Problem p03357 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <ciso646>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstring>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <utility>
#include <vector>
using namespace std;
typedef long long ll;
typedef unsigned int ui;
const ll mod = 1000000007;
typedef long double ld;
const ll INF = 1e+14;
typedef pair<int, int> P;
#define stop                                                                   \
  char nyaa;                                                                   \
  cin >> nyaa;
#define rep(i, n) for (int i = 0; i < n; i++)
#define per(i, n) for (int i = n - 1; i >= 0; i--)
#define Rep(i, sta, n) for (int i = sta; i < n; i++)
#define rep1(i, n) for (int i = 1; i <= n; i++)
#define per1(i, n) for (int i = n; i >= 1; i--)
#define Rep1(i, sta, n) for (int i = sta; i <= n; i++)
typedef complex<ld> Point;
const ld eps = 1e-8;
const ld pi = acos(-1.0);
typedef pair<ld, ld> LDP;
typedef pair<ll, ll> LP;
#define fr first
#define sc second
#define all(c) c.begin(), c.end()
#define pb push_back
#define int long long

void Yes() {
  cout << "Yes" << endl;
  exit(0);
}

void No() {
  cout << "No" << endl;
  exit(0);
}

signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  char c[4040];
  int a[4040];
  int dp[2020][2020]; // 左から(B,0),..,(B,i-1),
                      // (W,0),..(W,j-1)を目標順にするための最小swap数
  rep(i, 2 * n) {
    cin >> c[i] >> a[i];
    --a[i];
  }

  vector<vector<int>> black(n, vector<int>(n + 1, 0));
  //(B, i)の前に(B, iより大),(W, j以上)が何個あるか
  vector<vector<int>> white(n, vector<int>(n + 1, 0));
  //(W, i)の前に(W, iより大),(B, j以上)が何個あるか

  vector<int> bnum(n, 0);
  vector<int> wnum(n, 0);
  rep(i, 2 * n) {
    int num = a[i];
    char col = c[i];
    vector<int> bsum(n + 1, 0), wsum(n + 1, 0); // 前にあるi以上の個数
    per(j, n) {
      bsum[j] = bsum[j + 1] + bnum[j];
      wsum[j] = wsum[j + 1] + wnum[j];
    } // 後ろからみる
    per(j, n + 1) {
      if (col == 'B')
        black[num][j] = bsum[num + 1] + wsum[j];
      if (col == 'W')
        white[num][j] = wsum[num + 1] + bsum[j];
    }
    if (col == 'B') {
      bnum[num]++;
    } else {
      wnum[num]++;
    }
  }

  rep(i, 2020) {
    rep(j, 2020) { dp[i][j] = INF; }
  }
  dp[0][0] = 0;
  rep(i, n + 1) {
    rep(j, n + 1) {
      if (i < n)
        dp[i + 1][j] = min(dp[i + 1][j], dp[i][j] + black[i][j]);
      if (j < n)
        dp[i][j + 1] = min(dp[i][j + 1], dp[i][j] + white[i][j]);
    }
  }
  cout << dp[n][n] << endl;
  return 0;
}

Fixed submission:
#include <algorithm>
#include <bitset>
#include <ciso646>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstring>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <utility>
#include <vector>
using namespace std;
typedef long long ll;
typedef unsigned int ui;
const ll mod = 1000000007;
typedef long double ld;
const ll INF = 1e+14;
typedef pair<int, int> P;
#define stop                                                                   \
  char nyaa;                                                                   \
  cin >> nyaa;
#define rep(i, n) for (int i = 0; i < n; i++)
#define per(i, n) for (int i = n - 1; i >= 0; i--)
#define Rep(i, sta, n) for (int i = sta; i < n; i++)
#define rep1(i, n) for (int i = 1; i <= n; i++)
#define per1(i, n) for (int i = n; i >= 1; i--)
#define Rep1(i, sta, n) for (int i = sta; i <= n; i++)
typedef complex<ld> Point;
const ld eps = 1e-8;
const ld pi = acos(-1.0);
typedef pair<ld, ld> LDP;
typedef pair<ll, ll> LP;
#define fr first
#define sc second
#define all(c) c.begin(), c.end()
#define pb push_back
#define int long long

void Yes() {
  cout << "Yes" << endl;
  exit(0);
}

void No() {
  cout << "No" << endl;
  exit(0);
}

signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  char c[4040];
  int a[4040];
  int dp[2020][2020]; // 左から(B,0),..,(B,i-1),
                      // (W,0),..(W,j-1)を目標順にするための最小swap数
  rep(i, 2 * n) {
    cin >> c[i] >> a[i];
    --a[i];
  }

  vector<vector<int>> black(n, vector<int>(n + 1, 0));
  //(B, i)の前に(B, iより大),(W, j以上)が何個あるか
  vector<vector<int>> white(n, vector<int>(n + 1, 0));
  //(W, i)の前に(W, iより大),(B, j以上)が何個あるか

  vector<int> bnum(n, 0);
  vector<int> wnum(n, 0);
  rep(i, 2 * n) {
    int num = a[i];
    char col = c[i];
    vector<int> bsum(n + 1, 0), wsum(n + 1, 0); // 前にあるi以上の個数
    per(j, n) {
      bsum[j] = bsum[j + 1] + bnum[j];
      wsum[j] = wsum[j + 1] + wnum[j];
    } // 後ろからみる
    per(j, n + 1) {
      if (col == 'B')
        black[num][j] = bsum[num + 1] + wsum[j];
      if (col == 'W')
        white[num][j] = wsum[num + 1] + bsum[j];
    }
    if (col == 'B') {
      bnum[num]++;
    } else {
      wnum[num]++;
    }
  }

  rep(i, 2020) {
    rep(j, 2020) { dp[i][j] = INF; }
  }
  dp[0][0] = 0;
  rep(i, n + 1) {
    rep(j, n + 1) {
      if (i < n)
        dp[i + 1][j] = min(dp[i + 1][j], dp[i][j] + black[i][j]);
      if (j < n)
        dp[i][j + 1] = min(dp[i][j + 1], dp[i][j] + white[j][i]);
    }
  }
  cout << dp[n][n] << endl;
  return 0;
}