Problem p04013 (C++)

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

#define int long long
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()
#define rep(i, n) for (int i = 0; i < n; ++i)
#define rep1(i, n) for (int i = 1; i < n; ++i)
#define exrep(i, a, b) for (ll i = a; i < b; i++)
#define out(x) cout << x << endl
#define EPS (1e-7)
#define gearup                                                                 \
  ios_base::sync_with_stdio(0);                                                \
  cin.tie(0);                                                                  \
  cout.tie(0);

typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<char> vc;
typedef vector<bool> vb;
typedef vector<double> vd;
typedef vector<string> vs;
typedef vector<pair<int, int>> vpii;
typedef vector<vector<int>> vvi;
typedef vector<vector<char>> vvc;
typedef vector<vector<bool>> vvb;
typedef vector<vector<double>> vvd;
typedef vector<vector<string>> vvs;
typedef vector<ll> vl;
typedef vector<vector<ll>> vvl;
typedef vector<vector<vector<ll>>> vvvl;
ll MOD = 1000000007;
const long long L_INF = 1LL << 60;
const int INF = 2147483647; // 2^31-1
const double PI = acos(-1);
// cout<<fixed<<setprecision(10);

template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> void debug(T v) {
  rep(i, v.size()) cout << v[i] << " ";
  cout << endl;
}
const ll dx[8] = {1, 1, 0, -1, -1, -1, 0, 1};
const ll dy[8] = {0, 1, 1, 1, 0, -1, -1, -1};
// 以降 cin の入力元が 'input.txt' になる
// std::ifstream in("input.txt");
// std::cin.rdbuf(in.rdbuf());

ll dp[51][51][3000];
signed main() {
  gearup;
  int n, a;
  cin >> n >> a;
  vl x(n);
  rep(i, n) cin >> x[i];
  dp[0][0][0] = 1;
  rep(i, n) {       // 種類
    rep(j, n + 1) { // 何枚選ぶか
      rep(k, n * a + 1) {
        dp[i + 1][j][k] += dp[i][j][k];            // 選ばない
        dp[i + 1][j + 1][k + x[i]] += dp[i][j][k]; // 選ぶ
      }
    }
  }
  ll res = 0;
  rep(i, n) res += dp[n][i + 1][(i + 1) * a];
  out(res);
}

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

#define int long long
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()
#define rep(i, n) for (int i = 0; i < n; ++i)
#define rep1(i, n) for (int i = 1; i < n; ++i)
#define exrep(i, a, b) for (ll i = a; i < b; i++)
#define out(x) cout << x << endl
#define EPS (1e-7)
#define gearup                                                                 \
  ios_base::sync_with_stdio(0);                                                \
  cin.tie(0);                                                                  \
  cout.tie(0);

typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<char> vc;
typedef vector<bool> vb;
typedef vector<double> vd;
typedef vector<string> vs;
typedef vector<pair<int, int>> vpii;
typedef vector<vector<int>> vvi;
typedef vector<vector<char>> vvc;
typedef vector<vector<bool>> vvb;
typedef vector<vector<double>> vvd;
typedef vector<vector<string>> vvs;
typedef vector<ll> vl;
typedef vector<vector<ll>> vvl;
typedef vector<vector<vector<ll>>> vvvl;
ll MOD = 1000000007;
const long long L_INF = 1LL << 60;
const int INF = 2147483647; // 2^31-1
const double PI = acos(-1);
// cout<<fixed<<setprecision(10);

template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> void debug(T v) {
  rep(i, v.size()) cout << v[i] << " ";
  cout << endl;
}
const ll dx[8] = {1, 1, 0, -1, -1, -1, 0, 1};
const ll dy[8] = {0, 1, 1, 1, 0, -1, -1, -1};
// 以降 cin の入力元が 'input.txt' になる
// std::ifstream in("input.txt");
// std::cin.rdbuf(in.rdbuf());

ll dp[55][55][3000];
signed main() {
  gearup;
  int n, a;
  cin >> n >> a;
  vl x(n);
  rep(i, n) cin >> x[i];
  dp[0][0][0] = 1;
  rep(i, n) {       // 種類
    rep(j, n + 1) { // 何枚選ぶか
      rep(k, n * a + 1) {
        dp[i + 1][j][k] += dp[i][j][k];            // 選ばない
        dp[i + 1][j + 1][k + x[i]] += dp[i][j][k]; // 選ぶ
      }
    }
  }
  ll res = 0;
  rep(i, n) res += dp[n][i + 1][(i + 1) * a];
  out(res);
}