define dbg(...)                                                               \
  do {                                                                         \
    cerr << __LINE__ << ": ";                                                  \
    dbgprint(#__VA_ARGS__, __VA_ARGS__);                                       \
  } while (0);

using namespace std;

namespace std {
template <class S, class T> struct hash<pair<S, T>> {
  size_t operator()(const pair<S, T> &p) const {
    return ((size_t)1e9 + 7) * hash<S>()(p.first) + hash<T>()(p.second);
  }
};
template <class T> struct hash<vector<T>> {
  size_t operator()(const vector<T> &v) const {
    size_t h = 0;
    for (auto i : v)
      h = h * ((size_t)1e9 + 7) + hash<T>()(i) + 1;
    return h;
  }
};
} // namespace std
template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << "[ ";
  rep(i, v.size()) os << v[i] << (i == v.size() - 1 ? " ]" : ", ");
  return os;
}
template <class T> ostream &operator<<(ostream &os, const set<T> &v) {
  os << "{ ";
  for (const auto &i : v)
    os << i << ", ";
  return os << "}";
}
template <class T, class U>
ostream &operator<<(ostream &os, const map<T, U> &v) {
  os << "{";
  for (const auto &i : v)
    os << " " << i.first << ": " << i.second << ",";
  return os << "}";
}
template <class T, class U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  return os << "(" << p.first << ", " << p.second << ")";
}
void dbgprint(const string &fmt) { cerr << endl; }
template <class H, class... T>
void dbgprint(const string &fmt, const H &h, const T &...r) {
  cerr << fmt.substr(0, fmt.find(",")) << "= " << h << " ";
  dbgprint(fmt.substr(fmt.find(",") + 1), r...);
}
typedef long long ll;
typedef vector<int> vi;
typedef pair<int, int> pi;
const int inf = (int)1e9;
const double INF = 1e12, EPS = 1e-9;

int cntw[4001][4001], cntb[4001][4001];
int dp[2001][2001];

int main() {
  cin.tie(0);
  cin.sync_with_stdio(0);
  int n;
  cin >> n;
  vector<pi> in;
  vi pos(2 * n);
  rep(i, 2 * n) {
    char c;
    int p;
    cin >> c >> p;
    p--;
    in.emplace_back(c == 'B', p);
    pos[p + (c == 'B' ? n : 0)] = i;

    if (i) {
      rep(j, 2 * n) cntw[i][j] = cntw[i - 1][j];
      rep(j, 2 * n) cntb[i][j] = cntb[i - 1][j];
    }
    if (c == 'B')
      for (int j = p; j < 2 * n; j++)
        cntb[i][j]++;
    if (c == 'W')
      for (int j = p; j < 2 * n; j++)
        cntw[i][j]++;
  }

  // rep(i, 2 * n) rep(j, 2 * n) cerr<<cntw[i][j]<<(j==2*n-1?"\n":" ");

  rep(i, n + 1) rep(j, n + 1) dp[i][j] = inf;
  dp[0][0] = 0;
  rep(i, n + 1) rep(j, n + 1) {
    if (i < n) {
      int p = pos[i], w = cntw[p][2 * n - 1] - cntw[p][i],
          b = cntb[p][2 * n - 1] - (j ? cntb[p][j - 1] : 0);
      dp[i + 1][j] = min(dp[i + 1][j], dp[i][j] + w + b);
    }
    if (j < n) {
      int p = pos[j + n], w = cntw[p][2 * n - 1] - (i ? cntw[p][i - 1] : 0),
          b = cntb[p][2 * n - 1] - cntb[p][j];
      dp[i][j + 1] = min(dp[i][j + 1], dp[i][j] + w + b);
    }
  }
  // dbg(pos[0], cntw[pos[0]][2 * n - 1]);
  // rep(i, n + 1) rep(j, n + 1) cerr<<dp[i][j]<<(j==n?"\n":" ");
  cout << dp[n][n] << endl;

  return 0;
}